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Is attachment substrate a prerequisite for mussels to establish on soft-sediment substrate?
Journal of Experimental Marine Biology and Ecology 495 (2017) 83–88
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Is attachment substrate a prerequisite for mussels to establish on softsediment substrate?
MARK
Mark Wilcox⁎, Andrew Jeffs Leigh Marine Laboratory, Institute of Marine Science, University of Auckland, P.O. Box 349, Warkworth 0941, New Zealand
A R T I C L E I N F O
A B S T R A C T
Keywords: Attachment substrate Green-lipped mussels P. canaliculus Shellfish restoration Soft-sediment
It is unknown whether the presence of hard substrate is a necessary prerequisite for the attachment and establishment of mussels, especially on soft-sediment habitats where hard substrates are scarce. Therefore, we examined the importance of natural attachment substrates in the form of mussel shell and adult conspecifics for the establishment of juvenile and adult green-lipped mussels (Perna canaliculus) on soft-sediment. In field experiments where shell material was added as substrate to soft-sediment it made no difference to the subsequent retention of adult mussels. Laboratory experiments showed that juvenile mussels preferentially sought out, and attached to adult mussels compared to remaining on unmodified soft-sediment. Furthermore, juvenile mussels attached to live adult mussels had higher survival in the presence of a common sea star predator compared to juveniles on unmodified soft-sediment or attached to mussel shell. The results suggest that establishment of mussel beds on soft-sediment requires only adult mussels, which receive sufficient anchorage through attachment to neighbouring adults and in so doing, providing a stable, complex substrate suitable for improving the survival of establishing juvenile mussels by protecting them from sea star predators.
1. Introduction
habitat for settling larvae (Commito, 1987). For the many species of mussel that inhabit rocky coastal habitats, hard substrates for attachment are abundant. In addition to attaching to conspecifics, these mussels can attach directly to the primary substrate. On soft-sediment habitats, however, hard substrates for attachment are sparse and mussels rarely attach to the primary sediment (Bayne, 1964; Commito et al., 2005) as the byssal threads are often unable to attach to the small grain sizes which would not provide sufficient anchorage. In these environments, mussels rely on rocks, shells, and predominantly conspecifics for attachment (Commito et al., 2014) with experiments showing that recruiting mussels primarily use these attachment materials rather than bare soft-sediment (Commito et al., 2014; van der Heide et al., 2014). Whether by the dislodgement and transportation of adults or the settlement of larvae, the importance of substrates in the establishment of mussel beds on soft-sediment is not clear. The re-establishment of adult northern horse mussels, Modiolus modiolus, showed no increase in survival when transplanted onto shell cultch of either high or low relief when compared to bare soft-sediment (Fariñas-Franco et al., 2013). Likewise, the survival of transplanted adult blue mussels, Mytilus edulis, was not higher on natural fibre mats made of coir compared to those transplanted directly onto soft-sediment (de Paoli et al., 2015). This suggests that additional attachment substrates aside from conspecifics are likely unnecessary for the establishment of mussel beds
Epifaunal bivalves such as oysters and mussels anchor themselves to hard substrates through permanent cementation and detachable byssal threads, respectively. This process of attachment helps to reduce the likelihood of dislodgement and transport away from selected environments (Bell and Gosline, 1997; Hunt and Scheibling, 2001) which can result in mortality (Carrington et al., 2009; Petrović and Guichard, 2008). Bivalves are at particular risk of dislodgement during storm events as well as in areas of naturally strong hydrodynamic conditions (Carrington et al., 2009; Denny, 1987; Hunt and Scheibling, 2001; Petrović and Guichard, 2008). The hard substrates which these bivalves attach to include natural cobbles and bedrock, anthropogenic structures, as well as other organisms (Commito et al., 2014; Dankers et al., 2001; Dolmer and Frandsen, 2002; McGrorty et al., 1993; Southgate and Myers, 1985), of which conspecifics are a common substrate (Commito et al., 2014). The gregarious nature of many epifaunal bivalves often leads to the formation of extensive populations, known as beds, which occur both intertidally and subtidally within coastal ecosystems. The attachment of bivalves within these beds not only reduces the risk of dislodgement but has also been shown to reduce the risk of predation by crabs (Leonard et al., 1999). In addition, the complex substrate created by the aggregating bivalves provides a preferred
⁎
Corresponding author. E-mail addresses: [email protected] (M. Wilcox), a.jeff[email protected] (A. Jeffs).
http://dx.doi.org/10.1016/j.jembe.2017.07.004 Received 26 November 2016; Received in revised form 31 May 2017; Accepted 8 July 2017 0022-0981/ © 2017 Elsevier B.V. All rights reserved.
Journal of Experimental Marine Biology and Ecology 495 (2017) 83–88
M. Wilcox, A. Jeffs
2.3. Use of substrate by adult mussels in the field
by adult mussels. In contrast, larval recruitment of both northern horse mussels and blue mussels were both higher in the presence of adults compared with any other available substrate including shell cultch (Commito et al., 2014; Fariñas-Franco et al., 2013). The survival of seed mussels was also shown to be greatest when provided with more complex substrates (Frandsen and Dolmer, 2002). This suggests that on soft-sediment, available attachment substrates may provide critical habitat for establishing larval and juvenile mussels. Extensive mussel beds of the green-lipped mussel, Perna canaliculus, covering over 1300 km2 on soft-sediment in the Hauraki Gulf, New Zealand, were nearly extirpated by dredge fishing during the last century (Greenway, 1969; Reid, 1969). Despite the closure of the fishery in 1969 (Paul, 2012), there has been no sign of natural recovery to date. The removal of the adult mussel beds through fishing has subsequently led to the removal of much of the available hard substrate which could have contributed to the lack of recovery in this population. The aim of this study is therefore to determine whether attachment substrates are necessary for the establishment of mussel beds on soft-sediment. This was accomplished using a series of laboratory and field experiments examining particular benefits to establishing adult and juvenile mussels provided with two common attachment substrates found within natural soft-sediment mussel beds, i.e., adult mussels and mussel shell. This study will address the hypotheses of whether; (1) conspecific shell increases the persistence of adult mussels establishing on soft-sediment, (2) conspecific shell and/or adult conspecifics increases the persistence of juvenile mussels establishing on soft-sediment, (3) juvenile mussels establishing on soft-sediment preferentially attach to conspecific shells and/or adults, and (4) the attachment of establishing juvenile mussels to mussel shell and/or adult mussels increases survival in the presence of a common sea star predator. The results of these experiments will help to increase our understanding of why natural recovery fails in depleted mussel populations and will have implications for restoration initiatives in this and other mussel species.
On 26 November 2013, twenty 0.25 m2 (0.5 × 0.5 m) plots were established by divers in the field arranged in five rows, each containing four plots. Each plot was separated by a distance of 1.5 m and marked with a subsurface float. The crossed experimental design consisted of a substrate level of either unmodified soft-sediment or the addition of 60 clean adult mussel shells (80–100 mm SL) which was crossed with a predator exclusion or access level, with a total of five replicates per treatment. This number of adult mussel shells was used to ensure that they would provide sufficient attachment substrate for juvenile mussels in the experiment. Each of the five rows of plots contained one replicate of each treatment arranged in a random order. Predator exclusion plots were enclosed in a lightweight stainless steel frame covered with coarse plastic mesh (20 mm openings). The plastic mesh prevented large mobile predators, such as fish and lobster, from removing and consuming mussels from the plots, while not unduly restricting water flow to the mussels inside. The experimental design did not include an additional control treatment for possible artefacts caused by caging because the primary aim of the caging was to determine whether or not mussels were being lost from the experiment due to emigration or predation. Forty live adult green-lipped mussels were then transplanted into each plot to establish a density of mussels typical of the wide range of densities found in natural beds of these mussels (McLeod, 2009). After 50 days, the number of surviving mussels in each plot was enumerated by divers.
2.4. Use of substrate by juvenile mussels in the field On 26 November 2013, a total of fifteen 1.5 m2 circular plots were established by divers in three rows of five plots at the field site and marked with subsurface floats. The experiment consisted of three substrate treatments; 1) unmodified soft-sediment substrate, 2) addition of ≈ 250 adult mussel shells (80–100 mm SL) and, 3) addition of ≈ 1200 live adult mussels. Quantities of adult mussels and mussel shell ensured the entire plot was covered with the available attachment substrate and the density of live mussels was consistent with the densities found in natural beds of these mussels (McLeod, 2009). The three substrate treatments were each randomly allocated to five of the plots with no more than two of the same substrate treatment per row. In the laboratory, macroalgal material with attached juvenile mussels that had been previously collected from Ninety Mile Beach was divided into fifteen roughly equal bundles. Each bundle weighed 0.318 kg ( ± 0.013 SE) and based on mussel counts from weighed subsamples of mussel laden algae, each bundle contained on average 5298 mussels ( ± 353 SE). The bundles were then each enclosed within a biodegradable mesh sock (5–10 mm mesh size), commonly used for the deployment of juvenile mussels on seaweed in aquaculture operations (Jeffs et al., 1999), that helped to maintain the pre-measured quantities of juvenile mussels during transport. Bundles were transported to the site and secured by divers to the centre of each of the 15 experimental plots with a stainless steel pin driven 10 cm into the sediment. The mesh socks also helped to ensure the macroalgae with attached juvenile mussels remained within the positioned plot. The mesh size used did not unduly inhibit the movement of the juvenile mussels into and out of the sock, allowing the juvenile mussels to freely disperse onto the plot. After a period of 44 days each plot was surveyed by haphazardly placing a 0.0625 m2 quadrat within each quarter of the plot and quantifying the number of remaining juvenile mussels in situ (1–5 mm SL) within each of the four quadrats for each plot. Divers haphazardly placed the quadrat by releasing the frame from 1.5 m above the plot and allowing it to fall to the seabed. Clear visibility at this site allowed divers to observe the entire quadrat and identify the presence of mussels > 1 mm SL.
