Distribution, abundance and size-structure of cerithiid gastropods in sediments at One Tree Reef, southern Great Barrier Reef

J. Exp. Mar. Biol. Ecol., 151 (1991) 185-207 © 1991 Elsevier Science Publishers B.V. All rights reserved 0022-0981/91/$03.50

185

JEMBE 01641

Distribution, abundance and size-structure of cerithiid gastropods in sediments at One Tree Reef, southern Great Barrier Reef G.A. Skflleter Institute of Marine Ecology, University of Sydney, New South Wales, Australia (Received 7 August 1990; revision received 29 April 1991; accepted l0 May 1991) Abstract: The patterns of abundance of mollusc,~ in sediments at One Tree Reef, southern Great Barrier Reef were determined for three adjacent habitat~ in October 1985: a shallow subtidal sandflat, a deep channel, and the sloping, unstable edge separating the two. Deposit-feeding cerithiid gastropods, especially Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Brugui~'e, 1792) and R. vertagus (Linnaeus, 1758), were the most abundant molluscs present but these species were predominantly found in the coarse sandy sediments of the sandflat. The abundances and sizes of these three species were monitored seasonally from October 1985 to January 1988 on the sandflat. Densities fluctuated during the three years of the study and varied at a number of spatial scales. The density of R. aspera was generally greater at the beginning of the year, after recruitment, then gradually declined until the next period of recruitment. This pattern was not as evident for R fasciata or R. vertagus. Changes in size-frequency distributions through time suggest that movement of animals, either by migration and/or passive transport in the shifting sediments maytqontribute to the observed patterns. Comparisons are made between temperate and tropical soft-sediments habitats, especially in relation to the distribution of deposit-feeding organisms.

Key words: Abundance; Australia; Cerithiidae; Coral reef; Distribution; Rhinoclavis

INTRODU'.. ION

Many recent studies have provided detailed qualitative descriptions of the fauna that occupy sediments associated with coral reefs (review by Alongi, 1989). While providing information on the diversity of organisms in these habitats, such studies provide only basic quantitative information such as total numbers and/or biomass of the most common taxa. Some of these studies have provided detailed analyses of community structure (Faubel, 1984; Jones, 1984; Moriarty et al., 1985; Alongi, 1986; Warwick & Ruswahyuni, 1987; Jones et al., 1990), but in all cases these data were collected only on one occasion, and gave no information on possible changes in the populations through time. A few studies have examined changes in the fauna through time (e.g. meiofauna-Guzman et al., 1987; macrofauna- Riddle, 1988), but each of these studies dealt with "communities" at high taxonomic levels. It has been repeatedly demonstrated Correspondence address: G.A. Skiileter, University of North Carolina at Chapel Hill, Institute of Marine Sciences, Morehead City, NC 28557, USA.

186

G.A. SKILLETER

that explanations of factors structuring communities can be most profitably addressed through detailed studies of the population biology of the component species (Heck, 1979; Underwood, 1979; Peterson, 1982; Strong, 1983) but this approach has rarely been applied to studies of the fauna occupying sediments on coral reefs (Alongi, 1989). At the beginning of this study in October 1985, to the best of my knowledge, there were no published studies containing information on the abundance or diversity of molluscs in sediments on the Great Barrier Reef, Australia. Jones et al. (1990) described the molluscan assemblages in lagoonal sediments at One Tree Reef, in the southern Great Barrier Reef, as part of their investigations into the relationships between benthic feeding fishes and their prey. They found that there was significant variation in the distribution and abundance of species among five different habitats defined on the basis of geographic location within the lagoon, water depth and sediment characteristics. Rhinoclavis aspera (Linnaeus, 1758) (Family Cerithiidae), was one of the most abundant molluscs within the sediments at One Tree but this species was mainly found in the sediments of one habitat: a shallow subtidai sandflat on the southern side of the lagoon (Jones et al., 1990). Observations I made at the beginning of October 1985 indicated that a number of other cerithiid gastropods also appeared to be most abundant in this habitat compared with the other habitats in the lagoon. The first aim of this study was to document the spatial patterns of distribution and abundance of molluscs in the sediments at One Tree Reef and to determine which species were predominantly associated with the shallow sandflat habitat on the southern side of the lagoon. To maintain as much similarity as possible between this study and that of Jones et al. (1990) to allow comparison of results, I chose to examine these patterns at a number of different spatial scales ranging from a broad scale of different habitats ( 10 km 2) down to a fine scale of replicate samples (1 m 2) (see Jones et al., 1990, for greater detail). The three most abundant molluscs within the sediments of the sandflat were Rhinoclavis aspera, R. fasciata (Brugui6re, 1792) and R. vertagus (Linnaeus, 1758). The second aim was to provide a quantitative description of the temporal changes in the abundance and population size-structure of R. aspera, R. fasciata and R. vertagus. A study of these three species would allow comparisons of processes affecting the demography of these tropical, deposit-feeding gastropods, and those processes already studied in temperate areas for other deposit-feeding snails. MATERIALS AND METHODS STUDY SITE

