Does the slipper limpet (Crepidula fornicata, L.) impair oyster growth and zoobenthos biodiversity? A revisited hypothesis

Journal of Experimental Marine Biology and Ecology, 235 (1999) 105–124

L

Does the slipper limpet (Crepidula fornicata, L.) impair oyster growth and zoobenthos biodiversity? A revisited hypothesis Xavier de Montaudouin*, Corinne Audemard, Pierre-Jean Labourg ´ Biologique, UMR 5805 -Universite´ Bordeaux 1 -CNRS, 2, rue du Pr Jolyet, Laboratoire d’ Oceanographie F-33120 Arcachon, France Received 19 March 1998; received in revised form 12 September 1998; accepted 17 September 1998

Abstract The Prosobranch Gastropod Crepidula fornicata was introduced into Great Britain at the end of the 19th century from North America, upon imported oysters Crassostrea virginica. Since then, it has invaded sheltered coasts of the North West Atlantic and Mediterranean Sea. C. fornicata proliferation has often generated social conflicts due to three main causes: (1) trophic competition with other suspension feeders, e.g. the cultivated oyster Crassostrea gigas; (2) spatial competition with macrozoobenthos; and (3) enhancement of silt and clay sedimentation. The effects of C. fornicata on C. gigas growth and on macrozoobenthic density and diversity have been documented through field experiments in an oyster park of Arcachon Bay (France). Densities of C. gigas and biomass of C. fornicata were manipulated over a period of 247 days within field enclosures at low water level to test: (1) oyster growth, condition index and mortality; (2) zoobenthic community alterations (abundance, biomass, species richness). From this small-scale experiment, it was shown that none of these investigated parameters was significantly affected by the presence of C. fornicata. However, faunal assemblages were modified in enclosures compared to external bare sands, due to adding a hard substrata over a soft sediment.  1999 Elsevier Science B.V. All rights reserved. Keywords: Biodiversity; Crassostrea gigas; Crepidula fornicata; Interspecific competition; Field experiment; Macrofauna

1. Introduction Marine invasions by nonindigenous species are causing profound ecological changes *Corresponding author. Tel.: 133-5-56223908; fax: 33-5-56835104; e-mail: [email protected] 0022-0981 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0022-0981( 98 )00167-1

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in the ocean (Carlton and Geller, 1993). There are two types of invasions: range expansions by natural mechanisms, and introductions by human activity, often across natural barriers and vast distances (Carlton, 1989). These anthropogenic introductions are often accidental, with two main vectors: (1) transport of ballast water and plankton by ocean-going vessels (Williams et al., 1988; Carlton, 1989, 1992); (2) the movement of commercial oysters, which are ‘‘the greatest agency of all that spreads marine animals to new quarters of the world’’ (Elton, 1958 in Carlton, 1989). If a biological invasion is defined as ‘‘the arrival, establishment and subsequent diffusion of species in a community in which they did not previously exist in historical time’’ (Carlton, 1989), the occurrence of the Calyptraeidae Mesogastropod Crepidula fornicata (L.) (the American slipper limpet or slipper shell) suits this definition. C. fornicata was introduced into great Britain, along with the American oyster Crassostrea virginica, in the late 19th century (Barnes et al., 1973). Since then it has invaded most coastal embayments of the north-east Atlantic and west Mediterranean coasts (Hessland, 1951; Blanchard, 1995; Sauriau et al., 1998). Impressive biomasses were recorded, e.g. 583 g ´ dry weight m 22 in the bay of Marennes-Oleron (Deslous-Paoli, 1985; Sauriau et al., 1998). Due to their suspension-feeding regime (Walne, 1956) and to their predilection for the same habitats as oysters, C. fornicata quickly became an undesirable guest: ‘‘it has a detrimental effect upon oyster culture’’ (Chipperfield, 1951); ‘‘Crepidula as an oyster-pest’’ (Korringa, 1951; Walne, 1956); ‘‘it is a parasite’’ (Marteil, 1965), etc. In France, an order during a council in 1932 encouraged the destruction of C. fornicata (Marteil, 1965). In Helford River (Essex), British authorities even set a price on C. fornicata, viz 5 shillings per limpet in 1949, but only 1 penny in 1953 due to the increasing proliferation (Cole and Hancock, 1956). However, a few authors had a more moderate opinion about the effect of C. fornicata. Barnes et al. (1973), although they mentioned the potential competition of C. fornicata with oysters (Ostrea edulis), noted that in fact both populations did not show much overlap, and that C. fornicata provided the required niches for further hard-substrata species and that a rich association could be built on the initial basis of Crepidula alone. Peterson (1983) demonstrated with similar experiments that Crepidula onyx, in Mugu Lagoon (California), had a moderate effect on growth of the bivalve Chione undatella, and no significant effect on its survival. With a growth model coupling biological and hydrodynamical processes, Bacher (1991) demonstrated that the competition of C. fornicata on oyster growth was negligible compared with the effect of competition by oysters themselves (intraspecific competi´ tion), in the Bay of Marennes-Oleron (first oyster harvest in France). Hily (1991) made the hypothesis that C. fornicata has opportunistically increased in biomass partly in response to higher phytoplankton production, which should act as a mechanism of regulation preventing eutrophication. It is, however, surprising that hardly any of these statements (‘positive’ or ‘negative’) were really documented. The actual situation about the impact of C. fornicata could be summed up by Carlton’s statement (1992): ‘‘there is little experimental elucidation of the ecological impact of the introduced mollusks’’. The present study attempts to elucidate the effect of C. fornicata on a small-scale environment, through in situ experiments. Considering that C. fornicata are quoted to be a ‘plague’ for oysters, experiments took place in an oyster park, in Arcachon Bay, France (annual oyster production, 15 000 t). C. fornicata was first reported in this Bay in