2. Materials and methods 2.1. Field study site The Hauraki Gulf is located on the northeastern coast of the North Island of New Zealand. Field experiments were conducted in a sheltered coastal embayment on the northern section of Kawau Bay, in the Hauraki Gulf (36° 22′ 47″ S, 174° 49′ 02″ E). All experimental plots were situated on fine sand substrate at a depth of 4.1 to 4.9 m below chart datum. 2.2. Mussel sources Wild juvenile mussels (P. canaliculus) are regularly found attached to drifting algae (Alfaro et al., 2010) and were collected from Ninety Mile Beach (35° 02′ 08″ S, 173° 10′ 05″ E) and Muriwai Beach (36° 50′ 05″ S, 174° 27′ 59″ E) in northern New Zealand. All juvenile mussels (1–5 mm shell length (SL)) that were collected and used in the present study were housed in aquaria supplied with ambient seawater with aeration until utilised in field and laboratory experiments. Adult mussels were most easily obtained from the extensive aquaculture operations in the Hauraki Gulf, where juvenile mussels originating from wild sources at Ninety Mile Beach in northern New Zealand, are grown on suspended long lines until they reach commercial size (Jeffs et al., 1999). All the adult mussels (80–100 mm SL) utilised in these experiments were first cleared of all fouling organisms and kept in aquaria with flow-through seawater until deployment. Given that aquacultured mussels are the most readily available source of mussels for restoration, maintaining a similar size range of adult mussels throughout these experiments ensures the results will be applicable to future restoration initiatives. 84
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M. Wilcox, A. Jeffs
3.5–5 cm as measured from the tip of the longest arm to the opposite side of the oral disc. Into each container 100 juvenile mussels (1–5 mm SL, from Muriwai Beach) were placed and the number of surviving mussels was assessed after one week. The experiment was run for five consecutive weeks (i.e., a total of five experimental blocks) and commenced on 12 November 2015 and concluded on 24 December 2015. Novel sea stars and mussels were utilised in each successive experimental block. 2.7. Statistical analyses The data was inspected for deviations from normality and heterogeneity of variance firstly by visually inspecting quartile-quartile plots and plots of residuals versus fitted values (respectively) of models prior to running the analysis. For the experiments on substrate use by adult mussels, the data deviated from normality and were therefore fitted to a Poisson distribution using a general linear model (GLM) and tested using a Wald chi-square test to determine if differences in the number of surviving mussels were due to either the substrate and/or access by predators. For the experiment on selection of substrate by juvenile mussels, the data was normally distributed and thus were fitted to a Gaussian distribution using a linear model (LM) and tested using an ANOVA for differences in the percent of mussels among substrate treatments within each area separately. For the experiment assessing potential predation refuge from sea star predators, the data again deviated from normality and were therefore fitted to a Poisson distribution using a general linear mixed-effects model (GLMER) incorporating blocks as a random effect. A Wald chi-square test was utilised to determine if differences in mortality were attributed to the substrates and/or presence of predators. Significance was further examined using pairwise t-tests with a false discovery rate correction for multiple comparisons. For the field experiment on survival of juvenile mussels, results are presented using descriptive statistics only because there was very low recovery of juvenile mussels across all plots. All statistical tests were computed in R version 3.2.3 and RStudio version 0.99.879.
Fig. 1. Experimental laboratory setup of circular containers used in juvenile substrate selection experiments, depicting the (A) central, (B) intermediate, and (C) outer areas of the floor of the tank. Mud substrate was provided evenly across all areas, but different substrates were experimentally placed in the intermediate area, i.e., addition of live adult mussels versus addition of mussel shells versus no modifications (soft-sediment).
2.5. Substrate selection by juvenile mussels in the laboratory Nine round 60 l plastic tubs (320 mm diameter base, 500 mm high) were filled evenly across their base with natural sediment (sieved to a grain size < 1 mm) to a depth of 10 mm. The tubs were then filled with 20 l of filtered seawater (5 μm) which was aerated via a single airstone placed in the centre above the sediment. The bottom of each tub was sectioned into three concentric areas arranged by distance from the centre of the tub; 1) central area (0–5 cm), 2) intermediate area (5–10 cm) and, 3) outer area (beyond 10 cm and including the walls of the tub) (Fig. 1). The three treatments consisted of three modifications to the intermediate area only; either unmodified natural sediment substrate, adult mussels (nine mussels in groups of three), or mussel shell (18 clean adult mussel shells) placed as three groups of six shells. Live adult mussels and empty waste mussel shells from aquaculture operations both have the potential to be provided as substrate for mussel bed restoration initiatives. Three replicates per substrate treatment were used in each of two trials on 23 and 25 May 2015 for a combined total of six replicates per treatment using fresh mussels for each trial. For each replicate, 40 wild juvenile mussels of 1–5 mm shell length sourced from Ninety Mile Beach were placed within 2 cm of the central area of each tub. After 24 h the juvenile mussels in each of the designated areas were enumerated.
3. Results 3.1. Use of substrate by adult mussels in the field There were no significant effects of the provision of mussel shell as attachment substrate, the access or exclusion of predators, or their interaction on the survival of adult mussels across plots (Table 1). Survival rates across all replicates was high (mean survival 93.1 ± 0.9% SE) and no mussels out of the 40 mussels that were initially deployed into the plots were found outside of the plots indicating that little or no emigration occurred during the experiment.
2.6. Predation refuge provided by substrates in the laboratory 3.2. Use of substrate by juvenile mussels in the field Six 3.5 l square containers (180 × 180 × 120 mm) were filled evenly across the base with natural sediment (grain size < 1 mm) to a depth of 10 mm. Each container was supplied with flow-through (10 l h− 1), filtered seawater (5 μm) which was aerated via a single airstone. Water escaped through the screened (0.25 mm) outlets to the containers which were located 30 mm below the lip of the container, maintaining approximately 2 l of seawater within and a dry surface which prohibited mussels from escaping. The crossed experimental design consisted of a substrate factor of either unmodified soft-sediment, mussel shell (20 clean adult mussel shells), or adult mussels (10 adults in a single group) which was crossed with a predation factor, i.e., presence or absence of a sea star predator for a total of six experimental treatments. One container was set up for each of the six experimental treatments and those treatments containing a predator were supplied with a small eleven-armed sea star, Coscinasterias muricata, which are known to consume mussels. These sea stars ranged in size from
There was very low recovery of the juvenile mussels transplanted into each plot, with only two individuals being the highest recorded number of juveniles in any sampled quadrat. No juvenile mussels were found in any plots of the unmodified soft-sediment treatment. A total of three juvenile mussels were found across all plots in the mussel shell Table 1 Summary of statistical test for differences in survival of transplanted adult mussels in the presence/exclusion of predators and either provided addition attachment substrate or transplanted to the bare soft-sediment. Significance was determined at an α of 0.05.
Substrate Predation Substrate ∗ predation
85
Chi square
DF
p value
0.001 1.462 0.387
1 1 1
0.97 0.23 0.53
Journal of Experimental Marine Biology and Ecology 495 (2017) 83–88
M. Wilcox, A. Jeffs
Table 3 Results of chi-square tests examining mortality of juvenile mussels in the presence/absence of a sea star predator when provided different attachment substrates. Significance was determined at an α of 0.05. Wald chi-square test
DF
χ2 value
p value
Substrate Predation Substrate ∗ predation
2 1 2
39.76 321.12 13.31
< 0.001 < 0.001 0.001
percentage of mussels on the soft-sediment substrate (t = 2.45, DF = 15, p = 0.03). Juvenile mussels that had moved into the mussel shell treatment areas were always found attached to shells while juveniles in the adult mussel treatment were always found attached to the live adult mussels, either on their shells or among their byssal threads.
3.4. Predation refuge provided by substrates in the laboratory
Fig. 2. Percentage of juvenile mussels ( ± SE) occupying the central, intermediate, and outer areas of the juvenile substrate selection experiment when live adult mussels, mussel shell, or no additional substrate was provided in the intermediate area of the experimental tank (see Fig. 1). Significant differences between the three substrate treatments within each area of the tank are indicated by different letters above the bars.
