Food Selectivity of <I>Tripneustes Gratilla</I> (L.)

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BULLETIN OF MARINE SCIENCE, 70(3): 927–938, 2002 CORAL REEF PAPER

FOOD SELECTIVITY OF TRIPNEUSTES GRATILLA (L.) (ECHINODERMATA: ECHINOIDEA) IN OLIGOTROPHIC AND NUTRIENT-ENRICHED CORAL REEFS AT LA RÉUNION (INDIAN OCEAN)

Thierry Lison de Loma, Chantal Conand, Mireille Harmelin-Vivien and Enric Ballesteros

ABSTRACT Herbivorous echinoids can control algal populations and structure their composition in coral reef ecosystems. Food selectivity might play an important role in this structuration. The gut contents of a total number of 40 individuals of the edible sea urchin Tripneustes gratilla were analysed at two coral reefs of La Réunion. The sites differed in eutrophication level, one being oligotrophic, the other being nutrient enriched. Available algal food was quantified in each site and food selectivity of T. gratilla in the field was determined by the use of an electivity index. The diet of T. gratilla relied mainly on algae. However, a significant proportion of detritus was found in the digestive tracts of individuals at both sites. A strong food selectivity of T. gratilla was found at both sites. The tough leathery macroalga, Turbinaria ornata, was the most selected species at both sites. Calcareous algae were strongly avoided. Algal species were generally either highly selected, or highly avoided, and rarely consumed in proportion to their availability in the field. Significant differences in food selectivity were found between sites, some species being selected/ avoided at one site but not at the other. These differences might be linked to the nutrient contents of algae at both sites and/or related to different levels of herbivory between sites.

Echinoids are able to feed on almost everything that can be found in the field (cf review in De Ridder and Lawrence, 1982). However, the dominant regular echinoids on coral reefs are essentially herbivorous, eating seagrasses and algae (Lawrence, 1975; De Ridder and Lawrence, 1982). Availability of food types and habitat types might determine sea urchin diet (Dix, 1970; Vadas, 1977; Ayling, 1978), but sea-urchins are also highly selective under some ecological conditions (Lawrence, 1975; Schiel, 1982). There- fore, the nature of food ingested might result from a combination of the two factors: food availability and selectivity (Birkeland, 1989). Very few studies have quantified food selectivity of echinoids in the field, as gut contents only show what the urchin is eating and not what it prefers to eat (Lawrence, 1975). Echinoids have been frequently identified as major structuring organisms on coral reefs, through grazing impact (Ogden and Lobel, 1978; Sammarco, 1982; Steneck, 1988; Car- penter, 1990). Selection patterns of dominant echinoids should have a strong influence on this impact. Food selectivity might therefore play an important role in the control of algal community structures (Cole, 1993). The edible sea urchin Tripneustes gratilla, a common tropical sea urchin belonging to the Toxopneustidae family, is one of the most common herbivorous echinoids on the coral reefs of La Réunion (densities up to 5 ind m−2, Naim et al., 1997). Its diet has been determined in several other locations, by feeding observations in the field, and/or by digestive contents analyses (Herring, 1972; Nojima and Mukai, 1985; Klumpp et al., 1993; Maharavo et al., 1994). This first study at La Réunion, has been based on the

927 928 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 3, 2002 comparison of food availability and food ingested by T. gratilla in the field. The aims were (1) to determine the diet and (2) to quantify effectively the food selectivity of this species at La Réunion. At St. Gilles-La Saline, the most developed reef at La Réunion, algal and coral distri- butions have been related to the nutrient level of reef waters (Cuet et al., 1988; Naim, 1993; Semple, 1997). Two sites of the St. Gilles-La Saline reef complex (Fig. 1) are regularly surveyed by the Laboratory of Marine Ecology of the University of La Réunion. The oligotrophic site (site O) has been characterized as a reference site, and the nutrient enriched site (site E) as a degraded site (Table 1). Food selection by T. gratilla was stud- ied in both sites.