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1969 (Bachelet et al., 1980). It was suspected to have been introduced by accidental release with commercial oysters, after the second world war, unless it was directly introduced from America when attempts were made to settle the American oyster Crassostrea virginica (Fischer et al., 1865). The assessment of C. fornicata biomass in Arcachon Bay is in progress. Preliminary data showed that populations are still confined to a restricted area, where, however, the biomass can reach 10 kg fresh weight m 22 . The experimental design was set up in order to test the following hypotheses, which are commonly proposed: (1) C. fornicata is a trophic competitor of the cultivated oyster Crassostrea gigas, and affects its growth? (2) C. fornicata is a spatial competitor for macrozoobenthos? (3) The presence of C. fornicata enhances mud sedimentation modifying benthic communities and decreasing biodiversity?

2. Materials and methods

2.1. Study area The study site was a semi-exposed intertidal sand flat at Cap Ferret, in Arcachon Bay (448409N; 018109W), a 156-km 2 macrotidal embayment on the southwestern Atlantic coast of France (Fig. 1). The tidal amplitude ranges from 4.35 at spring tide to 0.95 m at neap tide (Gassiat, 1989). Cap Ferret is in the oceanic part of the Bay, with a constant salinity (34–36 psu) and a water temperature which varies seasonally between 9.5 and 21.08C (Bouchet, 1968). The experimental enclosures were situated in an oyster park, which was a sandflat protected from waves, boats and large predators by a fence of continuous wood poles (‘piniots’). For optimal oyster growth rates, the park lies between 0 and 1 m above chart datum. Enclosures were situated in a 400-m 2 area with bare fine sand (in the first 1 cm of sediment: mean grain size, 171 mm; silt-clay content, 4–8%; organic matter content, 0.6–0.9%), free of oysters. During experiments, Chlorophyll a and phaeopigment contents ranged from 1.0 to 3.8 mg l 21 and 0.3 to 4.0 mg l 21 in phytoplankton, respectively, and from 17.0 to 31.7 mg cm 22 and from 6.1 to 25.6 mg cm 22 , respectively in microphytobenthos (in the first 1 cm of sediment). Pigment contents were measured every month at the proximity of enclosures following Aminot (1983), based on the method of Lorenzen (1967).

2.2. Field experiment To test for interspecific interactions, the abundances of Crepidula fornicata and Crassostrea gigas were manipulated within 0.25-m 2 open-topped enclosures constructed to prevent transport of the molluscs by waves or currents. Each enclosure consisted of a 50 3 50 3 25-cm metal frame meshed with 10-mm plastic net, buried 10 cm and projecting 10 cm above the sediment. In September 1996, nine enclosures were installed in two rows parallel to the shoreline, at 0.8 m above chart datum: one row of five enclosures alternated with the four enclosures of the second row. The enclosures were regularly spaced with 1 m between them. Enclosures were randomly assigned to three

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Fig. 1. Map of Arcachon Bay at low tide, with location of the studied sites (w). Cap Ferret was the experimental site and Banc d’Arguin the origin site of Crepidula fornicata.

treatments (and three replicates) wherein molluscs were regularly disposed: (1) 45 C. gigas (73-mm shell hight, 180 ind m 22 , 6800 g fresh weight (FW) m 22 ) which represents a ‘normal’ density in oyster-farming; (2) 45 oysters mixed with 1500 g FW live C. fornicata, which represents a high but representative biomass for Arcachon Bay ( ¯ 940 ind m 22 , 6000 g fresh weight m 22 ); and (3) the same treatment as in (2) but the C. fornicata flesh were emptied after boiling and the stacks reconstituted with glue, in order to distinguish between the biological effect of C. fornicata and hydrodynamic effects due only to its physical presence. In order to quantify the spatial competition, the percentage of surface area occupied by C. fornicata and C. gigas in enclosures was assessed by analysis of photographs taken at the start and the completion of the experiment. The purpose of this study was to investigate the effect of C. fornicata in ‘oyster park’ condition, and consequently treatment with C. fornicata alone was not conducted.