There were significant differences in the mortality of green-lipped mussels for both the substrate and predation factors as well as their interaction (Table 3). All experimental combinations without a predator had consistently low levels of mortality of juvenile mussels and were not different among the three types of substrate (All p values > 0.6) (Fig. 3). The mortality of juvenile mussels in treatment combinations without a predator was also consistently lower than every treatment combination where a predator was present (All p values < 0.001). The experimental combinations with a predator and either the unmodified substrate or the shell substrate had similar high levels of mortality of juvenile mussels (t = 2.41, p = 0.216). However, the mortality of juvenile mussels in the experimental combination containing a predator and adult mussels as substrate was significantly lower than those containing a predator and either unmodified soft-sediment substrate or the shell substrate (p values < 0.001). There was no mortality of the adult mussels provided as substrate within this experiment and thus the lower mortality of the juvenile mussels was not due to the sea stars consuming the adult mussels instead of the juveniles. Sea stars were seen actively consuming juvenile mussels and all mussels were accounted for in every tank at the end of the experimental run. It was observed that juvenile mussels in the unmodified substrate treatment were either dispersed over the soft-sediment or clumped together whereas juvenile mussels in shell and adult mussel treatments were predominantly observed to be attached to the hard structures of the shells and byssal threads of adult
treatment, resulting in a mean density of 2.4 ± 1.7 mussels m− 2 (SE). These juvenile mussels were only found attached to the adult mussel shell which provided a very low relief and had mostly become buried by sediment. Only four juvenile mussels were found across all plots in the adult mussel treatment, resulting in a mean density of 3.2 ± 1.5 mussels m− 2 (SE). Unlike the mussel shells, the adult mussels provided higher relief for attachment of the juveniles, with all juvenile mussels being found at least 10 mm above the sediment.
3.3. Substrate selection by juvenile mussels in the laboratory Overall, the majority (58.6%) of the juvenile mussels remained in the central area after 24 h (Fig. 2), often attached to other juvenile mussels, and the percentage of mussels remaining in the central area did not differ among the treatments (Table 2). A minority of juvenile mussels (9.8%) moved into the outer area after 24 h and the percentage of mussels in this outer area was different among the three treatments, with a significantly smaller percentage of mussels occupying this area in the adult mussel versus soft-sediment control treatments (t = 3.07, DF = 15, p = 0.01) as well as the mussel shell versus soft-sediment control treatments (t = 2.97, DF = 15, p = 0.01), but there was no significant difference in the percentage of mussels between the adult mussel and mussel shell treatments (t = 0.11, DF = 15, p = 0.92). There were also significant differences in the percentage of juvenile mussels moving into the three different treatments in the intermediate area. The percentage of juveniles found on adult mussel substrate was greater than both the percentage of mussels located on mussel shell substrate (t = 2.43, DF = 15, p = 0.03) as well as the soft-sediment control substrate (t = 4.88, DF = 15, p < 0.001). The percentage of juveniles found on the mussel shell was in turn also greater than the Table 2 Summary of statistical tests for differences in proportion of mussels within designated areas when provided different attachment substrates. Significance was determined at an α of 0.05. Area
Source of variation
DF
Mean square
F value
p value
Centre area
Substrate Residual Substrate Residual Substrate Residual
2 15 2 15 2 15
0.166 0.046 0.339 0.028 0.045 0.007
3.62
0.052
11.89
< 0.001
6.09
0.012
Substrate area Outer area
Fig. 3. Mean mortality of juvenile mussels (SE) in predation experiment provided with either adult mussels, mussel shells, or unmodified substrate as available attachment substrates in both absence (control) and presence (predator) of a sea star predator. Significant differences between all treatment combinations are indicated by different letters above the bars.
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However, close observations of the experiment in the field indicated the lower elevation of the mussel shells above the sediment may have been insufficient, as most of the surviving mussels were nearly buried by sediment. Providing greater vertical elevation using piles of individual mussel shells is unlikely to result in less burial of establishing juveniles, given that the orientation of the piled shells facilitates the collection of sediment falling out of suspension regardless of the pile height (personal observation). Using mussel shell where the two valves have been attached and oriented to resemble live adult mussels may similarly reduce the risk of sedimentation to that of adult mussels. Any potential effects of substrate relief and orientation of the substrate on the survival of establishing juvenile mussels warrants further investigation. For juvenile P. canaliculus on soft-sediment, both adult mussels and mussel shell may offer suitable attachment substrate when compared to soft-sediment; however, live adult mussels provide some protection from sea star predation, are preferred by juvenile mussels as attachment substrate, and potentially reduce the risk of juveniles being smothered by sediment. Many mussel species, including P. canaliculus, are known to settle primarily into mussel beds (Alfaro, 2006; Commito et al., 2014; Lasiak and Barnard, 1995; McGrath et al., 1988; Reaugh et al., 2007) and juveniles mussels of M. edulis will move using passive transportation by hydrodynamic forces (Bayne, 1964; Navarrete et al., 2015; Reusch and Chapman, 1997) as well as active crawling to adult mussel beds (Capelle et al., 2014; Commito et al., 2014; Côté and Jelnikar, 1999). Despite the presence of other potential hard substrates within the environment, mussel distribution tends to be associated with the presence of conspecifics (Commito et al., 2014). Given the importance of adult mussels to the establishment of juvenile mussels, the dislodgement of adult mussels from pre-existing mussel beds may therefore be a critical process for the establishment of new mussel beds on soft-sediment environments. The recovery of P. canaliculus populations over the last half century within the Hauraki Gulf, New Zealand, has therefore been hampered by the lack of this important substrate. The removal of beds has occluded much of the potential for establishing new mussel beds. Adult mussels that have become dislodged from mussel farms are most likely to be the source for the establishment of any new adult mussels beds. The loss of natural mussel beds removes much of the historically available attachment substrate in the form of adult mussels which has likely contributed to the lack of population recovery observed for green-lipped mussels; an effect that is likely to have occurred in other species of mytilid where beds have been damaged by anthropogenic impacts. Restoration initiatives targeting lost oyster beds have predominantly used the provision of additional larval settlement and attachment substrate (Brumbaugh et al., 2006; Luckenbach et al., 1999). Restoration of mussels however, is unlikely to be enhanced by the supply of additional substrate for establishing adult green-lipped mussels transplanted onto soft-sediment habitats. The deployment of adult mussels directly onto the sediment as is done in many bottom culture aquaculture practices (Dolmer et al., 2012; Ysebaert et al., 2009) is likely sufficient to establish new mussel beds. Soft-sediments offer no structural support to establishing juvenile mussels and thus any attempts to restore mussel beds with mussels of this smaller size will require the addition of some form of substrate to maximize their retention and survival. Of the tested substrates in this study, adult mussels appear be the preferential substrate and thus the restoration of mussel beds is likely best directed towards restoring adult mussels rather than using juvenile mussels or trying to enhance natural settlement alone.
mussels. 4. Discussion The value of hard substrates for establishing mussels on soft-sediments appears to differ between adults and juveniles. In this study, establishing adult green-lipped mussels showed no increase in persistence on soft-sediment whether additional hard substrate for attachment (i.e., mussel shell material) was available or not. High returns of adult mussels regardless of the treatment suggest that mortality and emigration from experimental plots was negligible. For P. canaliculus, as well as other bed-forming mytilid species, attachment to live conspecifics appears to be of primary importance for anchoring and subsequent retention in soft-sediment habitats (de Paoli et al., 2015; Fariñas-Franco et al., 2013). The clumping of adult mussels facilitated by mutual byssal attachment to surrounding live mussels likely provides greater anchorage on soft-sediments when compared to attaching to lighter mussel shell. When in natural soft-sediment beds, individual M. edulis were shown to form the greatest number of byssal attachments to conspecifics despite the presence of other solid substrates, such as pebbles and shell hash (Commito et al., 2014). Hydrodynamic forcing during the study period was also negligible, however, the effects of high wave action on the persistence of mussel clumps at this scale would likely not have differed. Other experiments using caged mussels in the same location were ripped from the sediment during a storm event and transported offshore even when anchored 0.25 m into the sediment (personal observation). Potential critical sizes of mussel patches and any potential effects that additional attachment substrates may provide to prevent dislodgement during hydrodynamic forcing are still unclear and warrant further investigation. In contrast, attachment substrate is of importance for establishing juvenile green-lipped mussels. In the laboratory, significantly greater numbers of juvenile green-lipped mussels placed on soft-sediment moved and attached to adult mussels and mussel shells. In addition, juvenile mussels moving into areas containing mussel shell or adults were always observed attached to these hard structures. A similar pattern was observed in laboratory predation experiments, with juvenile mussels found predominantly attached to these hard structures (> 70%, personal observation) rather than remaining on the soft-substrate. Likewise, in the field experiment the juvenile green-lipped mussels were only observed attached to these hard substrates after being transplanted onto soft-sediment, with none located on the softsediment. In M. edulis, naturally recruiting juvenile mussels in soft-sediment habitats also attached preferentially to hard substrates such as adult mussels and coir ropes (Commito et al., 2014; van der Heide et al., 2014). The use of adult mussels as an attachment substrate for juvenile mussels was shown to reduce predation by sea stars on the juvenile mussels. In contrast, the survival of juvenile mussels was not improved when juvenile mussels utilised mussel shell as an attachment substrate, with similar levels of mortality when compared to juvenile mussels on bare soft-sediment. The mechanism responsible for this difference in survival of mussels between the three substrates is unclear. One possibility is that the complex matrix of adult mussels that are tightly bound by byssal threads may restrict the predatory probing of sea stars and increase the amount of search effort they expend in order to locate juvenile mussels. Blue mussels, M. edulis, exhibit decreased mortality from green crab predation, Carcinus maenas, when transplanted onto structurally more complex shell or adult mussel substrates when compared to mussels transplanted onto structurally simple substrates (Frandsen and Dolmer, 2002). This difference can be at least partly attributed to a greater time spent by green crabs searching for their prey on these more structurally complex habitats. Observations of the field experimental plots also suggested that the vertical elevation provided by attachment substrate may have enabled the remaining juvenile mussels to avoid smothering by sediment.