This study was done at One Tree Reef (23°30'S • 152°06'E), on the eastern edge of the Capricorn Group of reefs on the southern Great Barrier Reef (Fig. 1). One Tree Reef is a lagoonal platform reef with three lagoons and has been described in detail by Davies et al. (1976). The reef crest is continuous around the first lagoon, which is 10 km 2

DISTRIBUTION AND ABUNDANCE OF CERITHIID GASTROPODS

187

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188

G.A. S K I L L E T E R

in area and is bounded on the eastern and southern sides by subtidal sheets of sand and has a centre that consists of a maze of reticulated and linear patch reefs (Marshall & Davies, 1982). Sites for sampling were located in the largest of the three lagoons. Three different habitats were examined. The subtidal sand sheet on the southern side of the reef, hereafter called the sandflat, and the deep channel adjacent to this sandflat, called the south channel, correspond to the south shallow and south deep habitats defined by Jones et al. (1990). In between these two habitats the edge of the sandflat slopes at 30 ° into the deeper water of the southern channel. The sediment on this slope is constantly moving under the influence of wave and tide-generated currents. At the base of the slope the sediment is more compacted and less mobile. This sloping edge was defined as a third habita~i (Fig. 1). A B U N D A N C E AND SPECIES DIVERSITY OF MOLLUSCS - O C T O B E R 1985

To determine the patterns of distribution and abundance of molluscs within the sandflat, edge and south channel habitats, the densities of molluscs were estimated at a number of different spatial scales within each habitat in October 1985 using a hierarchical sampling design. I compared abundances among: (i)random 100-m2 locations within habitats, each ~ 150 m apart; (ii) random 1-m2 sites within locations; (iii) random 225-cm2 cores within sites. The sandflat habitat was divided into three zones based on physical characteristics and distance from the reef flat (Fig. 1). The "outer" zone (closest to the reef crest on the windward side) is the shallowest section of the sandflat, and is littered with large fragments of coral debris and extends ~ 1/3 the way across the sandflat. The "middle" zone which has a more uniform cover of bare sand with occasional patches of rubble rising a few centimetres from the surface, covers the central 1/3 of the sandflat. The "inner" zone (closest to the lagoon) gradually increases in the depth of water and sediment, and is characterised by the presence of a large number of coral patch-reefs, which grow up to a height where the tops of the reef are barely covered during neap tides. The "inner" zone covers the final 1/3 of the sandflat. These broad differences in the physical character of these zones on the sandflat, and the edge and south channel habitats were used to define five zones (outer, middle, inner, edge and channel) moving away from intertidal carbonate pavement and into the lagoon. Four haphazardly selected locations were sampled in each of the zones on the sandflat. In the edge zone and the south channel, only two locations could be sampled. Within each of the 16 locations, six haphazardly selected sites were chosen and four replicate cores were collected within each site. Each core was collected by means of a diver-operated compressed air-powered suction lift, which deposited the sediment into a mesh bag (mesh size = I ram). The corer was a galvanised steel cylinder with an area of 225 cm 2. Samples were collected to a depth of 7 cm which was the minimal depth of sediment occurring at all locations.