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The C. gigas chosen were ¯ 18 months old (mean shell height, 73 mm). At this age, potential growth is considered to be high (Deslous-Paoli, 1982; Maurer, 1989; Raillard et al., 1993) and mortality very low, which were two important points in order to study growth effects at constant density. At the beginning of the experiment (September 1996), all C. gigas were measured for shell height and length with Vernier callipers to the nearest 0.1 mm, and their fresh ( 5 wet) weight to the nearest 0.1 mg. Thirty individuals from the initial population were sacrificed in order to determine flesh dry weight (FWi ) to the nearest 0.1 mg. Individual inside shell volume (Vi ) was calculated as the difference between the volume of a closed live C. gigas and the volume of the empty open shell by immersion in a graduate beaker filled with water. Condition index was calculated as: CI 5 FWi (mg) /Vi (ml) 3 10 3 (Medcoff and Needler, 1941). At the end of the experiment (June 1997, i.e. 247 days later), C. gigas were similarly analysed, for each of the three treatments. Height, length, fresh and dry weight, condition index were compared using a two-way nested ANOVA with ‘treatment’ as a fixed factor and ‘enclosure’ as a nested random factor (Sokal and Rohlf, 1981; Underwood, 1981). Homogeneity of variances was previously verified by the Cochran test (Winer, 1971) and normality was assumed. Following the same procedure, C. gigas mortality was compared between treatments using a one-way ANOVA, with enclosures as a random factor. C. fornicata were collected in a proximal station, Banc d’Arguin (Fig. 1), just prior to the experiment. This site was chosen because the population of C. fornicata lived under similar conditions to those they were expected to be exposed to in their new location: similar sand, water mass and tidal position. Therefore, the stress of transplantation could be minimized. The size frequency distribution of this population was determined, using their longest linear dimension. Macrozoobenthos was studied through a monthly survey, and through a more exhaustive sampling at the end of the experiment for total macrofauna. For the monthly survey, two replicate cores of 7 cm diameter and 5 cm depth were taken within each enclosure and six cores outside the enclosures. The latter six cores were not considered as true control (a real control would be enclosures without both C. gigas and C. fornicata) but as a comparison between homogeneous (bare sand outside the enclosures) and heterogeneous (enclosures with oysters and slipper limpets inside) substrates. However, for convenience, it will be termed hereafter ‘control’. Samples were sieved through a 500-mm mesh, fixed in 4% formalin and stained with Rose Bengal. The small size of cores and sieve’s mesh were, respectively, chosen to avoid major disturbance inside enclosures during experiments and to collect macroinfaunal juveniles. In the laboratory, the individuals were sorted, identified to the lowest possible taxon and enumerated. For each month, density was compared using a one-way ANOVA, with ‘treatment’ as a fixed factor (the two core samples were pooled and the mean value was used as the estimator of benthic abundance). Cores taken outside the enclosures (‘controls’) were included in the test. Whenever an ANOVA was significant, the treatments were separated by Tukey a posteriori test (Day and Quinn, 1989) because species composition was roughly the same in all treatments and control. Data were log(x11) transformed when the homogeneity of variance was not obtained. It was sufficient, in our study to achieve homogeneity. In order to assess the effect of our treatments on zoobenthic assemblages, an analysis of correspondences was conducted.

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The choice of this inertia method was motivated by the choice of the x 2 as the distance. This distance allows to compare structure of communities between treatments (including months) and eliminates the numerous ‘double absence’ of species between treatments which are consequently not considered as similarities (Benzecri, 1973; Chardy et al., 1976). The data matrix was first reduced by elimination of rare species (relative abundance ,0.1%). This matrix was built with the mean density of each species in each treatment for each month, providing 20 rows (treatments) and 24 lines (species, representing 99% of the total number of individuals). The final sample were collected using a 0.04-m 2 quadrat from the center of each enclosure to the depth of 15-cm. An additional three quadrats were collected outside the enclosures in order to conduct the same comparisons realised with recruitment cores. Due to the large volume of sediment and macroalgae collected, samples were sieved on 1-mm mesh and treated as described above. Consequently, results from the monthly survey (sieved with 500-mm mesh) and results from the final sample (June) were analysed separately. Biomass was determined as ash-free dry weight, after previous decalcification of taxa with an external skeleton in a 10% HCl solution. Density was compared with a one-way ANOVA, with treatment as a fixed factor (control included). A posteriori test was not performed because differences of density (when they were significant) were due to different species, and a comparison of means would have been ecologically meaningless. As for monthly survey and following the same procedure, an analysis of correspondences was performed with enclosures (and control) in rows and species in lines (12330 data matrix). At the end of the experiment, superficial sediment was sampled in every enclosure (three3three samples) and in the control (three samples). Sediment organic matter content was calculated as loss of dry weight at 5508C for 2 h and median grain size and silt and clay contents were compared between treatments and control with a one-way ANOVA.