5. Conclusion Our understanding of the processes that affect the establishment of mussel beds on soft-sediment are still limited. However, the current study suggests that the presence of attachment substrates appear to be of necessity for the establishment of mussel beds on soft-sediment and that adult conspecifics may be of particular importance as an 87
Journal of Experimental Marine Biology and Ecology 495 (2017) 83–88
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de Paoli, H., van de Koppel, J., van der Zee, E., Kangeri, A., van Belzen, J., Holthuijsen, S., van den Berg, A., Herman, P., Olff, H., van der Heide, T., 2015. Processes limiting mussel bed restoration in the Wadden-Sea. J. Sea Res. 103, 42–49. Denny, M.W., 1987. Lift as a mechanism of patch initiation in mussel beds. J. Exp. Mar. Biol. Ecol. 113, 231–245. Dolmer, P., Frandsen, R., 2002. Evaluation of the Danish mussel fishery: suggestions for an ecosystem management approach. Helgol. Mar. Res. 56, 13–20. Dolmer, P., Christensen, H.T., Hansen, B.W., Vismann, B., 2012. Area-intensive bottom culture of blue mussels Mytilus edulis in a micro-tidal estuary. Aquac. Environ. Interact. 3, 81–91. Fariñas-Franco, J.M., Allcock, L., Smyth, D., Roberts, D., 2013. Community convergence and recruitment of keystone species as performance indicators of artificial reefs. J. Sea Res. 78, 59–74. Frandsen, R.P., Dolmer, P., 2002. Effects of substrate type on growth and mortality of blue mussels (Mytilus edulis) exposed to the predator Carcinus maenas. Mar. Biol. 141, 253–262. Greenway, J.P.C., 1969. Survey of mussels (Mollusca: Lamellibranchia) in the Firth of Thames, 1961–1967. New Zeal. J. Mar. Fresh 3, 304–317. Hunt, H.L., Scheibling, R.E., 2001. Predicting wave dislodgment of mussels: variation in attachment strength with body size, habitat, and season. Mar. Ecol. Prog. Ser. 213, 157–164. Jeffs, A.G., Holland, R., Hooker, S., Hayden, B., 1999. Overview and bibliography of research on the greenshell mussel, Perna canaliculus, from New Zealand waters. J. Shellfish Res. 18, 1–14. Lasiak, T.A., Barnard, T.C.E., 1995. Recruitment of the brown mussel Perna perna onto natural substrata: a refutation of the primary/secondary settlement hypothesis. Mar. Ecol. Prog. Ser. 120, 147–153. Leonard, G.H., Bertness, M.D., Yund, P.O., 1999. Crab predation, waterborne cues, and inducible defenses in the blue mussel, Mytilus edulis. Ecology 80, 1–14. Luckenbach, M., Mann, R., Wesson, J., 1999. Oyster Reef Habitat Restoration: A Synopsis and Synthesis of Approaches. Virginia Institute of Marine Science Press. Gloucester Point, Virginia. McGrath, D., King, P.A., Gosling, E.M., 1988. Evidence for the direct settlement of Mytilus edulis larvae on adult mussel beds. Mar. Ecol. Prog. Ser. 47, 103–106. McGrorty, S., Goss-Custard, J., Clarke, R., 1993. Mussel Mytilus edulis (Mytilacea) dynamics in relation to environmental gradients and intraspecific interactions. Aquat. Ecol. 27, 163–171. McLeod, I.M., 2009. Green-lipped mussels, Perna canaliculus. In: Soft-sediment Systems in Northeastern New Zealand. University of Auckland, Auckland, New Zealand, pp. 113. Navarrete, S., Largier, J., Vera, G., Tapia, F., Parragué, M., Ramos, E., Shinen, J., Stuardo, C., Wieters, E., 2015. Tumbling under the surf: wave-modulated settlement of intertidal mussels and the continuous settlement-relocation model. Mar. Ecol. Prog. Ser. 520, 101–121. Paul, L.J., 2012. A History of the Firth of Thames Dredge Fishery for Mussels: Use and Abuse of a Coastal Resource. Ministry of Agriculture and Forestry, Wellington, New Zealand. Petrović, F., Guichard, F., 2008. Scales of Mytilus spp. population dynamics: importance of adult displacement and aggregation. Mar. Ecol. Prog. Ser. 356, 203–214. Reaugh, K.E., Harris, J.M., Branch, G.M., 2007. Further refutation of the primary-secondary settlement hypothesis for the brown mussel Perna perna. Afr. J. Mar. Sci. 29, 545–549. Reid, B., 1969. Mussel Survey Hauraki Gulf and Firth of Thames 1958 New Zealand Marine Department. (Wellington, New Zealand). Reusch, T.B.H., Chapman, A.R.O., 1997. Persistence and space occupancy by subtidal blue mussel patches. Ecol. Monogr. 67, 65–87. Southgate, T., Myers, A.A., 1985. Mussel fouling on the Celtic Sea Kinsale field gas platforms. Estuar. Coast. Shelf Sci. 20, 651–659. van der Heide, T., Tielens, E., van der Zee, E.M., Weerman, E.J., Holthuijsen, S., Eriksson, B.K., Piersma, T., van de Koppel, J., Olff, H., 2014. Predation and habitat modification synergistically interact to control bivalve recruitment on intertidal mudflats. Biol. Conserv. 172, 163–169. Ysebaert, T., Hart, M., Herman, P.M.J., 2009. Impacts of bottom and suspended cultures of mussels Mytilus spp. on the surrounding sedimentary environment and macrobenthic biodiversity. Helgol. Mar. Res. 63, 59–74.
attachment surface to both adult and juvenile establishing mussels. Clumps of dislodged mussels that have been transported away from parent mussel beds may be a particularly important precursor for the establishment of new mussel beds. Although additional attachment substrates in the form of shell did not increase persistence in the current study, it is still uncertain whether the presence of additional attachment substrate and mussel beds size may interact to reduce the erosion of establishing mussel beds during periods of hydrodynamic stress. For establishing juvenile mussels, sedimentation may be a significant barrier to establishing mussel beds and it is uncertain how various substrates and configurations of those substrates may alleviate this potential source of mortality. Acknowledgments This work was funded by a grant from the J. S. Watson Trust and with support from the Mussel Restoration Trust (reviveourgulf.org.nz). Thanks to Jan Hesse, Brady Doak, Max Schofield, and Paul Caiger who helped with the field work, Peter Browne and Errol Murray who helped in the construction of equipment, Shane Kelly for his help in the development of the project, and the many volunteers at the Leigh Marine Laboratory for their help in cleaning the mussels. References Alfaro, A.C., 2006. Population dynamics of the green-lipped mussel, Perna canaliculus, at various spatial and temporal scales in northern New Zealand. J. Exp. Mar. Biol. Ecol. 334, 294–315. Alfaro, A.C., McArdle, B., Jeffs, A.G., 2010. Temporal patterns of arrival of beachcast green-lipped mussel (Perna canaliculus) spat harvested for aquaculture in New Zealand and its relationship with hydrodynamic and meteorological conditions. Aquaculture 302, 208–218. Bayne, B.L., 1964. Primary and secondary settlement in Mytilus edulis L. (Mollusca). J. Anim. Ecol. 33, 513–523. Bell, E.C., Gosline, J.M., 1997. Strategies for life in flow: tenacity, morpholometry, and probability of dislodgment of two Mytilus species. Mar. Ecol. Prog. Ser. 159, 197–208. Brumbaugh, R.D., Beck, M.W., Coen, L.D., Craig, L., Hicks, P., 2006. A Practitioners' Guide to the Design and Monitoring of Shellfish Restoration Projects: An Ecosystem Services Approach. The Nature Conservancy, Arlington, VA. Capelle, J.J., Wijsman, J.W.M., Schellekens, T., van Stralen, M.R., Herman, P.M.J., Smaal, A.C., 2014. Spatial organisation and biomass development after relaying of mussel seed. J. Sea Res. 85, 395–403. Carrington, E., Moeser, G.M., Dimond, J., Mello, J.J., Boller, M.L., 2009. Seasonal disturbance to mussel beds: field test of a mechanistic model predicting wave dislodgement. Limnol. Oceanogr. 54, 978–986. Commito, J.A., 1987. Adult-larval interactions: predictions, mussels and cocoons. Estuar. Coast. Shelf Sci. 25, 599–606. Commito, J.A., Celano, E.A., Celico, H.J., Como, S., Johnson, C.P., 2005. Mussels matter: postlarval dispersal dynamics altered by a spatially complex ecosystem engineer. J. Exp. Mar. Biol. Ecol. 316, 133–147. Commito, J.A., Commito, A.E., Platt, R.V., Grupe, B.M., Dow Piniak, W.E., Gownaris, N.J., Reeves, K.A., Vissichelli, A.M., 2014. Recruitment facilitation and spatial pattern formation in soft-bottom mussel beds. Ecosphere 5, 1–20. Côté, I.M., Jelnikar, E., 1999. Predator-induced clumping behaviour in mussels (Mytilus edulis Linnaeus). J. Exp. Mar. Biol. Ecol. 235, 201–211. Dankers, N., Brinkman, A., Meijboom, A., Dijkman, E., 2001. Recovery of intertidal mussel beds in the Wadden sea: use of habitat maps in the management of the fishery. In: Coastal Shellfish—A Sustainable Resource. Springer, pp. 21–30.