DISTRIBUTION AND ABUNDANCE OF CERITHIID GASTROPODS

189

Even in areas that had sediment > 7 cm deep, molluscs were rarely found below this depth. Sediment particles and animals < 1 mm in size were not retained in the sample. The cores were transferred in the mesh bags to the laboratory where they were kept in running seawater until they were sorted (within 24 h). All specimens were identified and counted. There was some difficulty in differentiating among species of lucinid bivalves, but this did not affect estimates of species diversity because these species only ever occurred as a single specimen in any sample. ABUNDANCE OF R ltlNO CL A V IS THROUGH TIME

To describe the temporal changes in the abundance of Rhinoclavis aspera, R. fasciata and R. vertagus on the sandflat their densities were estimated at twelve locations seasonally from October 1985 to January 1988. Two of the locations (Locations 1 and 3 - Fig. 1) sampled in October 1985 could not be resampled in February 1986 because all the sediment was gone, leaving a bare coral substratum. These two locations were examined on each subsequent field trip but the depth of sediment never exceeded 1-2 cm at either location. In February 1986, two new locations were included in the sampling programme. SIZES OF SNAILS

To determine the size-structure of populations of R. aspera, R.fasciata and R. vertagus I measured all the specimens collected in the two sampling programmes. I used maximal width of the shell as the measured variable because the shells were considerably eroded, probably due to movements within the sediments, and shell length was considered to be unsuitable as a measure of size. For R aspera, the data from all sites within any location were pooled to provide an overall size.-frequency distribution for that location. This was necessary to obtain a sample size large enough for analysis. At each time and location, all the individuals of R. aspera collected in the cores during the primary sampling programme were measured so that changes in size-frequency distributions could be compared among locations. Although R. fasciata and R. vertagus were among the most abundant molluscs in the study area (R. aspera being the most abundant), the number of animals collected during sampling was too small to allow individual size-frequency distributions to be produced at each of the 12 locations. Therefore, the data from all 12 locations were pooled to produce a single estimate of the size-structure for each species within the study area. Cohorts within the size-frequency distributions were identified using the cohort extraction procedure of Cassie (1954). This procedure provides estimates of the proportion of animals in each cohort and the mean size of the animals in that cohort. STATISTICAL ANALYSES The data from the initial October 1985 survey on abundances of species, and the total number of species were analysed using three-factor, mixed-model, nested ANOVA with

190

G.A. S K I L L E T E R

factors Zones (fixed), Locations (nested within zones - random) and Sites (nested within Zones and Locations - random). To maintain a balanced design for the analyses, the two locations from each of the edge and south channel zones were pooled as a single zone. The analyses, therefore, examined differences among four zones (1, outer sandflat; 2, middle sandflat; 3, inner sandflat; 4, area off the sandflat). Data for R. aspera, R. fasciata, R. vertagus and C. tenellum were transformed to IOge(X + 1) before analysis to conform to assumptions of homoscedasticity ( C o c h r ~ ' s test, • = 0.05; Underwood, 1981). Data for E. rhomboides and the number of species in each sample were untransformed as these data conformed to assumptions of homoscedasticity. The data for temporal changes in the abundance of Rhinoclavis were analysed using a three-factor, mixed-model, nested ANOVA with factors Time (fixed), Locations (fixed) and Sites (nested within Times and Locations - random). Only the data for the period February 1986 to October 1987 (7 times) were analysed to maintain a symmetrical design because not all locations could be sampled in October 1985 or January 1988. A subset of the total number of locations sampled is presented in the figures. The data for the locations presented are representative of all the locations sampled. Data were transformed to loge(x + 1) before analysis to conform to assumptions ofhomoscedasticity. ....

RESULTS

A B U N D A N C E A N D SPECIES D I V E R S I T Y O F M O L L U S C S - O C T O B E R 1985

A total of 38 species of molluscs was identified from the 384 cores collected in October 1985 (Table I). Only five of these species occurred in > 10% of the samples and were considered sufficiently abundant for their distributions to be analysed separately. Four of these species belonged to the Family Cerithiidae (Rhinoclavis aspera, R. fasciata, R. vertagus and Cerithium teneilum). The fifth species was a small teUinid bivalve, Exotica rhomboides. The other species occurred in only a small proportion of samples, and many were only represented by one or two specimens (Table I). The densities of R. aspera did not differ significantly among the zones across the sandflat and into the lagoon, but there were significant differences among the locations within the zones (ANOVA; P < 0.01; Table II; Fig. 2). Student-Neuman-Keuls (SNK) tests differentiated the 16 locations into four groups (Table III) but these groupings did not correspond to any obvious physical differences in the habitat as distance increased from the reef crest. There were also significant differences among the sites within the locations (at a spatial scale of a few metres; Table II). The abundances of R.fasciata varied among the zones (ANOVA; zones P < 0.05; Table II) but multiple comparison tests (SNK) were unable to identify the significant differences among the four zones (Table III). There were no significant differences among the different locations within the four zones (ANOVA; Table II; Fig. 2) but there