3. Results

3.1. Population of C. fornicata Biomass, density and egg brooding by females were selected as parameters to verify that the C. fornicata population in the enclosures was ‘fit’. The biomass of C. fornicata in the enclosures remained stable during the experiment (only 5.5% decrease), which warranted a constant treatment (growth was not investigated). Density reduction was accentuated (235%) but involved mainly small individuals (Fig. 2). In June 1997, 30% of females were brooding (Table 1). Consequently, the population of C. fornicata, although it was transplanted from Banc d’Arguin to Cap Ferret (experiment site), was assumed to fulfil experimental conditions.

3.2. Effect of C. fornicata on C. gigas From September 1996 to June 1997 (i.e. 247 days), C. gigas mean height increased

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Fig. 2. Length frequency distribution of Crepidula fornicata at the native site (Arguin, natural evolution of the population) and the transplant site (Cap Ferret, evolution of the enclosed population) between September 1996 and June 1997.

(from 73 to 93 mm, length from 40 to 60 mm, fresh weight from 38 to 65 g and dry flesh weight from 0.05 to 2.43 g (Fig. 3). The condition index increased by 2.8-fold from 39.25 to 110.00 g ml 21 (Fig. 3), mainly due to gonadal maturation during spring

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Table 1 Mean abundance (61 S.E.), biomass (fresh weight, 61 S.E.) and total percentage of brooding females of Crepidula fornicata populations at the beginning and the end of the experiment, in the native (Arguin, n5four 1-m 2 quadrats) and transplant (Cap Ferret, n53 0.25-m 2 enclosures) site Arguin

Cap Ferret

Sept. 1996

June 1997

Sept. 1996

June 1997

Abundance (ind m ) 66 (25) Biomass (g FW m 22 ) 421 (137) Brooding females (%) 6

297 (105) 310 (120) 19

941 (10) 6000 (0) 6

607 (20) 5673 (406) 30

22

(Maurer, 1989). Final C. gigas mean abundance in enclosures was 43 individuals (Fig. 3), corresponding to a moderate mortality of 4.4%. None of these parameters (dimensions, condition index, survival) was significantly affected by the treatments in the enclosures (P.0.05) (Table 2), although survival was abnormally low in a single enclosure of the ‘C. gigas and dead C. fornicata’ treatment (78%). However, although the oyster condition index was not significantly affected by the treatments (P.0.05), it was 20% lower in the treatment with live C. fornicata than in the other treatments. Most of the decrease of ‘free’ sediment surface area during the experiment was due to C. gigas growth (Table 3), with oysters occupying ¯38% of the sediment surface at the beginning of the experiment and 67% at the end of the experiment. The surface occupied by C. fornicata during the experiment was decreased. This result and the decrease of the sediment surface occupied by dead C. fornicata (Table 3) were due to the burial of a few specimens.

3.3. Effect of C. fornicata on macrozoobenthos (monthly survey of the top 5 cm) From January to May 1997, the density of total benthic macrofauna in the first 2 cm of sediment fluctuated between 38 and 83 ind 38 cm 22 (Fig. 4a), with no significant

Fig. 3. Compared biological parameters of Crassostrea gigas individuals between the beginning of the experiment in September 1996 (start) and the end of the experiment in the three enclosed treatments. The measured parameters (11 S.D.) were: shell height (mm), shell length (mm), total fresh weight (FW, g), flesh dry weight (DW, 100 g), condition index (CI, mg / ml), survival (%).

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Table 2 Results of two-way nested ANOVAs comparing the effect of Crepidula fornicata presence (fixed factor) and enclosure (nested random factor) on biological variables measured on Crassostrea gigas at the end of the field experiment Variables

Sources of variation

df

F

Shell height

C. fornicata presence Enclosure /C. fornicata Error C. fornicata presence Enclosure /C. fornicata Error C. fornicata presence Enclosure /C. fornicata Error C. fornicata presence Enclosure /C. fornicata Error C. fornicata presence Enclosure /C. fornicata Error C. fornicata presence Error

2 6 171 2 6 171 2 6 171 2 6 171 2 6 171 2 6

0.73 ns 0.65 ns

Shell length

Fresh weight

Flesh dry weight

Condition index

Mortality

0.18 ns 1.06 ns 0.79 ns 0.53 ns 1.99 ns 1.25 ns 2.91 ns 1.70 ns 4.16 ns

Effect of C. fornicata presence on C. gigas mortality was assessed with a one-way ANOVA. df, degrees of freedom; ns, not significant (P.0.05).