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Is attachment substrate a prerequisite for mussels to establish on softsediment substrate?
MARK
Mark Wilcox⁎, Andrew Jeffs Leigh Marine Laboratory, Institute of Marine Science, University of Auckland, P.O. Box 349, Warkworth 0941, New Zealand
A R T I C L E I N F O
A B S T R A C T
Keywords: Attachment substrate Green-lipped mussels P. canaliculus Shellfish restoration Soft-sediment
It is unknown whether the presence of hard substrate is a necessary prerequisite for the attachment and establishment of mussels, especially on soft-sediment habitats where hard substrates are scarce. Therefore, we examined the importance of natural attachment substrates in the form of mussel shell and adult conspecifics for the establishment of juvenile and adult green-lipped mussels (Perna canaliculus) on soft-sediment. In field experiments where shell material was added as substrate to soft-sediment it made no difference to the subsequent retention of adult mussels. Laboratory experiments showed that juvenile mussels preferentially sought out, and attached to adult mussels compared to remaining on unmodified soft-sediment. Furthermore, juvenile mussels attached to live adult mussels had higher survival in the presence of a common sea star predator compared to juveniles on unmodified soft-sediment or attached to mussel shell. The results suggest that establishment of mussel beds on soft-sediment requires only adult mussels, which receive sufficient anchorage through attachment to neighbouring adults and in so doing, providing a stable, complex substrate suitable for improving the survival of establishing juvenile mussels by protecting them from sea star predators.
1. Introduction
habitat for settling larvae (Commito, 1987). For the many species of mussel that inhabit rocky coastal habitats, hard substrates for attachment are abundant. In addition to attaching to conspecifics, these mussels can attach directly to the primary substrate. On soft-sediment habitats, however, hard substrates for attachment are sparse and mussels rarely attach to the primary sediment (Bayne, 1964; Commito et al., 2005) as the byssal threads are often unable to attach to the small grain sizes which would not provide sufficient anchorage. In these environments, mussels rely on rocks, shells, and predominantly conspecifics for attachment (Commito et al., 2014) with experiments showing that recruiting mussels primarily use these attachment materials rather than bare soft-sediment (Commito et al., 2014; van der Heide et al., 2014). Whether by the dislodgement and transportation of adults or the settlement of larvae, the importance of substrates in the establishment of mussel beds on soft-sediment is not clear. The re-establishment of adult northern horse mussels, Modiolus modiolus, showed no increase in survival when transplanted onto shell cultch of either high or low relief when compared to bare soft-sediment (Fariñas-Franco et al., 2013). Likewise, the survival of transplanted adult blue mussels, Mytilus edulis, was not higher on natural fibre mats made of coir compared to those transplanted directly onto soft-sediment (de Paoli et al., 2015). This suggests that additional attachment substrates aside from conspecifics are likely unnecessary for the establishment of mussel beds
Epifaunal bivalves such as oysters and mussels anchor themselves to hard substrates through permanent cementation and detachable byssal threads, respectively. This process of attachment helps to reduce the likelihood of dislodgement and transport away from selected environments (Bell and Gosline, 1997; Hunt and Scheibling, 2001) which can result in mortality (Carrington et al., 2009; Petrović and Guichard, 2008). Bivalves are at particular risk of dislodgement during storm events as well as in areas of naturally strong hydrodynamic conditions (Carrington et al., 2009; Denny, 1987; Hunt and Scheibling, 2001; Petrović and Guichard, 2008). The hard substrates which these bivalves attach to include natural cobbles and bedrock, anthropogenic structures, as well as other organisms (Commito et al., 2014; Dankers et al., 2001; Dolmer and Frandsen, 2002; McGrorty et al., 1993; Southgate and Myers, 1985), of which conspecifics are a common substrate (Commito et al., 2014). The gregarious nature of many epifaunal bivalves often leads to the formation of extensive populations, known as beds, which occur both intertidally and subtidally within coastal ecosystems. The attachment of bivalves within these beds not only reduces the risk of dislodgement but has also been shown to reduce the risk of predation by crabs (Leonard et al., 1999). In addition, the complex substrate created by the aggregating bivalves provides a preferred
⁎
Corresponding author. E-mail addresses: [email protected] (M. Wilcox), a.jeff[email protected] (A. Jeffs).
http://dx.doi.org/10.1016/j.jembe.2017.07.004 Received 26 November 2016; Received in revised form 31 May 2017; Accepted 8 July 2017 0022-0981/ © 2017 Elsevier B.V. All rights reserved.
Journal of Experimental Marine Biology and Ecology 495 (2017) 83–88
M. Wilcox, A. Jeffs
2.3. Use of substrate by adult mussels in the field
by adult mussels. In contrast, larval recruitment of both northern horse mussels and blue mussels were both higher in the presence of adults compared with any other available substrate including shell cultch (Commito et al., 2014; Fariñas-Franco et al., 2013). The survival of seed mussels was also shown to be greatest when provided with more complex substrates (Frandsen and Dolmer, 2002). This suggests that on soft-sediment, available attachment substrates may provide critical habitat for establishing larval and juvenile mussels. Extensive mussel beds of the green-lipped mussel, Perna canaliculus, covering over 1300 km2 on soft-sediment in the Hauraki Gulf, New Zealand, were nearly extirpated by dredge fishing during the last century (Greenway, 1969; Reid, 1969). Despite the closure of the fishery in 1969 (Paul, 2012), there has been no sign of natural recovery to date. The removal of the adult mussel beds through fishing has subsequently led to the removal of much of the available hard substrate which could have contributed to the lack of recovery in this population. The aim of this study is therefore to determine whether attachment substrates are necessary for the establishment of mussel beds on soft-sediment. This was accomplished using a series of laboratory and field experiments examining particular benefits to establishing adult and juvenile mussels provided with two common attachment substrates found within natural soft-sediment mussel beds, i.e., adult mussels and mussel shell. This study will address the hypotheses of whether; (1) conspecific shell increases the persistence of adult mussels establishing on soft-sediment, (2) conspecific shell and/or adult conspecifics increases the persistence of juvenile mussels establishing on soft-sediment, (3) juvenile mussels establishing on soft-sediment preferentially attach to conspecific shells and/or adults, and (4) the attachment of establishing juvenile mussels to mussel shell and/or adult mussels increases survival in the presence of a common sea star predator. The results of these experiments will help to increase our understanding of why natural recovery fails in depleted mussel populations and will have implications for restoration initiatives in this and other mussel species.
On 26 November 2013, twenty 0.25 m2 (0.5 × 0.5 m) plots were established by divers in the field arranged in five rows, each containing four plots. Each plot was separated by a distance of 1.5 m and marked with a subsurface float. The crossed experimental design consisted of a substrate level of either unmodified soft-sediment or the addition of 60 clean adult mussel shells (80–100 mm SL) which was crossed with a predator exclusion or access level, with a total of five replicates per treatment. This number of adult mussel shells was used to ensure that they would provide sufficient attachment substrate for juvenile mussels in the experiment. Each of the five rows of plots contained one replicate of each treatment arranged in a random order. Predator exclusion plots were enclosed in a lightweight stainless steel frame covered with coarse plastic mesh (20 mm openings). The plastic mesh prevented large mobile predators, such as fish and lobster, from removing and consuming mussels from the plots, while not unduly restricting water flow to the mussels inside. The experimental design did not include an additional control treatment for possible artefacts caused by caging because the primary aim of the caging was to determine whether or not mussels were being lost from the experiment due to emigration or predation. Forty live adult green-lipped mussels were then transplanted into each plot to establish a density of mussels typical of the wide range of densities found in natural beds of these mussels (McLeod, 2009). After 50 days, the number of surviving mussels in each plot was enumerated by divers.
2.4. Use of substrate by juvenile mussels in the field On 26 November 2013, a total of fifteen 1.5 m2 circular plots were established by divers in three rows of five plots at the field site and marked with subsurface floats. The experiment consisted of three substrate treatments; 1) unmodified soft-sediment substrate, 2) addition of ≈ 250 adult mussel shells (80–100 mm SL) and, 3) addition of ≈ 1200 live adult mussels. Quantities of adult mussels and mussel shell ensured the entire plot was covered with the available attachment substrate and the density of live mussels was consistent with the densities found in natural beds of these mussels (McLeod, 2009). The three substrate treatments were each randomly allocated to five of the plots with no more than two of the same substrate treatment per row. In the laboratory, macroalgal material with attached juvenile mussels that had been previously collected from Ninety Mile Beach was divided into fifteen roughly equal bundles. Each bundle weighed 0.318 kg ( ± 0.013 SE) and based on mussel counts from weighed subsamples of mussel laden algae, each bundle contained on average 5298 mussels ( ± 353 SE). The bundles were then each enclosed within a biodegradable mesh sock (5–10 mm mesh size), commonly used for the deployment of juvenile mussels on seaweed in aquaculture operations (Jeffs et al., 1999), that helped to maintain the pre-measured quantities of juvenile mussels during transport. Bundles were transported to the site and secured by divers to the centre of each of the 15 experimental plots with a stainless steel pin driven 10 cm into the sediment. The mesh socks also helped to ensure the macroalgae with attached juvenile mussels remained within the positioned plot. The mesh size used did not unduly inhibit the movement of the juvenile mussels into and out of the sock, allowing the juvenile mussels to freely disperse onto the plot. After a period of 44 days each plot was surveyed by haphazardly placing a 0.0625 m2 quadrat within each quarter of the plot and quantifying the number of remaining juvenile mussels in situ (1–5 mm SL) within each of the four quadrats for each plot. Divers haphazardly placed the quadrat by releasing the frame from 1.5 m above the plot and allowing it to fall to the seabed. Clear visibility at this site allowed divers to observe the entire quadrat and identify the presence of mussels > 1 mm SL.