MATERIALS AND METHODS

STUDY SITES.—Located on the west coast of La Réunion Island (55°32'E, 21°07'S) in the western Indian Ocean, the Saint Gilles-La Saline fringing reef complex is composed of two distinct fringing reefs, the Saint-Gilles reef and the La Saline Reef separated by the Hermitage pass (Fig. 1). It is exposed to intense hydrodynamic conditions resulting from south-east tradewinds and south-west dominant swells during austral winter, and from cyclonic swells during austral summer (Gabrié and Montaggioni, 1982). Spatially uneven nutrient-enriched submarine groundwater discharges (SGD) affect the reefs differentially, resulting in the formation of oligotrophic and eutrophic reef sites (Cuet et al., 1988; Naim, 1993). The choice of the two sites on this reef was motivated by previous results on the chemical composition of reef waters and their impacts on the structure and the metabolism of benthic communities (Mioche and Cuet, 1999). Benthic communities and nutrient enrichment patterns in these two sites of the reef complex are well known (Table 1). Toboggan (site O) is oligotrophic, has clear waters and relatively healthy coral communities. Planch’Alizés (site E for enriched) is considered an eutrophic site, as higher levels of nutrients, particularly ni- trates, are brought by SGD. It has more turbid waters and is characterized by macroalgae and cyanophytes assemblages competing with coral colonies (Naim, 1993). Site O and site E are 2 km apart, and are submitted to the same light exposure. Sampling stations are located in the back-reef zone, where the highest densities of T. gratilla are found. The respective area of site O and site E are 16,250 m2 and 31,250 m2. Algal biomasses (in dry weight) are higher at site E, with 66.6 kg (4.1 g m−2) at site O and 231.3 kg (7.4 g m−2) at site E (calculated from Semple, 1997), the difference resulting from both a significant difference in algal biomass density and surface area. The mean sea-urchin densities are 6.8 ind m−2 at site O and 5.0 ind m−2 at site E, both populations being mainly composed of Echinometra mathaei (Conand et al., 1998) followed by T. gratilla (Lison de Loma, 2000). Site O and site E are geomorphologically similar, with sand and scattered coral colonies (Montaggioni and Faure, 1980).

Table 1. Comparison of chemical (mean ± SD) and biological features of site O (oligotrophic) and site E (enriched) (from Chabanet et al. 1995).

Ssites Feeature Back reef zon

SOite O N 3 0.73 ± 0.35 µM

PO4 0.23 ± 0.06 µM s2alinity 35.1 ± 0.12 beenthic communities no fleshy alga

SOite E N 3 3.23 ± 0.99 µM

PO4 0.36 ± 0.18 µM s8alinity 34.6 ± 0.15 beenthic communities high coverage of fleshy alga LISON DE LOMA ET AL: FOOD SELECTIVITY OF TRIPNEUSTES GRATILLA AT LA RÉUNION 929

Figure 1. Sampling sites on Saint Gilles-La Saline fringing reef complex (site O: oligotrophic site, site E: enriched site, RN1: national road)

DATA COLLECTION.—The relative cover of benthic organisms in the back-reef communities of both sites was quantified in February 1998 (austral summer), using a quadrat method (Sala and Ballesteros, 1997). Forty quadrats of 25 cm × 25 cm, divided into 25 subquadrats of 5 cm × 5 cm, were positioned randomly within the back-reef at each site. The number of subquadrats in which a species appeared, was recorded and used as the unit of abundance. Abundances obtained in each quadrat were summed. Maximum abundance of a species was thus 1000 (= 40 × 25), and minimum abundance was 0 when the considered species was absent from all the quadrats. Mean relative abundances (in %) of each species encountered were calculated from this data. To determine feeding habits of T. gratilla, 20 individuals per site were collected during the same period of February 1998, from the entire area from which the subquadrats were taken, and put 930 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 3, 2002 straight on ice. The sea-urchins collected had a mean (± SD) diameter (at ambitus) of 7.5 ± 0.7 cm at site O and 8.1 ± 0.8 cm at site E, which in both cases correspond to adults of at least 1.5 yrs (Dafni, 1992). They were transferred to the laboratory within 30 minutes, and dissected without disrupting the digestive walls. The digestive tract of each individual was removed. Stomachs con- tained large amounts of mucus and digestive secretions, making algal species identifications haz- ardous. Thus, stomach contents were not used for analyses, and the contents of the first festoon of the intestine (Hyman, 1955), where algal material was clearly identifiable, were collected. The food contents were spread in a Petri dish, and algal species composition was determined under a dissecting microscope at 40× with a transmitted light source. At least five 40× microscopic fields were randomly selected for each sample and a visual estimate of percentage cover of each species in each microscopic field was obtained. The relative abundances (in %) of each species were calculated as the mean relative percentage cover of the n (≥5) microscopic fields surveyed. Occasional presence of a species in a sample was given an arbitrary value of 0.1% relative abundance. Finally, the mean relative abundance (± SD) of each algal species in the gut was calculated using the relative abundance of each species in the gut contents of the twenty T. gratilla in each site. DATA ANALYSES.—Correlation between relative abundance of algal species in the field and in the gut contents of T. gratilla was tested using the Spearman rank non parametric analysis, due to the nature of the data. The cumulative relative abundance of all algal species was tested for difference between sites E and O using the Student t-test. All statistical procedures were performed at a 0.05 confidence level. Food selectivity was quantified by comparing the mean cover percentage of each algal species in the field to its mean cover percentage in the food content of T. gratilla. Electivity indices were calculated for each algal species present in the field and/or in the digestive contents of T. gratilla. Algal species were then ranked by their index value. Ivlev’s E electivity index has been frequently used in food selection studies (Lechowicz, 1982). E is calculated for each algal species using the following formula.