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G.A. SKILLETER TABLE I1

A N O V A on the mean numbers of animals at three different spatial scales in October 1985. Data for

R. aspera, R. fasciata, R. vertagus and C. tenellum were transformed to loge (x + I). Data for E. rhomboides and the mean number of species were untransformed, F values are shown. In this and subsequent tables: * P < 0.05; ** P < 0.01; NS, not significant (P > 0.05). Source Zones: Z Location: L (Z) Sites: {Z x L) Residual

df

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Species

3 12 80 288

3.07 Ns 36.58** 2.21"*

4.80* 1.48Ns 1.66"

1.51Ns 1.92" 1.41"

22.51"* 0.66 Ns 2.82**

1.29Ns 29.31"* 1.38"

0,46 Ns 29.31"* 1.68"

MS estimates for Zones and Locations (Z) were pooled because F zones were not significant at P > 0.25 (Winer, 1971). Ra, R. aspera; Rf, R. fasciata; Rv, R. vertagus; Ct, C. teneilum; Er, E. rhomboides; species, total number of species in sample.

were significant differences among the sites within locations (ANOVA; Table II). There were no significant differences in the density of R. vertagus among the zones but the densities of animals varied among the locations (ANOVA; locations within zones; P < 0.05; Table II; Fig. 2). Multiple comparison tests (SNK tests) were unable to identify any significant differences among the 16 locations (Table III). Again, there were significant differences at the smaller spatial scale of sites within each location (Table II). C. tenellum showed a striking pattern of distribution among the zones. The "outer" sandflat had significantly more animals present than found in other parts of the sandflat or in the lagoon (ANOVA; zones; P < 0.01; Table II; SNK tests - Table III). C. tenellum was not recorded at the locations along the prograding edge of the sandflat or in the south channel. C. tenellum was also absent from several of the locations on the sandflat (Fig. 2). There were significant differences at the smaller spatial scale of sites within locations (Table I!). There were no significant differences in the abundance of E. rhomboides among the zones across the sandflat and into the lagoon (ANOVA; P > 0.05; Table II) but density of E. rhomboides did vary significantly among the different locations (ANOVA; Table II; Fig. 2). SNK tests differentiated the locations into three groups (Table liD. E. rhomboides was not found in the sediments on the edge of the sandflat so these two locations formed one group. The other two groups did not correspond to any obvious changes in the physical characteristics of the habitats, nor were the locations within those groups close together. The sloping edge habitat formed an obvious discontinuity in the distribution of this bivalve. Total numbers of species of molluscs There were no significant differences in the number of species among the different zones (ANOVA; Zones; P > 0.05; Table II) but there were significant differences

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Fig. 2. Mean ( + SE) densities ofR. aspera, R.fasciata, R. vertagus, C. tenellum, E. rhomboides and the total numbers of species of molluscs (gastropods and bivalves) at the 16 locations sampled during October 1985 (n = 24 x 225 cm 2 cores; four from each of six sites). Outer, middle and inner are the three zones on the sandflat. Edge is the edge habitat and chann, is the south channel habitat.

among the locations within the zones (Table II). Multiple comparison tests (SNK), comparing the number of species at each of the sixteen locations, showed that the two locations in the channel had significantly more species present than the locations on the edge and on the sandflat (Table III). Only three species, R. aspera, R.fasciata and R. vertagus, were found in the sediments along the prograding edge of the sandflat. In many samples collected from the edge habitat only one of these species was present. PATTERNS OF ABUNDANCE AND SIZE OF RHINOCLA VIS T H R O U G H TIME

The abundance of R. aspera varied significantly during the period of the study but the changes through time were not identical at all locations (ANOVA; Time x Location interaction; Table IV). There were also significant differences among the sites within

DISTRIBUTION AND ABUNDANCE OF CERITHIID GASTROPODS

195

TABLE IV Results of three-factor ANOVA comparing densities of R. aspera, R.fasciata and R. vertagus in 255-cm 2 cores at 12 locations on the sandflat on seven occasions from February 1986 to October 1987. Data were transformed to log,, (x + I). Source