2. Materials and methods 2.1. Field study site The Hauraki Gulf is located on the northeastern coast of the North Island of New Zealand. Field experiments were conducted in a sheltered coastal embayment on the northern section of Kawau Bay, in the Hauraki Gulf (36° 22′ 47″ S, 174° 49′ 02″ E). All experimental plots were situated on fine sand substrate at a depth of 4.1 to 4.9 m below chart datum. 2.2. Mussel sources Wild juvenile mussels (P. canaliculus) are regularly found attached to drifting algae (Alfaro et al., 2010) and were collected from Ninety Mile Beach (35° 02′ 08″ S, 173° 10′ 05″ E) and Muriwai Beach (36° 50′ 05″ S, 174° 27′ 59″ E) in northern New Zealand. All juvenile mussels (1–5 mm shell length (SL)) that were collected and used in the present study were housed in aquaria supplied with ambient seawater with aeration until utilised in field and laboratory experiments. Adult mussels were most easily obtained from the extensive aquaculture operations in the Hauraki Gulf, where juvenile mussels originating from wild sources at Ninety Mile Beach in northern New Zealand, are grown on suspended long lines until they reach commercial size (Jeffs et al., 1999). All the adult mussels (80–100 mm SL) utilised in these experiments were first cleared of all fouling organisms and kept in aquaria with flow-through seawater until deployment. Given that aquacultured mussels are the most readily available source of mussels for restoration, maintaining a similar size range of adult mussels throughout these experiments ensures the results will be applicable to future restoration initiatives. 84
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3.5–5 cm as measured from the tip of the longest arm to the opposite side of the oral disc. Into each container 100 juvenile mussels (1–5 mm SL, from Muriwai Beach) were placed and the number of surviving mussels was assessed after one week. The experiment was run for five consecutive weeks (i.e., a total of five experimental blocks) and commenced on 12 November 2015 and concluded on 24 December 2015. Novel sea stars and mussels were utilised in each successive experimental block. 2.7. Statistical analyses The data was inspected for deviations from normality and heterogeneity of variance firstly by visually inspecting quartile-quartile plots and plots of residuals versus fitted values (respectively) of models prior to running the analysis. For the experiments on substrate use by adult mussels, the data deviated from normality and were therefore fitted to a Poisson distribution using a general linear model (GLM) and tested using a Wald chi-square test to determine if differences in the number of surviving mussels were due to either the substrate and/or access by predators. For the experiment on selection of substrate by juvenile mussels, the data was normally distributed and thus were fitted to a Gaussian distribution using a linear model (LM) and tested using an ANOVA for differences in the percent of mussels among substrate treatments within each area separately. For the experiment assessing potential predation refuge from sea star predators, the data again deviated from normality and were therefore fitted to a Poisson distribution using a general linear mixed-effects model (GLMER) incorporating blocks as a random effect. A Wald chi-square test was utilised to determine if differences in mortality were attributed to the substrates and/or presence of predators. Significance was further examined using pairwise t-tests with a false discovery rate correction for multiple comparisons. For the field experiment on survival of juvenile mussels, results are presented using descriptive statistics only because there was very low recovery of juvenile mussels across all plots. All statistical tests were computed in R version 3.2.3 and RStudio version 0.99.879.
Fig. 1. Experimental laboratory setup of circular containers used in juvenile substrate selection experiments, depicting the (A) central, (B) intermediate, and (C) outer areas of the floor of the tank. Mud substrate was provided evenly across all areas, but different substrates were experimentally placed in the intermediate area, i.e., addition of live adult mussels versus addition of mussel shells versus no modifications (soft-sediment).
2.5. Substrate selection by juvenile mussels in the laboratory Nine round 60 l plastic tubs (320 mm diameter base, 500 mm high) were filled evenly across their base with natural sediment (sieved to a grain size < 1 mm) to a depth of 10 mm. The tubs were then filled with 20 l of filtered seawater (5 μm) which was aerated via a single airstone placed in the centre above the sediment. The bottom of each tub was sectioned into three concentric areas arranged by distance from the centre of the tub; 1) central area (0–5 cm), 2) intermediate area (5–10 cm) and, 3) outer area (beyond 10 cm and including the walls of the tub) (Fig. 1). The three treatments consisted of three modifications to the intermediate area only; either unmodified natural sediment substrate, adult mussels (nine mussels in groups of three), or mussel shell (18 clean adult mussel shells) placed as three groups of six shells. Live adult mussels and empty waste mussel shells from aquaculture operations both have the potential to be provided as substrate for mussel bed restoration initiatives. Three replicates per substrate treatment were used in each of two trials on 23 and 25 May 2015 for a combined total of six replicates per treatment using fresh mussels for each trial. For each replicate, 40 wild juvenile mussels of 1–5 mm shell length sourced from Ninety Mile Beach were placed within 2 cm of the central area of each tub. After 24 h the juvenile mussels in each of the designated areas were enumerated.
3. Results 3.1. Use of substrate by adult mussels in the field There were no significant effects of the provision of mussel shell as attachment substrate, the access or exclusion of predators, or their interaction on the survival of adult mussels across plots (Table 1). Survival rates across all replicates was high (mean survival 93.1 ± 0.9% SE) and no mussels out of the 40 mussels that were initially deployed into the plots were found outside of the plots indicating that little or no emigration occurred during the experiment.
2.6. Predation refuge provided by substrates in the laboratory 3.2. Use of substrate by juvenile mussels in the field Six 3.5 l square containers (180 × 180 × 120 mm) were filled evenly across the base with natural sediment (grain size < 1 mm) to a depth of 10 mm. Each container was supplied with flow-through (10 l h− 1), filtered seawater (5 μm) which was aerated via a single airstone. Water escaped through the screened (0.25 mm) outlets to the containers which were located 30 mm below the lip of the container, maintaining approximately 2 l of seawater within and a dry surface which prohibited mussels from escaping. The crossed experimental design consisted of a substrate factor of either unmodified soft-sediment, mussel shell (20 clean adult mussel shells), or adult mussels (10 adults in a single group) which was crossed with a predation factor, i.e., presence or absence of a sea star predator for a total of six experimental treatments. One container was set up for each of the six experimental treatments and those treatments containing a predator were supplied with a small eleven-armed sea star, Coscinasterias muricata, which are known to consume mussels. These sea stars ranged in size from
There was very low recovery of the juvenile mussels transplanted into each plot, with only two individuals being the highest recorded number of juveniles in any sampled quadrat. No juvenile mussels were found in any plots of the unmodified soft-sediment treatment. A total of three juvenile mussels were found across all plots in the mussel shell Table 1 Summary of statistical test for differences in survival of transplanted adult mussels in the presence/exclusion of predators and either provided addition attachment substrate or transplanted to the bare soft-sediment. Significance was determined at an α of 0.05.
Substrate Predation Substrate ∗ predation
85
Chi square
DF
p value
0.001 1.462 0.387
1 1 1
0.97 0.23 0.53
Journal of Experimental Marine Biology and Ecology 495 (2017) 83–88
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Table 3 Results of chi-square tests examining mortality of juvenile mussels in the presence/absence of a sea star predator when provided different attachment substrates. Significance was determined at an α of 0.05. Wald chi-square test
DF
χ2 value
p value
Substrate Predation Substrate ∗ predation
2 1 2
39.76 321.12 13.31
< 0.001 < 0.001 0.001
percentage of mussels on the soft-sediment substrate (t = 2.45, DF = 15, p = 0.03). Juvenile mussels that had moved into the mussel shell treatment areas were always found attached to shells while juveniles in the adult mussel treatment were always found attached to the live adult mussels, either on their shells or among their byssal threads.
3.4. Predation refuge provided by substrates in the laboratory
Fig. 2. Percentage of juvenile mussels ( ± SE) occupying the central, intermediate, and outer areas of the juvenile substrate selection experiment when live adult mussels, mussel shell, or no additional substrate was provided in the intermediate area of the experimental tank (see Fig. 1). Significant differences between the three substrate treatments within each area of the tank are indicated by different letters above the bars.