Ei = (ri – pi) / (ri + pi) Eq. 1

where ri is the proportion of food i in the diet of the animal, and pi the proportion of food i in the environment (Ivlev, 1961). E varies theorically between +1 and –1, and zero is obtained when random feeding occurs (food consumed in proportion to its availability in the field). However, the statistical properties of this index imply that the expected value of E will not be zero for all values of r and p (Strauss, 1979). Moreover, this index is affected by the relative abundance of food types, not enabling meaningful between-sample comparisons (Lechowicz, 1982). Thus, we chose to use the relativized electivity index E* (Vanderploeg and Scavia, 1979), as it is analogous to Ivlev’s E, but it is based on the number of available food types and does not have the highlighted drawbacks of Ivlev’s index. While the E* index is assymetrical, it is thought to provide the single best, not perfect, electivity index (Lechowicz, 1982). It is calculated for each algal species, using the following formula.

Ei* = (Wi – (1/n) ) / (Wi + (1/n) ) Eq. 2

where Wi = (ri / pi) / (∑i ri / pi) and ri and pi are the same as in (1), and n = number of kinds of food items.

RESULTS

Among the diverse organisms present in the benthic communities of the back-reef, forty algal species were identified (Table 2) in both sites. Seven of these species were found in algal turfs. Sixteen species (40% of the total number of species) represented 87.7% of the total abundance in site E and 75.8% in site O (Fig. 2). LISON DE LOMA ET AL: FOOD SELECTIVITY OF TRIPNEUSTES GRATILLA AT LA RÉUNION 931

Table 2. Algal species and associated benthos present in the benthic communities of the back-reef and in the digestive tract of Tripneustes gratilla,é.on the St. Gilles-La Saline fringing coral reefs (La R union)

Species Benthic Digestive tract communities CHLOROPHYTA Boergesenia forbesii (**Harvey) J. Feldmann Boodlea composita (*Harvey) Brand Caulerpa racemosa (åForssk l) J. Agardh v .lamourouxii (åForssk l*) J. Agard h Cladophora sp. * Cladophora socialis Kützin g * Cladophoropsis cf. herpestica (**Montagne) Howe Dictyosphaeria cavernosa (åBForssk lø) r**gese n Enteromorpha clathrata (*Roth) Greville Neomeris van-bosseae H*owe Acetabularia parvula S**olms-Laubach Cladophoropsis javanica (üK t**zing) P. Silv a Valonia aegagropila C**. Agardh

RHODOPHYTA Amphiroa fragilissima (**Linnaeus) Lamouroux Ceramium sp. **(Turf algae) Champia parvula ()C. Agardh) Harvey **(Turf algae Coelothrix irregularis (ønHarvey) B r**gese Corallophila apiculata ()Yamada) R. Norris **(Turf algae Gelidiella acerosa (åForssk l**) J. Feldmann and Hame l Gelidiopsis intricata ()C. Agardh) Vickers **(Turf algae Gelidium pusillum (**Stackhouse) Le Jolis Gracilaria spinuligera Bør**gese n Herposiphonia s)p. **(Turf algae Hydrolithon onkodes (*Heydrich) Penrose and Woelkerling Hypnea spinella (ügC. Agardh) K t*zin Hypnea valentiae ()Turner) Montagne **(Turf algae Jania adhaerens L**amouroux Laurencia distichophylla J**. Agardh Laurencia flexilis S*etchell Laurencia perforata (*Bory) Montagne Mesophyllum erubescens (**Foslie) Lemoine Peyssonnelia s*p. (calcareous) Peyssonnelia cf. conchicola P*iccone and Grunow Polysiphonia sp. 1 **(Turf algae) Polysiphonia sp. 2 * Stylonema alsidii (Zanardini) Drew * Tolypiocladia calodictyon (üHarvey ex K t**zing) P. Silv a Unidentified Melobesiae *