difference (P.0.05) between treatments, or compared to the control (Table 4). The two first axes of the analyse of correspondences accounted for 56.5% of the total inertia (Fig. 5). Axis 1 (29.7% inertia) isolated very clearly the controls (left position) from the three treatments (right position), for each month. The Axis 2 (26.8% inertia) followed the seasonal gradient with an homogeneous group from January to March. In April, a separation of the control was observed and this ‘spring effect’ was confirmed with a distinct subgroup in May for both control and treatments. Between treatments, the analysis of correspondences showed no obvious trend. About 68% of the total macrofauna density was due to annelid density whose trend was consequently the same as that for total densities (Fig. 4b; Table 4). The dominant species was the oligochaete Table 3 Percentage (61 S.D.) of surface area occupied by Crassostrea gigas and Crepidula fornicata, at the beginning (September 1996) and at the end (June 1997) of the experiment, in the three different treatments Treatment in enclosures

Species

Beginning of experiment

End of experiment

1. C. gigas

C. gigas Total C. gigas C. fornicata Total C. gigas C. fornicata Total

38.2 38.2 34.3 20.7 55.0 34.7 20.0 54.7

72.0 72.0 66.5 18.9 85.4 63.2 15.9 79.2

2. C. gigas and live C. fornicata

3. C. gigas and dead C. fornicata

(60.9)% (60.9)% (60.8)% (60.5)% (61.1)% (62.0)% (60.4)% (62.4)%

(65.1)% (65.1)% (62.3)% (61.8)% (60.5)% (66.3)% (64.6)% (62.7)%

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Fig. 4. Abundance (ind 38 cm 22 , 11 S.E.) of macrofauna collected in the first 2 cm of sediment and sieved with 500-mm mesh, in the control and the three enclosed treatments, from January to May 1997 (n56). When ANOVA is significant, arrows gather treatments with different densities (P,0.05), following a posteriori Tukey test.

Tubificoides benedeni, throughout the experiment duration and in all treatments (20– 63% of total abundance) (Table 5). Molluscs represented 29% of the total macrofauna (Fig. 4c). Most of them (86%) were juvenile bivalves (probably most of them were Ruditapes spp.). On three occasions (January, February and May), the density of these bivalves was significantly higher (P,0.05) within the enclosures as compared to the

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Table 4 Results of one-way ANOVAs comparing the effect of Crepidula fornicata presence (fixed factor) on total macrozoobenthos in the first 5 cm of sediment (500-mm mesh size) and for the three main zoological groups Zool. group

Month

Source of variation

df

F

Zool. group

Source of variation

df

F

Total

Jan. 97

Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error

3 8 3 8 3 8 3 8 3 8 3 8 3 8 3 8 3 8 3 8

0.07 ns

Annelids

Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error Treatment Error

3 8 3 8 3 8 3 8 3 8 3 8 3 8 3 8 3 8 3 8

0.45 ns

Feb. 97 Mar. 97 Apr. 97 May 97 Molluscs

Jan. 97 Feb. 97 Mar. 97 Apr. 97 May 97

0.61 ns 0.40 ns 3.88 ns 1.50 ns 5.83* 2.88 ns 11.55** 2.43 ns 5.32*

Crustaceans

0.41 ns 0.14 ns 1.92 ns 2.62 ns 0.28 ns 1.92 ns 4.00 ns 1.18 ns 1.08 ns

df, degrees of freedom; ns, not significant (P.0.05); *P,0.05; **P,0.01).

Fig. 5. Treatment and control plots of the results of the analysis of correspondences on axes 1 and 2, from the monthly macrozoobenthic survey. The shape of the symbols represents each month (from January to May). White symbols are controls (C) and black symbols are treatments (O, oysters alone; D, oysters and dead slipper limpets; L, oysters and live slipper limpets).

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Table 5 Percentage of the total zoobenthic abundance (sieved with 500-mm mesh) of the five dominant species in the ‘control’ and in the three different treatments, from January to May 1997 in the first 15 cm of sediment (1-mm mesh size) Treatment

Species

Jan.

Feb.

Mar.

Apr.