There were significant differences in the mortality of green-lipped mussels for both the substrate and predation factors as well as their interaction (Table 3). All experimental combinations without a predator had consistently low levels of mortality of juvenile mussels and were not different among the three types of substrate (All p values > 0.6) (Fig. 3). The mortality of juvenile mussels in treatment combinations without a predator was also consistently lower than every treatment combination where a predator was present (All p values < 0.001). The experimental combinations with a predator and either the unmodified substrate or the shell substrate had similar high levels of mortality of juvenile mussels (t = 2.41, p = 0.216). However, the mortality of juvenile mussels in the experimental combination containing a predator and adult mussels as substrate was significantly lower than those containing a predator and either unmodified soft-sediment substrate or the shell substrate (p values < 0.001). There was no mortality of the adult mussels provided as substrate within this experiment and thus the lower mortality of the juvenile mussels was not due to the sea stars consuming the adult mussels instead of the juveniles. Sea stars were seen actively consuming juvenile mussels and all mussels were accounted for in every tank at the end of the experimental run. It was observed that juvenile mussels in the unmodified substrate treatment were either dispersed over the soft-sediment or clumped together whereas juvenile mussels in shell and adult mussel treatments were predominantly observed to be attached to the hard structures of the shells and byssal threads of adult
treatment, resulting in a mean density of 2.4 ± 1.7 mussels m− 2 (SE). These juvenile mussels were only found attached to the adult mussel shell which provided a very low relief and had mostly become buried by sediment. Only four juvenile mussels were found across all plots in the adult mussel treatment, resulting in a mean density of 3.2 ± 1.5 mussels m− 2 (SE). Unlike the mussel shells, the adult mussels provided higher relief for attachment of the juveniles, with all juvenile mussels being found at least 10 mm above the sediment.
3.3. Substrate selection by juvenile mussels in the laboratory Overall, the majority (58.6%) of the juvenile mussels remained in the central area after 24 h (Fig. 2), often attached to other juvenile mussels, and the percentage of mussels remaining in the central area did not differ among the treatments (Table 2). A minority of juvenile mussels (9.8%) moved into the outer area after 24 h and the percentage of mussels in this outer area was different among the three treatments, with a significantly smaller percentage of mussels occupying this area in the adult mussel versus soft-sediment control treatments (t = 3.07, DF = 15, p = 0.01) as well as the mussel shell versus soft-sediment control treatments (t = 2.97, DF = 15, p = 0.01), but there was no significant difference in the percentage of mussels between the adult mussel and mussel shell treatments (t = 0.11, DF = 15, p = 0.92). There were also significant differences in the percentage of juvenile mussels moving into the three different treatments in the intermediate area. The percentage of juveniles found on adult mussel substrate was greater than both the percentage of mussels located on mussel shell substrate (t = 2.43, DF = 15, p = 0.03) as well as the soft-sediment control substrate (t = 4.88, DF = 15, p < 0.001). The percentage of juveniles found on the mussel shell was in turn also greater than the Table 2 Summary of statistical tests for differences in proportion of mussels within designated areas when provided different attachment substrates. Significance was determined at an α of 0.05. Area
Source of variation
DF
Mean square
F value
p value
Centre area
Substrate Residual Substrate Residual Substrate Residual
2 15 2 15 2 15
0.166 0.046 0.339 0.028 0.045 0.007
3.62
0.052
11.89
< 0.001
6.09
0.012
Substrate area Outer area
Fig. 3. Mean mortality of juvenile mussels (SE) in predation experiment provided with either adult mussels, mussel shells, or unmodified substrate as available attachment substrates in both absence (control) and presence (predator) of a sea star predator. Significant differences between all treatment combinations are indicated by different letters above the bars.
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However, close observations of the experiment in the field indicated the lower elevation of the mussel shells above the sediment may have been insufficient, as most of the surviving mussels were nearly buried by sediment. Providing greater vertical elevation using piles of individual mussel shells is unlikely to result in less burial of establishing juveniles, given that the orientation of the piled shells facilitates the collection of sediment falling out of suspension regardless of the pile height (personal observation). Using mussel shell where the two valves have been attached and oriented to resemble live adult mussels may similarly reduce the risk of sedimentation to that of adult mussels. Any potential effects of substrate relief and orientation of the substrate on the survival of establishing juvenile mussels warrants further investigation. For juvenile P. canaliculus on soft-sediment, both adult mussels and mussel shell may offer suitable attachment substrate when compared to soft-sediment; however, live adult mussels provide some protection from sea star predation, are preferred by juvenile mussels as attachment substrate, and potentially reduce the risk of juveniles being smothered by sediment. Many mussel species, including P. canaliculus, are known to settle primarily into mussel beds (Alfaro, 2006; Commito et al., 2014; Lasiak and Barnard, 1995; McGrath et al., 1988; Reaugh et al., 2007) and juveniles mussels of M. edulis will move using passive transportation by hydrodynamic forces (Bayne, 1964; Navarrete et al., 2015; Reusch and Chapman, 1997) as well as active crawling to adult mussel beds (Capelle et al., 2014; Commito et al., 2014; Côté and Jelnikar, 1999). Despite the presence of other potential hard substrates within the environment, mussel distribution tends to be associated with the presence of conspecifics (Commito et al., 2014). Given the importance of adult mussels to the establishment of juvenile mussels, the dislodgement of adult mussels from pre-existing mussel beds may therefore be a critical process for the establishment of new mussel beds on soft-sediment environments. The recovery of P. canaliculus populations over the last half century within the Hauraki Gulf, New Zealand, has therefore been hampered by the lack of this important substrate. The removal of beds has occluded much of the potential for establishing new mussel beds. Adult mussels that have become dislodged from mussel farms are most likely to be the source for the establishment of any new adult mussels beds. The loss of natural mussel beds removes much of the historically available attachment substrate in the form of adult mussels which has likely contributed to the lack of population recovery observed for green-lipped mussels; an effect that is likely to have occurred in other species of mytilid where beds have been damaged by anthropogenic impacts. Restoration initiatives targeting lost oyster beds have predominantly used the provision of additional larval settlement and attachment substrate (Brumbaugh et al., 2006; Luckenbach et al., 1999). Restoration of mussels however, is unlikely to be enhanced by the supply of additional substrate for establishing adult green-lipped mussels transplanted onto soft-sediment habitats. The deployment of adult mussels directly onto the sediment as is done in many bottom culture aquaculture practices (Dolmer et al., 2012; Ysebaert et al., 2009) is likely sufficient to establish new mussel beds. Soft-sediments offer no structural support to establishing juvenile mussels and thus any attempts to restore mussel beds with mussels of this smaller size will require the addition of some form of substrate to maximize their retention and survival. Of the tested substrates in this study, adult mussels appear be the preferential substrate and thus the restoration of mussel beds is likely best directed towards restoring adult mussels rather than using juvenile mussels or trying to enhance natural settlement alone.
mussels. 4. Discussion The value of hard substrates for establishing mussels on soft-sediments appears to differ between adults and juveniles. In this study, establishing adult green-lipped mussels showed no increase in persistence on soft-sediment whether additional hard substrate for attachment (i.e., mussel shell material) was available or not. High returns of adult mussels regardless of the treatment suggest that mortality and emigration from experimental plots was negligible. For P. canaliculus, as well as other bed-forming mytilid species, attachment to live conspecifics appears to be of primary importance for anchoring and subsequent retention in soft-sediment habitats (de Paoli et al., 2015; Fariñas-Franco et al., 2013). The clumping of adult mussels facilitated by mutual byssal attachment to surrounding live mussels likely provides greater anchorage on soft-sediments when compared to attaching to lighter mussel shell. When in natural soft-sediment beds, individual M. edulis were shown to form the greatest number of byssal attachments to conspecifics despite the presence of other solid substrates, such as pebbles and shell hash (Commito et al., 2014). Hydrodynamic forcing during the study period was also negligible, however, the effects of high wave action on the persistence of mussel clumps at this scale would likely not have differed. Other experiments using caged mussels in the same location were ripped from the sediment during a storm event and transported offshore even when anchored 0.25 m into the sediment (personal observation). Potential critical sizes of mussel patches and any potential effects that additional attachment substrates may provide to prevent dislodgement during hydrodynamic forcing are still unclear and warrant further investigation. In contrast, attachment substrate is of importance for establishing juvenile green-lipped mussels. In the laboratory, significantly greater numbers of juvenile green-lipped mussels placed on soft-sediment moved and attached to adult mussels and mussel shells. In addition, juvenile mussels moving into areas containing mussel shell or adults were always observed attached to these hard structures. A similar pattern was observed in laboratory predation experiments, with juvenile mussels found predominantly attached to these hard structures (> 70%, personal observation) rather than remaining on the soft-substrate. Likewise, in the field experiment the juvenile green-lipped mussels were only observed attached to these hard substrates after being transplanted onto soft-sediment, with none located on the softsediment. In M. edulis, naturally recruiting juvenile mussels in soft-sediment habitats also attached preferentially to hard substrates such as adult mussels and coir ropes (Commito et al., 2014; van der Heide et al., 2014). The use of adult mussels as an attachment substrate for juvenile mussels was shown to reduce predation by sea stars on the juvenile mussels. In contrast, the survival of juvenile mussels was not improved when juvenile mussels utilised mussel shell as an attachment substrate, with similar levels of mortality when compared to juvenile mussels on bare soft-sediment. The mechanism responsible for this difference in survival of mussels between the three substrates is unclear. One possibility is that the complex matrix of adult mussels that are tightly bound by byssal threads may restrict the predatory probing of sea stars and increase the amount of search effort they expend in order to locate juvenile mussels. Blue mussels, M. edulis, exhibit decreased mortality from green crab predation, Carcinus maenas, when transplanted onto structurally more complex shell or adult mussel substrates when compared to mussels transplanted onto structurally simple substrates (Frandsen and Dolmer, 2002). This difference can be at least partly attributed to a greater time spent by green crabs searching for their prey on these more structurally complex habitats. Observations of the field experimental plots also suggested that the vertical elevation provided by attachment substrate may have enabled the remaining juvenile mussels to avoid smothering by sediment.