PHAEOPHYTA Feldmannia indica (Sonder) Womersley and Bailey * Lobophora variegata (**Lamouroux) Womersley ex Oliveira Padina boryana T**hivy Sphacelaria tribuloides M)eneghini **(Turf algae Turbinaria ornata (**Turner) J. Agardh

CYANOPHYTA Blennothrix cantharidosma (ákMontagne) Anagnostidis and Kom r*e Lynbya s**p. Schizothrix calcicola (**C. Agardh) Gomont Symploca hydnoides (ügHarvey) K t*zin

CORALS Acropora s*p. Montipora circumvallata (*Ehrenberg) Pavona s*p. Porites s*p. Porites s*p. 1 (branched) Porites s*p. 2 (massive) Millepora exaesa (å)Forssk l*

SPONGES Cliona inconstans (*Dendy) Unidentified sponge * *: species present in the considered compartment, algal authorities from Silva et al., 1996. 932 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 3, 2002

Figure 2. Mean cover percentage of the main algal species present in the benthic communities of the St. Gilles-La Saline back-reefs (site O: oligotrophic site, site E: eutrophic site; only species with >1% cover are figured).

While the red alga Hypnea valentiae was the most common species and had almost the same relative abundance in both sites, clear differences in algal cover were observed between sites. Algal communities were dominated by turf algae, such as Ceramium sp., Champia parvula, Corallophila apiculata, Gelidiopsis intricata and Herposiphonia sp. in site O, whereas macroalgae such as Gelidiella acerosa, Gelidium pusillum, Jania adhaerens, Lobophora variegata and calcareous red algae, dominated the communities in site E. In the gut contents of T. gratilla, 35 algal species were identified. Twelve of them, (34% of the total number of species) represented respectively 98.5% and 98.7% of the abundance of all species ingested in site E and site O (Fig. 3). A significant proportion of detritus was noted in the gut contents of sea urchins in both sites, with respectively 34.9 ± 22.6% and 43.2 ± 15.7% of relative abundance in site E and in site O. Thus, the real proportion of algae on the total ingested material was 65.1 ± 22.6% in site E and 56.8 ± 15.7% in site O, the difference being not significant (t = 1.33, P = 0.19, df = 38). The proportions of algal species ingested clearly differed between sites. In site O, Hypnea valentiae was dominant in the diet, with 79.0 ± 25.2% of the total abundance of the ingested algal food, followed by J. adhaerens and Padina boryana. In site E, Turbinaria ornata was the most grazed alga, although its relative abundance in the field was very low (< 0.5 %), followed by H. valentiae and G. acerosa. J. adhaerens and L. variegata, although relatively abundant on the reef, were less ingested than the previous species. Neither calcareous algae, nor corals nor sponges, were found in the gut contents of individuals from either sites. Turf algae other than H. valentiae, were little ingested. LISON DE LOMA ET AL: FOOD SELECTIVITY OF TRIPNEUSTES GRATILLA AT LA RÉUNION 933

Figure 3. Mean cover percentage of main algal species found in the digestive tract of Tripneustes gratilla sampled in the St. Gilles-La Saline back-reef (site O: oligotrophic site, site E: eutrophic site; species with >1% cover are figured).

A significant correlation was found between the relative abundance of algal species in the field and in the gut contents of T. gratilla at site E (z = 2.14, P = 0.03) but not at site O (z = 1.61, P = 0.11). A common pattern in the selectivity of T. gratilla appeared in both sites, independently of food availability. Calcareous algae (Melobesiae) were clearly avoided (Table 3). The brown alga L. variegata and algal turf species other than H. valentiae were also poorly ingested compared to their availability in the field. The most selected alga was T. ornata in both sites. Selectivity varied between sites for several algal species. Sphacelaria tribuloides, Ceramium sp., and G. acerosa were lightly avoided in site E, while they were heavily avoided in site O. Other species such as P. boryana, H. valentiae, J. adhaerens, and G. pusilllum were avoided in site E and selected in site O. Most of the identified species were, however, either highly selected or highly avoided, and very few were ingested in proportion to their availability on the reef.