R, aspera

df MS

Time: T Location: L T x L Sites (T x L) Residual

6 11 66 420 1512

R. fasciata F

49.52 133.08 5.21 0.93 0.22

53.24** 142.89"* 5.60** 4.25**

R. vertagus

MS

F

MS

F

0.74 7.37 0.88 0.26 0.19

2.86** 28.47** 3.41"* 1,34"*

2.46 8.78 1.37 0.42 0.22

9.02** 20.86** 3,25** 1.90"*

Results of SNK tests on loge (x + !) data for R. aspera (SE -- 0.20) to examine the significant interaction between Time and Location. Numbers refer to the Locations on the sandflat listed in order of increasing mean density. Locations joined by the same line have means which are not significantly different (P > 0,05). Feb 1986 May1986 Sep 1986 Dec 1986 Jan 1987 Jun 1987 Oct 1987

7 12 8 8 9 7 7.

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1.2 9 1.2 13 4 8 2

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9 _ 5 11 5_ 11 5 11 5 11 6 5 11 11 5

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each location (ANOVA; Sites (T x L); Table IV). Peaks in abundance usually occurred in the first few months of the year after which densities gradually declined until the next period of juvenile recruitment (Fig. 3). Locations 5, 6, 10 and 11 had consistently greater densities of R. ,pera than other areas ofthe sandflat from May 1986 to October 1987 (SNK tests for Time × Location interaction; P < 0.05; Table IV). These four locations are all grouped together on the sandflat (Fig. 1) but there were no obvious physical differences at these locations compared with surrounding areas of the sandflat. The size-frequency distributions for R. aspera were exa~Jined at each of the twelve locations but only the data from two of these locations are shown (Figs. 4, 5). These are representative of the changes in :he population size-structure at all the locations sampled (Skilleter, 1990). At the beginning of each year a new cohort of small ( < 4 mm maximal width of shell) R. aspera appeared in the population on the sandflat. There were virtually no animals of this size anywhere on the sandflat in the spring of the previous year (Table V) and extensive searching in other habitats, such as under coral patch reefs, in intertidal areas or in the lagoon, failed to locate juveniles which may have been cryptic. The appearance of these small animals in the summer of each year, therefore, represents recruitment of juveniles to the population after larvae had arrived from the plankton, settled in the sediments and gone through a period of growth and mortality. The number of animals found in this 0 + year class varied among locations

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Fig. 3. Mean ( + SE) seasonal abundances ofR. aspera on the sand flat at eight locations on the sandflat (n = 24 x 225-cm 2 cores; four from each of six sites). * The approximate time ofappearance of recruits (Feb 1986, Jan 1987, Jan 1988) determined from the size-frequency distributions at that location.

TABLE V Comparisons of tile number of R. aspera < 4 mm (maximal width of shell) in spring (October-December) and summer (January) for 1986/87 and 1987/88. Locations I and 3 could not be sampled aRer October 1985. NS, not sampled in January 1988. Location

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aspera at Location 6 from October 1985 to January

1988.

on the sandflat and also varied among years (Table V). There was no evidence for multiple periods of recruitment during the year at any of the locations on the sandflat. There is evidence to suggest that movement of animals, as a result of their migrations and/or due to passive transport in the sediment during storms, may have an effect on the size-frequency distributions documented at some locations. For example, in October 1987 at Location 7 (Fig. 5A) there were virtually no R. aspera present. In January 1988, the density had markedly increased as a result of not only the arrival of the 0 + year class, but also the recruitment of adults (animals > 5.5 mm width). As a further example

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of this, there were few R. aspera remaining at Location 13 in September 1986, and they were all > 9.0 mm (Fig. 5B). In December 1986, however, the density of R. aspera had increased as a result of the input of animals of 6.0-11.0 mm in size. These changes in the population size-structure could only have occurred as a result of the movement of adult animals into these locations. These abundances of R. fasciata and R. vertagus varied among the 12 locations but the fluctuations in abundance did not occur at the same time at all locations (ANOVAs; T x L interaction; Table Ill and also Figs. 6, 7). Many ofthe 12 locations sampled only supported small densities of either species during the course of this study. The data for these locations are not shown here: only the data for those locations where R. fasciata or R. vertagus occurred consistently are shown (Figs. 6 and 7, respectively). The