May

Tubificoides benedeni (O) Myriochele oculata (P) Bivalve juvenile (M) Heteromastus filiformis (P) Pygospio elegans (P) Prionospio malmgreni (P) Cerastoderma edule (M) C. gigas Tubificoides benedeni (O) Bivalve juvenile (M) Myriochele oculata(P) Iphinoe trispinosa (C) Bittium reticulatum (M) Capitella capitata (P) Heteromastus filiformis (P) Pygospio elegans (P) C. gigas and Tubificoides benedeni (O) live C. fornicata Bivalve juvenile (M) Myriochele oculata (P) Bittium reticulatum (M) Heteromastus filiformis (P) Capitella capitata (P) Amphipod sp. Tubificoides benedeni (O) Bivalve juvenile (M) C. gigas and Myriochele oculata (P) dead C. fornicata Capitella capitata (P) Iphinoe trispinosa (C) Heteromastus filiformis (P) Cerastoderma edule (M) Mytilus edulis (M)

51 19 8 7

49 13 16 2 5

63 13 5 5 5

43 (1) 12 (2)

29 (1) 13 (4) 10 (5)

‘Control’

(1) (2) (3) (4)

(1) (3) (2) (5) (4)

(1) (2) (4) (3) (5)

6 (5) 12 (3)

15 (2)

4 (5) 35 34 11 4 4

(1) (2) (3) (4) (5)

31 (2) 41 (1) 9 (3)

48 (1) 20 (2) 13 (3)

3 (4)

4 (4)

7 37 10 13

(4) (1) (5) (2)

10 (4) 10 (3)

48 23 10 4 4

(1) (2) (3) (4) (5)

3 46 22 9 4

(5) (1) (2) (3) (5)

6 (4) 47 26 12 3 3

(1) (2) (3) (4) (5)

14 (3) 20 (2) 42 (1)

41 29 14 3

(1) (2) (3) (5)

5 (4)

4 50 13 11

(5) (1) (2) (3)

53 (1) 4 (5) 15 (2)

5 (5) 5 (4)

11 (3) 8 (4)

50 24 11 4

(1) (2) (3) (4)

4 (5)

59 7 11 4

(1) (3) (2) (5)

7 (4)

4 6 4 32 31 7

(5) (3) (4) (1) (2) (3)

5 (4) 5 (5) 31 (1) 29 (2)

7 (4) 7 (5) 9 (3)

Rank is in brackets. C, crustacean; M, mollusc; O, oligochaete; P, polychaete.

controls, with no difference (P.0.05) between treatments (Table 4). A posteriori Tukey test revealed that most contrasts (P,0.05) were observed between ‘control’ and ‘oyster alone’ (Fig. 6), which was consistent with analysis of correspondences (Fig. 5, Axis 1). Although this result is difficult to interpret, it can be suggested that ‘oyster’ treatment was a good compromise for bivalve recruitment, with lower hydrodynamics and sufficient bare sand surface. Crustaceans were not abundant with 1.7 ind 38 cm 22 (Fig. 4d) and showed no differences (P.0.05) with treatment (Table 4). Concerning the species composition, the three dominant ‘species’ were similar in the three treatments during the whole experiment (Table 5): from January to March, Tubificoides benedeni, Myriochele oculata and the group of juvenile bivalves. In April and May, the species rank after the second was moderately altered following treatments, but with the constant

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Fig. 6. Abundance (a, ind m 22 , 11 S.E.), biomass (b, g m 22 , 11 S.E.) and species richness (c) of macrofauna sampled 15 cm deep in the sediment and sieved with 1000-mm mesh, in the control and the three enclosed treatments, at the end of the experiment (May 1997) (n53).

occurrence of the polychaetes Heteromastus filiformis and Capitella capitata. The control community differed mainly from the community inside the enclosures by the moderate presence of juvenile bivalves (15 and 27% of the total abundance, respectively) and by the absence of Capitella capitata (compared to the enclosures wherein this species was constant and represented up to 8% of the total density) (Table 5).

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3.4. Effect of C. fornicata on macrozoobenthos community ( final sampling, in the top 15 cm) At the completion of the experiment, in June 1997, the total density of macrozoobenthos was similar (P.0.05) in the three treatments and the control (Table 6), with an average of 9556 ind m 22 . There were no significant differences (P.0.05) in mollusc densities (Table 6) between treatments, suggesting that the higher recruitment rate of bivalves in enclosures observed in January and March 1997 (Table 4) had not been retained as an adult difference. Indeed, even with 1-mm mesh, these juveniles would have been large enough in June to be sampled. For the two more highly represented zoological groups, annelids and crustaceans, densities had been affected (P,0.05) by treatment (Table 6; Fig. 6a). In this case, the species compositions were of major importance to interpret the observed trends. At this time of the year, green algae had attached to the hard substrates, i.e. shells of slipper limpets and oysters, and the enclosure frames. In the analysis of correspondences, the two first axes accounted for 71.4% of the total inertia (Fig. 7). Axis 1, with a high 58.8% inertia separated very clearly the different treatments from the three replicated controls. Enclosures O1 (oysters alone) and D1 (oysters and dead slipper limpets) were isolated by the unexplained lack of Capitella capitata (Polychaete) which was a dominant species in the other enclosures (spatial heterogeneity?). The control C3 was isolated by a relatively low density of the dominant Oligochaete Tubificoides benedeni. Thus, different communities settled in the enclosures and in the controls, although the oligochaete Tubificoides benedeni remained a dominant species in both cases (Table 7). Within the enclosures, the next dominant species were the crustacean amphipod Gammarus spp. and Melita palmata, the polychaetes Capitella capitata and Heteromastus filiformis. In the control, the dominant species were the polychaetes Myriochele oculata, Pygospio elegans and the bivalves Ruditapes pullastra, Abra spp. The density differences for the rest of the zoological groups (‘others’ in Table 6) (P,0.001) has no biological significance due to the very