DISCUSSION

This study provides the first data on food composition of T. gratilla on the coral reefs of La Réunion. It also stands as one of the very few works providing quantitative data on food selection by this species. The data presented here reflect only a snapshot summer situation, and more data are needed in winter. However, the summer season is character- 934 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 3, 2002

Table 3. Ranks and values of the relativized electivity index (E*) for Tripneustes gratilla on the fé.ringing coral reefs of St. Gilles-La Saline (La R union)

SEpecies SEite EO* site SOite E* site Turbinaria ornata 1 +100.94 +1.0 Laurencia distichophylla absent 23+0.8 Sphacelaria tribuloides (T) 27−0.3 71−0.7 Padina boryana 30−0.5 43+0.6 Ceramium s)p. (T 45−0.6 194 −0.9 Gelidiella acerosa 58−0.6 93−0.8 Hypnea valentiae (T) 64−0.7 63+0.5 Corallophila apiculata (T) 79−0.7 181 −0.8 Polysiphonia s)p. (T 81−0.8 142 −0.9 Schizothrix calcicola 93−0.8 163 −0.9 Jania adhaerens 140 −0.8 31+0.8 Champia parvula (T) 151 −0.8 106 −1.0 Lobophora variegata 182 −0.8 83−0.8 Gelidium pusillum 193 −0.8 56+0.5 Gelidiopsis intricata (T) 164 −0.9 106 −1.0 Herposiphonia s)p. (T 195 −0.9 105 −1.0 Hydrolithon onkodes (C) 106 −1.0 106 −1.0 Mesophyllum erubescens (C) 106 −1.0 180 −0.8 Peyssonnelia cf. conchicola (C) 106 −1.0 106 −1.0 Unidentified Melobesiae (C) 106 −1.0 106 −1.0 (C): calcareous alga, (T): turf alga

ized by dense macroalgal populations (Naim, 1993; Semple, 1997), and thus remains an important season for the dynamics of coral reefs at La Réunion. The diet of T. gratilla in other coral reef locations seems to be mainly composed of seagrasses, with a preference for the genera Thalassia, Halodule and Halophila (Mortensen, 1943; Herring, 1972; Nojima and Mukai, 1985; Jafari and Mahasneh, 1987; Klumpp et al., 1993; Maharavo et al., 1994). The same affinity for seagrasses was noted in the vicariant Caribbean species T. ventricosus (McPherson, 1965; Moore and McPherson, 1965; Ogden, 1976; Ogden and Lobel, 1978). No extensive seagrass meadows are found in La Réunion and T. gratilla relies essentially on algae. Detritus might also be a significant food re- source, as high percentages of calcium carbonate were mesured in the digestive contents of T. gratilla at La Réunion (Lison de Loma, 2000), but this remains to be confirmed. The availability of a few algal species in the field was proportional to their relative abundance in the digestive tract of T. gratilla, as suggested by the differing results on available food between sites. However, most of the algal species eaten by T. gratilla were either selected or avoided, as shown by E* indices. This contrasts with several previous results (Ogden, 1976; Birkeland, 1989), but corroborates the idea that echinoids are selective animals under conditions of abundant and diverse food (Lawrence, 1975; De Ridder and Lawrence, 1982). Calcareous and turf algae were avoided in both sites in La Réunion. Calcareous algae are known to deter feeding by sea urchins, toughness being the main reason for this deterrence (Pennings and Svedberg, 1993). However, this avoidance seems to be an adult characteristic as juveniles of several species of regular echinoids feed upon calcareous algae, or on the biofilm covering them. LISON DE LOMA ET AL: FOOD SELECTIVITY OF TRIPNEUSTES GRATILLA AT LA RÉUNION 935