5. Conclusion Our understanding of the processes that affect the establishment of mussel beds on soft-sediment are still limited. However, the current study suggests that the presence of attachment substrates appear to be of necessity for the establishment of mussel beds on soft-sediment and that adult conspecifics may be of particular importance as an 87
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de Paoli, H., van de Koppel, J., van der Zee, E., Kangeri, A., van Belzen, J., Holthuijsen, S., van den Berg, A., Herman, P., Olff, H., van der Heide, T., 2015. Processes limiting mussel bed restoration in the Wadden-Sea. J. Sea Res. 103, 42–49. Denny, M.W., 1987. Lift as a mechanism of patch initiation in mussel beds. J. Exp. Mar. Biol. Ecol. 113, 231–245. Dolmer, P., Frandsen, R., 2002. Evaluation of the Danish mussel fishery: suggestions for an ecosystem management approach. Helgol. Mar. Res. 56, 13–20. Dolmer, P., Christensen, H.T., Hansen, B.W., Vismann, B., 2012. Area-intensive bottom culture of blue mussels Mytilus edulis in a micro-tidal estuary. Aquac. Environ. Interact. 3, 81–91. Fariñas-Franco, J.M., Allcock, L., Smyth, D., Roberts, D., 2013. Community convergence and recruitment of keystone species as performance indicators of artificial reefs. J. Sea Res. 78, 59–74. Frandsen, R.P., Dolmer, P., 2002. Effects of substrate type on growth and mortality of blue mussels (Mytilus edulis) exposed to the predator Carcinus maenas. Mar. Biol. 141, 253–262. Greenway, J.P.C., 1969. Survey of mussels (Mollusca: Lamellibranchia) in the Firth of Thames, 1961–1967. New Zeal. J. Mar. Fresh 3, 304–317. Hunt, H.L., Scheibling, R.E., 2001. Predicting wave dislodgment of mussels: variation in attachment strength with body size, habitat, and season. Mar. Ecol. Prog. Ser. 213, 157–164. Jeffs, A.G., Holland, R., Hooker, S., Hayden, B., 1999. Overview and bibliography of research on the greenshell mussel, Perna canaliculus, from New Zealand waters. J. Shellfish Res. 18, 1–14. Lasiak, T.A., Barnard, T.C.E., 1995. Recruitment of the brown mussel Perna perna onto natural substrata: a refutation of the primary/secondary settlement hypothesis. Mar. Ecol. Prog. Ser. 120, 147–153. Leonard, G.H., Bertness, M.D., Yund, P.O., 1999. Crab predation, waterborne cues, and inducible defenses in the blue mussel, Mytilus edulis. Ecology 80, 1–14. Luckenbach, M., Mann, R., Wesson, J., 1999. Oyster Reef Habitat Restoration: A Synopsis and Synthesis of Approaches. Virginia Institute of Marine Science Press. Gloucester Point, Virginia. McGrath, D., King, P.A., Gosling, E.M., 1988. Evidence for the direct settlement of Mytilus edulis larvae on adult mussel beds. Mar. Ecol. Prog. Ser. 47, 103–106. McGrorty, S., Goss-Custard, J., Clarke, R., 1993. Mussel Mytilus edulis (Mytilacea) dynamics in relation to environmental gradients and intraspecific interactions. Aquat. Ecol. 27, 163–171. McLeod, I.M., 2009. Green-lipped mussels, Perna canaliculus. In: Soft-sediment Systems in Northeastern New Zealand. University of Auckland, Auckland, New Zealand, pp. 113. Navarrete, S., Largier, J., Vera, G., Tapia, F., Parragué, M., Ramos, E., Shinen, J., Stuardo, C., Wieters, E., 2015. Tumbling under the surf: wave-modulated settlement of intertidal mussels and the continuous settlement-relocation model. Mar. Ecol. Prog. Ser. 520, 101–121. Paul, L.J., 2012. A History of the Firth of Thames Dredge Fishery for Mussels: Use and Abuse of a Coastal Resource. Ministry of Agriculture and Forestry, Wellington, New Zealand. Petrović, F., Guichard, F., 2008. Scales of Mytilus spp. population dynamics: importance of adult displacement and aggregation. Mar. Ecol. Prog. Ser. 356, 203–214. Reaugh, K.E., Harris, J.M., Branch, G.M., 2007. Further refutation of the primary-secondary settlement hypothesis for the brown mussel Perna perna. Afr. J. Mar. Sci. 29, 545–549. Reid, B., 1969. Mussel Survey Hauraki Gulf and Firth of Thames 1958 New Zealand Marine Department. (Wellington, New Zealand). Reusch, T.B.H., Chapman, A.R.O., 1997. Persistence and space occupancy by subtidal blue mussel patches. Ecol. Monogr. 67, 65–87. Southgate, T., Myers, A.A., 1985. Mussel fouling on the Celtic Sea Kinsale field gas platforms. Estuar. Coast. Shelf Sci. 20, 651–659. van der Heide, T., Tielens, E., van der Zee, E.M., Weerman, E.J., Holthuijsen, S., Eriksson, B.K., Piersma, T., van de Koppel, J., Olff, H., 2014. Predation and habitat modification synergistically interact to control bivalve recruitment on intertidal mudflats. Biol. Conserv. 172, 163–169. Ysebaert, T., Hart, M., Herman, P.M.J., 2009. Impacts of bottom and suspended cultures of mussels Mytilus spp. on the surrounding sedimentary environment and macrobenthic biodiversity. Helgol. Mar. Res. 63, 59–74.
attachment surface to both adult and juvenile establishing mussels. Clumps of dislodged mussels that have been transported away from parent mussel beds may be a particularly important precursor for the establishment of new mussel beds. Although additional attachment substrates in the form of shell did not increase persistence in the current study, it is still uncertain whether the presence of additional attachment substrate and mussel beds size may interact to reduce the erosion of establishing mussel beds during periods of hydrodynamic stress. For establishing juvenile mussels, sedimentation may be a significant barrier to establishing mussel beds and it is uncertain how various substrates and configurations of those substrates may alleviate this potential source of mortality. Acknowledgments This work was funded by a grant from the J. S. Watson Trust and with support from the Mussel Restoration Trust (reviveourgulf.org.nz). Thanks to Jan Hesse, Brady Doak, Max Schofield, and Paul Caiger who helped with the field work, Peter Browne and Errol Murray who helped in the construction of equipment, Shane Kelly for his help in the development of the project, and the many volunteers at the Leigh Marine Laboratory for their help in cleaning the mussels. References Alfaro, A.C., 2006. Population dynamics of the green-lipped mussel, Perna canaliculus, at various spatial and temporal scales in northern New Zealand. J. Exp. Mar. Biol. Ecol. 334, 294–315. Alfaro, A.C., McArdle, B., Jeffs, A.G., 2010. Temporal patterns of arrival of beachcast green-lipped mussel (Perna canaliculus) spat harvested for aquaculture in New Zealand and its relationship with hydrodynamic and meteorological conditions. Aquaculture 302, 208–218. Bayne, B.L., 1964. Primary and secondary settlement in Mytilus edulis L. (Mollusca). J. Anim. Ecol. 33, 513–523. Bell, E.C., Gosline, J.M., 1997. Strategies for life in flow: tenacity, morpholometry, and probability of dislodgment of two Mytilus species. Mar. Ecol. Prog. Ser. 159, 197–208. Brumbaugh, R.D., Beck, M.W., Coen, L.D., Craig, L., Hicks, P., 2006. A Practitioners' Guide to the Design and Monitoring of Shellfish Restoration Projects: An Ecosystem Services Approach. The Nature Conservancy, Arlington, VA. Capelle, J.J., Wijsman, J.W.M., Schellekens, T., van Stralen, M.R., Herman, P.M.J., Smaal, A.C., 2014. Spatial organisation and biomass development after relaying of mussel seed. J. Sea Res. 85, 395–403. Carrington, E., Moeser, G.M., Dimond, J., Mello, J.J., Boller, M.L., 2009. Seasonal disturbance to mussel beds: field test of a mechanistic model predicting wave dislodgement. Limnol. Oceanogr. 54, 978–986. Commito, J.A., 1987. Adult-larval interactions: predictions, mussels and cocoons. Estuar. Coast. Shelf Sci. 25, 599–606. Commito, J.A., Celano, E.A., Celico, H.J., Como, S., Johnson, C.P., 2005. Mussels matter: postlarval dispersal dynamics altered by a spatially complex ecosystem engineer. J. Exp. Mar. Biol. Ecol. 316, 133–147. Commito, J.A., Commito, A.E., Platt, R.V., Grupe, B.M., Dow Piniak, W.E., Gownaris, N.J., Reeves, K.A., Vissichelli, A.M., 2014. Recruitment facilitation and spatial pattern formation in soft-bottom mussel beds. Ecosphere 5, 1–20. Côté, I.M., Jelnikar, E., 1999. Predator-induced clumping behaviour in mussels (Mytilus edulis Linnaeus). J. Exp. Mar. Biol. Ecol. 235, 201–211. Dankers, N., Brinkman, A., Meijboom, A., Dijkman, E., 2001. Recovery of intertidal mussel beds in the Wadden sea: use of habitat maps in the management of the fishery. In: Coastal Shellfish—A Sustainable Resource. Springer, pp. 21–30.
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