Table 6 Results of one-way ANOVAs comparing the effect of Crepidula fornicata presence (fixed factor) on macrozoobenthic density at the end of the field experiment (1-mm mesh size) Zoological groups

Sources of variation

df

Annelid

C. fornicata Error C. fornicata Error C. fornicata Error C. fornicata Error C. fornicata Error

3 8 3 8 3 8 3 8 3 8

Mollusc Crustacean Others Total

presence presence presence presence presence

df, degrees of freedom; ns, not significant (P.0.05). *P,0.05; ***P,0.001.

F 5.80* 2.48 ns 5.52* 19.14*** 4.60*

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Fig. 7. Treatment and control plots of the results of the analysis of correspondences on axes 1 and 2, from the final macrozoobenthic samples (June). The shape of the symbols represents each month (from January to May). C, control; O, oysters alone; D, oysters and dead slipper limpets; L, oysters and live slipper limpets. The three replicates are plotted.

low number of individuals present (Fig. 6a). In terms of biomass (Fig. 6b), values were highly variable with high standard errors, due to the occasional presence of large individuals such as the gastropod Hinia reticulata, the crab Carcinus maenas or the sea star Asterias rubens. Species richness ranged between 34 and 37 in enclosures compared to 21 in the control sediments. This difference was mainly due to the presence of Crustaceans and Echinoderms in enclosures (Fig. 6c).

Table 7 Percentage of the total zoobenthic abundance (sieved with 1-mm mesh) of the five dominant species in the ‘control’ and in the three different treatments, at the end of the experiment (June 1997) Species

‘Control’

C. gigas

C. gigas and live C. fornicata

C. gigas and dead C. fornicata

Tubificoides benedeni (O) Myriochele oculata (P) Pygosio elegans (P) Ruditapes pullastra (M) Abra sp. (M) Gammarus sp. (C) Capitella capitata (P) Heteromastus filiformis (P) Melita palmata (C)

45 (1) 18 (2) 13 (3) 7 (4) 5 (5)

32 (1)

22 (2)

21 (2)

12 (2) 10 (3) 10 (4) 5 (5)

27 (1) 17 (3) 14 (4) 5 (5)

26 (1) 11 (4) 11 (3) 6 (5)

Rank is in brackets. C, crustacean; M, mollusc; O, oligochaete; P, polychaete.

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3.5. Effect of C. fornicata on sediment The main characteristics of the sediment, i.e. organic matter content, median grain size and silt and clay content did not significantly change (P.0.05) in the control between the beginning and the end of experiments, or amongst different enclosures (not illustrated). The average values were respectively 1.41%, 178 mm and 6.3%. However, we are aware that with three replicates the power of the test was reduced, and that microscale environments in and between shells were not taken into account.

4. Discussion

4.1. Enclosure artifact Growth of Crassostrea gigas was monitored in enclosures, installed on the tidal flats. Similar in situ experiments have already been performed to investigate similar questions concerning competition between molluscs. Although the scale of such treatments appears dramatically ‘reduced’ in the environment, it has already supplied surprisingly large effects concerning bivalve growth (Reise, 1985; Peterson and Black, 1988; Dobbinson et al., 1989; de Montaudouin and Bachelet, 1996; de Montaudouin, 1997). It is however not possible to exclude that enclosures affected oyster growth. Like any device installed in the marine environment, enclosures modify hydrodynamics and sedimentation rates (Eckman, 1983; Hannan, 1984; Butman, 1986), and may also create unrecognized effects considered as ‘hidden treatments’ (Huston, 1997). Nevertheless, this artifact does not preclude a test of the influence of Crepidula fornicata on C. gigas growth, because the necessary independent treatments were identically enclosed. Concerning the effect of C. fornicata on macrozoobenthic biodiversity, two levels of analysis must be distinguished: (1) biodiversity in an oyster-flat in the presence or absence of C. fornicata. At this first level, as previously mentioned, all appropriate treatments were similarly enclosed and allowed to assess whether the presence of C. fornicata, added to oysters, modified zoobenthic community, (2) biodiversity in bare sand compared to biodiversity in heterogeneous substrates (sand, enclosures and large epifaunal molluscs). At this second level, the experimental design (see Section 2) does not distinguish between ‘enclosure effects’ and ‘oyster and / or slipper limpet effects’, although both have certainly a similar influence on hydrodynamics and sediment deposition processes.