The most positively-selected algae was T. ornata, a tough leathery macroalga, which was uncommon in the field but occasionally very abundant in the digestive tract of T. gratilla. This finding contrasts with previous conclusions that this functional group of algae resisted herbivory almost as well as calcified algae (Littler et al., 1983; Steneck, 1994). Aquarium experiments have confirmed the preference of T. gratilla for this species of alga (Lison de Loma, 2000). For some algal species, selection varied between sites. The significant correlation between the relative abundance of algal species in the field and in the gut contents of individuals at site E, but not at site O, indicates that the diet might be more selective at site O. Conclusions issued from available food in benthic communities must be looked at care- fully for several reasons. First, echinoids might not be restricted to food in situ, as they may also feed on captured drift material (Lawrence, 1975). At La Réunion, T. gratilla is invariably covered with fragments of shell, stone and alga. This covering behaviour in sea urchins might favor direct food use from drift material (Régis and Thomassin, 1982). Secondly, the reliability of E* indices is poor when calculated on food types with low levels of availability (Lechowicz, 1982). Thus, E* indices have been considered only for relatively abundant species. However, differential selection observed between sites also concerned abundant algae. Several factors may contribute, at least partially, to the selection observed. Nutrient content has been identified as one of the main factors involved in food preference in echinoids (Emlen, 1973; Lawrence, 1975), and other herbivores (Mattson, 1980; Nicotri, 1980; Choat and Clements, 1998). The food of T. gratilla has been analysed for nutrient concentrations on both sites, and was significantly richer in site E (Lison de Loma, 2000). In that case, and assuming that digestibility of the same species should not differ between sites, site differences in food selection might be linked to unequal nutrient content of the same species between sites. Nutrient enrichment of reef waters, as at La Réunion, has been shown elsewhere to increase both nitrogen content of seagrasses and levels of herbivory on them (McGlathery, 1995). Several other works on algae tend to confirm such effects (Littler et al., 1986; Lapointe et al., 1994). However, the tissue enrichment in nutrient would have to be species-differential, i.e. the nutrient content of some species is more related to the environment than the content of other species. Indeed, had all the species responded in the same way to increased nutrient exposure, no differences between sites would have been found. These hypotheses deserve more study, and further experimentation should be carried out in several reef sites differing in eutrophication levels. Another element potentially entering the game is the level of herbivory. The regulation of algal communities by herbivores has been well documented on coral reefs (review in Steneck, 1988), where sea urchins are significant actors (Benayahu and Loya, 1977; Sammarco, 1982; Carpenter, 1990), particularly in overfished reefs (Hay, 1984; McClanahan and Shafir, 1990). Moreover, herbivory can regulate the morphological plasticity of reef algae (Steneck and Adey, 1976; Hay, 1981) and temperate algae (van Alstyne, 1999). Sea urchins are able to distinguish between growth forms of algae (Cole, 1993). For instance, a morphological shift in the reef alga Padina jamaicensis, from a highly branched turf shape to an erect foliose morphology was observed with a decrease in herbivory intensity (Lewis et al., 1987). At La Réunion, site E is characterized by low densities of sea urchins and high densities are found at the oligotrophic site (Conand et al., 1998), resulting in lower levels of herbivory in site E. This could explain that algal communities were dominated by turfs at site O, whereas they were mainly fleshy algae at 936 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 3, 2002 site E. Higher abundances of herbivory-resistant forms of algae should be found in site E compared to site O. The fact that several algae were more selected by T. gratilla in site E than in site O could reflect a morphological change of those species between sites. This point deserves further investigations.

ACKNOWLEDGMENTS

Our thanks go to T. Roux for help in the field and in the laboratory. This research was supported by a grant from the French Ministry of Education, Technology and Research. Study conducted at the Laboratoire d’Ecologie Marine, Université de La Réunion, 15 av. René Cassin 97715 St-Denis messag Cedex 9, La Réunion.

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DATE SUBMITTED: January 26, 2001. DATE ACCEPTED: August 10, 2001.

ADDRESSES: (T.L.de L., C.C.) Laboratoire d’Ecologie Marine, Université de La Réunion, 15 av. René Cassin 97715 St-Denis messag Cedex 9, La Réunion. (M.H.V.) Centre d’Océanologie de Marseille, CNRS UMR 6540, Station Marine d’Endoume, 13007 Marseille, France. (E.B.) Centre d’Estudis Avançats de Blanes, CSIC, C. Sta. Bàrbara s/n. E-17300 Blanes, Spain. (T.L.de L., CORRESPONDING AUTHOR) E-mail: .