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Fig. 6. Mean ( + sE) seasonal a b u n d a n c e s of R.fasciata on the sandflat at six locations on the sandflat. O t h e r details as in Fig. 3.

abundance of each species also varied significantly at the smaller spatial scale of 10's of metres (ANOVA; Sites (T x L); Table III). Fhe size-frequency distributions for R. fitsciata and R. vertagus, constructed from the pooled samples collected on each occasion, were characterised by the presence of two cohorts of animals (Figs. 8, 9). The smaller cohort, which was usually well defined at the beginning ofthe year, quickly merged with the cohort oflarger animals. Sample sizes for each species, at individual locations, were too small to allow comparisons of the proportion of animals in the 0 + year class among locations on the sandflat. The main difference between the size-frequency distributions for R. vertagus and those for R. aspera and R.fasciata is in the presence of small individuals of R. vertagus throughout the year. Whether this was due to continuous recruitment during the year, or relatively slow growth of some individuals was not determined.

DISCUSSION

The southern subtidal sandflat at One Tree Reef is a distinct habitat forming an obvious break between the intertidal, carbonate reef flat and the deeper waters of the

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main lagoon (Marshall & Davies, 1982). The sandflat habitat is exposed to a gradient in the rate of water flow generated by different stages of the tide. The direction of water movement, at all stages of the tide, is away from the carbonate reef flat and towards the lagoon and the velocity of the water increases as it crosses the sandflat (Davies & West, 1981). There is also a flux of sediment towards the lagoon resulting in the edge of the sandflat (the edge habitat in this study) migrating into the lagoon in the direction of water movement. These gradients in water flow and sediment flux parallel a gradual change in the physical characteristics of the sandflat moving from the intertidal region to the lagoon. These differences were used to divide the sandflat habitat into three separate zones which were contrasted with two other habitats; the edge of the sandflat and the south channel. Although none of the species investigated showed a pattern of abundance that could easily be related to this gradient in the physical environment on the sandflat, there is evidence to suggest that some species showed a zonation at the broader spatial scale of habitats. In October 1985, R. aspera and C. tenellum were predominantly associated with the sandflat habitat and were either absent or at very small densities in the sediments of the edge or channel habitats. R. fasciata and R. vertagus had similar densities in each of the three habitats examined in October 1985 but densities in the

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lagoon were significantly less than on the sanaflat on other occasions (Skilleter, in prep.). This is consistent with the results from the surveys by Jones et al. (1990) who found that densities of Rhinoclavis were generally less in the lagoonal habitats compared with the sandflat habitat (Ferrell, pers. comm.). E. rhomboides also reached greater densities in the sandflat habitat. Interestingly, all of the most common species, predominantly associated with the coarse sandy sediments of the sandflat habitat are deposit-feeders and these species numerically dominated the molluscan fauna on the sandflat. This appears to be contrary

202

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to the findings of most studies from temperate regions (Sanders, 1958; Rhoads & Young, 1970; Levinton, 1972; Holland & Polgar, 1976; Whitlatch, 1977) and the few studies in subtropical regions (Peterson, 1977; Peterson & Black, 1987) where sandy sediments are dominated by suspension-feeding fauna (especially bivalves). Very few suspension-feeding bivalves were found in the sediments on the southern sandflat at One Tree Reef. Deposit-feeders are usually found in the finer sands and muds (Sanders, 1958, 1960; Rhoads & Young, 1970; Levinton, 1972; Holland & Polgar, 1976; Whitlatch, 1977, 1981) presumably because of the greater availability of food, such as