4.2. Oyster growth Crassostrea gigas growth in this study was unaffected by the presence of Crepidula fornicata, although a high (but realistic) biomass of this gastropod was manipulated. Additionally, growth was obviously not affected by the treatments, with a very similar rate (0.08 mm / day) to what has been observed in exploited condition, in Arcachon Bay (Maurer, 1989). Therefore, if C. fornicata is considered as a ‘potential’ competitor of oysters due to its suspension-feeding regime (Walne, 1956; Jørgensen, 1966; Coum,

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1979), it was not a very ‘effective’ competitor under the experimental conditions. Indeed, competition can not be separated from the notion of ‘carrying capacity’ of the ´ habitat (Heral et al., 1989), and oyster parks are logically installed in areas of high phytoplankton production (i.e. with high biotic capacity for suspension-feeders). This was verified in the present study where chlorophyll a concentrations in the water column were high. Consequently, the theory of a deleterious effect of C. fornicata on oyster growth must be considered with the respect to biotic capacity of the habitat. Peterson (1983) demonstrated that Chione undatella (bivalve) carrying attached Crepidula onyx had a lower growth than those without. It seems likely that in this particular case, Crepidula had competed with their host for suspended food particles. In this enclosures experiment (Peterson, 1983), however, snails were located at a position on the shell directly adjacent to the Chione’s siphons, implying that partial local clearance of the intake water was possible. This was not the case in the present study, where Crepidula was not attached to oysters but disposed nearby.

4.3. Macrozoobenthic biodiversity With respect to macrofauna diversity, it must be taken into account that in the present study Crepidula fornicata was considered as a biological invader settled in an other biological invader population (the Japanese oyster, Crassostrea gigas), the latter having been deliberately introduced. Oyster-parks are already a modified artificial ecosystem. At the same site (Cap Ferret), Castel et al. (1989) compared macrofauna densities in bare sands and oyster flats, and found higher abundances in the oyster-parks (2905 ind m 22 compared to 1898 ind m 22 ). Faunal assemblages also differed. Still at this site, none of the five dominant species found by Bachelet and Dauvin (1993) outside oyster parks were found inside the parks of the present study. As observed by Castel et al. (1989), the studied oyster park was mainly characterized by the presence of oligochaetes (Table 5). In the present studied environment (an oyster park, but in a bare sand area - see Section 2), the zoobenthic community was different in control sediment and within experimental enclosures, but without significant differences between the experimental treatments. Compared to bare sediments which were mainly colonized by annelids and bivalves, the complex structure of enclosures, oysters (and sometimes C. fornicata) attracted a particular fauna composed of amphipods (Gammarus spp.) and capitellid polychaetes. However, in the present experiments, it was not possible to determine whether these changes in macrofaunal populations were due to the physical presence / protection provided by the enclosures or to associated green macroalgae which attached to the enclosures and lied over the sediment. For example, Capitella capitata is a well-known inhabitant of sediment under algal mats (Nicholls et al., 1981; Thrush, 1986; Hull, 1987). It is probable that all these parameters (enclosures, oysters, slipper limpet, macroalgae) act in similar fashion, i.e. increasing the complexity of the habitat, adding a new type of substrate (hard) and leading to a higher diversity of niches (Barnes et al., 1973; Woodin, 1978). The presence of C. fornicata in enclosures, although it increased the occupied surface area 1.6-fold at the beginning of the experiment and 1.3-fold at the end (Table 3), did not affect the benthic community. In this treatment, there was no spatial competition, the habitat becoming only a little more heterogeneous. Similar

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results were found by Black and Peterson (1988), who showed through experimental design that large suspension-feeders (Katelysia spp.) did not alter density and biodiversity of smaller macrofaunal invertebrates. These authors postulated that competition is largely ineffective in structuring communities of benthic infauna in soft substrata. In conclusion, our results show that under the conditions of our experiments, C. fornicata had no major influence in the local density or diversity of smaller coexisting macroinvertebrates (on spatial scales of 0.25 m 2 ) and did not affect the growth of 18-month-old oysters.

Acknowledgements This study was partly supported by Agence de l’Eau and Syndicat Intercommunal du Bassin d’Arcachon. Thanks are due to Mr J. Desbots who kindly gave us the opportunity to investigate private oyster parks. Professor P. Chardy helped us in data analysis and two anonymous Referees improved the manuscript. We are grateful to IFREMERArcachon for supplying temperature, salinity and phytoplankton data and to Dr D. Welsh who (once more!) corrected the English.

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