DISIRIBUTION AND ABUNDANCE OF CERITHIID GASTROPODS

203

bacteria and detritus (Sanders, 1958; NeweU, 1965; Levinton, 1972; Dale, 1974; Whitlatch, 1980). There are examples where this apparent pattern of suspension-feeders in sandy sediments and deposit-feeders in fine sediments breaks down (e.g. Sanders et al., 1962; Rhoads & Young, 1970; Aller & Dodge, 1974; Maurer et al., 1979). This occurred in places where sediment was exposed to strong water currents and/or there were very coarse sediments in pockets surrounded by finer sands. The sandflat habitat at One Tree is exposed to strong water currents and the surface of the sediment often has a rippled appearance indicative of constant reworking of the sediments often to depths of 10 cm (Frith, 1985). The constant physical disturbance ofthe sediments may explain why there are few suspension-feeding bivalves in this habitat but does not provide an explanation for why Rhinoclavis spp. are less abundant in the finer sediments of the lagoon, a habitat where they have significantly greater rates of growth than on the sandflat (Skilleter, in prep.). The dominance of R. aspera among the molluscs in the sediments of back reef areas is not confined to One Tree Reef. Kay (1971) and Kay & Switzer (1974) noted that R. aspera (asper in those papers) was the dominant mollusc in the back reef sediments at Fanning Island in thecentral Pacific, but densities did not exceed ~ 7 0 . m -2 (compared with over 2500. m - 2 at One Tree). Taylor (1971) also noted that R. aspera was very abundant in unvegetated sands of reefs in the western Indian Ocean, but gave no estimates of density. Densities of R. fasciata and R. vertagus were similar to each other in each of the habitats examined and there was no evidence of spatial segregation of these species. This is in stark contrast to the situation reported by Peterson & Black (1987) who noted a striking segregation pattern exhibited by R. fasciata and R. vertagus in Shark Bay, Western Australia. R. fasciata was only found at a subtidal site (at densities of up to 12.5.m -2) whereas R. vertagus was only found at an intertidal site (density of 0.4. m - 2). Neither of these species appear to occur in the pockets of intertidal sand at One Tree Reef. Cerithium tenellum occurred at densities greater than those observed for R. fasciata or R. vertagus at locations closest to the reef crest. The sediments in these areas are littered with broken coral rubble and large fragments of shell washed off the intertidal platform. At night, C. tenellum aggregates around these patches of coarse material, grazing over the surface of the rubble. Egg masses are attached to these surfaces during early summer (pers. obs.). C. tenellum was not found on the edge of the sandflat, nor in the southern channel. At least 12 species occurred in the sediments of the channel that were not recorded on the sandflat a few metres away. The reduced number of species on the sandflat may have been related to the reduced stability of the sediments in this ha.bRat. The stability of sediments has been suggested as being important for a variety of organisms (e.g. Sanders, 1958; Boaden, 1968; Aller & Dodge, 1974; Palmer & Brandt, 1981) and increasing sediment instability has been shown to cause a reduction in the number of

204

G.A. SKILLETER

species in other habitats (e.g. Maurer et al., 1979). Only Rhinoclavis was found in the sediments ofthe extremely unstable edge habitat. E. rhomboides occurred on the sandflat and in the lagoon but not in the sediments of the edge habitat. The density of all three species of Rhinoclavis fluctuated through time, with peaks in density usually, but not always, coinciding with recruitment of juveniles to the populations after arrival from the plankton in summer. Many other species of molluscs common on the sandflat and in the lagoon also showed increases in density in summer, after recruitment ofjuveniles (pers. obs. ). Berry & Othman (1983) found similar seasonal fluctuations in the density of Umbonium vestiarium, a gastropod occupying sandy beaches in tropical Malaysia but there have been few other studies which have investigated seasonal changes in abu,~darice of fauna of tropical sediments (AIongi, 1989). Greater numbers of R. aspera consistently occurred at Locations 5, 6, 10 and 11, compared with other parts of the sandflat. These locations are all grouped together but there were no obvious differences in the grain-size and sorting of the sediments between these and other locations (SkiUeter, 1990). The sediments varied among the locations on the sandflat, and changed through time, but there were no correlations between either grain-size or sorting and density of Rhinoclavis (Hansen & Skilleter, 1991). These four locations did not consistently support greater numbers of R. fasciata or R. vertagus than other parts of the sandflat. Hansen & Skilleter (1991) found a significant correlation between the density of R. aspera and the rates of bacterial production at five locations on the sandflat, including Locations 5, 6 and 10, but there were no relationships found in that study with other measures of abundance of food such as concentration of chlorophyll a, numbers of bacteria or concentration of organic carbon and nitrogen. Changes in the sediments during the year (generally they are coarser in summer; Skilleter, 1990) indicated that the animals living in and on this substratum were exposed to a number of changing conditions which could affect their population dynamics. During storms, especially when strong winds from the south-east coincided with spring tides, the sediment could be scoured to depths in excess of 5 cm, and this often exposed animals which were then carried along in the prevailing currents. The effects of storms on the sediments, and associated fauna were seen e~rly in this study when two of the original locations sampled in October 1985 were severely scoured with complete loss of sand. Disturbances from storms and strong water currents can have marked effects on benthic communities in temperate regions (Eagle, 1975; Boesch et al., 1976a; Rees et ai., 1977) and in freshwater streams (Hoopes,, 1974; Sagar, 1986). Savidge & Taghon (1988) found that rapid rates of recolonisation of artificial pits were probably a function of the rate at which animals were moved with the sediment. Such effec~:s may explain the large increases in the density of adult R. aspera at some locations although similar increases in density of R. fasciata and R. vertagus were not observed at these locations at the same time. The differential patterns of recruitment of R. aspera among locations on the sandflat may be a response to sediment movement on the sandflat and the presence of different amounts of fine sediment at different locations. Jones etal. (1988) suggested that

DISTRIBUTION AND ABUNDANCE OF CERITHIID GASTROPODS

205

changes in the sediment inside predator exclusion cages on the sandflat at One Tree, as a result ofgreater deposition and/or retention of fine particles in the cages, may have led to reduced recruitment of R. aspera inside the fences compared with o p e n ~ ~ O i ~ areas. Experimental manipulations of the substratum are necess~y ~to determine whether these patterns of rec~Jitn-ient and subsequent abundance of Rhinoclavis are a function of the type of sediment present at different locations on the sandflat and in different habitats in the lagoon. •A!ongi (i 989), in his review of the ecology of tropical, soft-sediment benthos~ called for more detailed studies on the benthic communities of coral reefs with emphasis on the spatial and seasonal variability of at least the dominant organisms. Here, I have presented data showing that for one habitat, a shallow subtidal sandflat, the most abundant molluscs are all deposit-feeders despite living in an environment of coarse sands and strong water currents" the type of environment usually dominated by suspension-feeding bivalves in temperate regions. I have also presented some of the first data documenting spatial and seasonal variability in the abundance and size-structure of populations of tropical, coral reef gastropods providing a starting point for more detailed studies on the factors affecting the population dynamics of these animals. ACKNOWLEDGEM ENTS

This study was supported by a Commonwealth Postgraduate Research Award and grants from the Australian Museum, Australian Coral Reef Society, Linnean Society of New South Wales, and the Institute of Marine Ecology, University of Sydney. I wish to thank C. Sowden, R. Webster and J. Wildforster for help in the field and K. Astles, M.G. Chapman, D.J. Ferrell, C. H. Peterson, K. L. Skil!eter, A.J. Underwood and an anonymous reviewer for comments on earlier drafts of the manuscript. REFERENCES Aller, R. C. and R. E. Dodge, 1974• Animal-sediment relations in a tropical lagoon, Discovery Bay, Jamaica. J. Mar. Res., Vol. 32, pp. 209-232. Alongi, D.M., 1986• Population structure and trophic composition of the free living nematodes inhabiting carbonate sands of Davies Reef, Great Barrier Reef, Australia. Aust. J. Mar. Freshwater Res., Vol. 37, pp. 609-619. Alongi, D.M., 1989. Ecology oftropical soft-bottom benthos: a review with emphasis on emerging concepts• Rev. Biol. Trop., Vol. 37, pp. 85-100• Berry, A.J. & Zamri Bin Othman, 1983. An annual cycle of recruitment, growth and production in a Malaysian population ofthe trochacean gastropod Umbonium vestiarium (L.). Estuarine Coastal SheifSci., Vol. 17, pp. 357-363. Boaden, P.J.S., 1968. Water movement - a dominant factor in interstitial ecology. Sarsia, Vol. 34, pp. 125-136. Boesch, D. F., R.J. Diaz & R.W. Virnstein, 1976. Effects oftropical storm Agnes on soft-bottom macrobenthic communities of the James and York estuaries and the lower Chesapeake Bay. Chesr:peake Sci., Vol. 17, pp. 246-259. ,,.,-,. o,,,.,,. i~,u~do,,l,.~ paper iii the analysis of size-frequency distributions. Aust. J. Caggie, R •,.,., s~ ~o~A e . . . . . .,.o,.o . . . ,~,,---~,--g:,:--. Mar. Freshwater Res., Vol. 429 pp- 783-794.

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