Homage to Ramon Marga/et,· or, Why there is such p/easure in studying nature 243 (J. D. Ros & N. Prat, eds.). 1991 . Oec% gia aquatica, 10: 243-254 STRUCTURE OF 'CNIDARIAN POPULATIONS IN MEDITERRANEAN SUBLITTORAL BENTHIC COMMUNITIES AS A RESUL T OF ADAPTATION TO DIFFERENT ENVIRONMENTAL CONDITIONS 2 JOSEP-MARIA GILIl & ENRIe BALLESTEROs .• 1 Insti t de Ciencies del Mar (CSIC). Passeig Nacional, s/n. 08039 Barcelona. Spain 2 � Centre d'Estudis Avanc;:atsde Blanes (CSIC). Camí de Santa Barbara, s/n. 17300 Blanes. Spain Received: March 1990 SUMMARY The presence and abundance of Cnidarians, extremely cornmon in Mediterranean sublittoral benthic communities, exerts a profound effect on the entire structure of such communities, although the roles of Anthozoans and Hydrozoans differ in accordance with their different biological characteristics. Structural differences in benthic cnidarian populations were observed in three sublittoral cornmunities located within 100 m of each other yet differing considerably in light intensity and water movement. Species-area and Shannon's diversity-area curves were computed for each community from reticulate samples constituted by 18 subsamples 2 of 289 cm . Species-area curves were fitted to semilogarithmic functions, while diversity-area curves were fitted to Michaelis-Menten functions by the method of least squares. Species richness, species distribution, alpha-diversity, and pattem diversity were estimated from the fitted curves. Patch size for Anthozoans was smaller than that for Hydrozoans in all three communities. Diversity was greatest in the cornmunities with the lowest and with the highest light intensity and water flow levels, but species richness was highest in the intermediate community. The most structured community (with the most heterogeneous species distribution pattem) had the largest organisms with the longest life spans. It is suggested that the difference of water flow conditions in these three cornmunities leads to the development of different spatial pattems of individuals and colonies. The size, shape and small-scale distribution of these organisms are probably correlated to their ability of enhancing prey capture under different water flowconditions. KEY WORDS: Cnidarians, benthos, population structure, diversity-area curves, Westem Mediterranean. INTRODUCTION sciaphilic cornmumtIes where algal growth is limited by low light irradiance. In such Cnidarians are extremely abundant in places, diffe rent cornmunities develop in nearly all Mediterranean marine benthic relation to light intensity. Strong changes in cornmumtIes. However, the highest irradiance can occur in short spaces where number of species is found on rocky holes and caves open on rocky walls. Light sublittoral bottoms, where cnidarians make attenuation from outside to inside the caves up a substantial proportion of the biomass involves a gradient in species composition of such cornmunities (TRUE, 1970; GILI & along an horizontal axis, a gradient which ROS, 1985a). Cnidarians constitute one of is enhanced by the decrease in water the dominant group of benthic organisms in hydrodynamism along the same axis 244 GILI & BALLESTEROS (RIEDL, 1966). This gradual change in small colonies or less aggregated species composition has led Mediterranean individuals alter the flow over the benthic biologists to define three different substratum, turbulence and pattern of communities (PÉRES & PICARD, 1964; partic1e transport (ECKMAN, 1983; ROS et al., 1985; GILI & ROS, 1985b) OKAMURA, 1984). Our hypothesis is that named (from outside to inside) the different hydrodinamical regimes in coralligenous community, semidark cave these three benthic communities, partly community and dark cave community. because of resulting differences in food According to previous observations supply, do condition the spatial distribution made in sublittoral communities (RIEDL, of the species, and therefore the population 1966; GILI et al., 1986; ZABALA et aL., structure and patch size. 1989), the water flow is intense and The structure of a community is bidirectional in the coralligenous, moderate determined by the spatial distribution of the and unidirectional at the entrance of the species within it. Studies designed to find a caves and reduced even though constant representative sampling area, based on inside the cave. This decrease in water flow species-area curves, diversity-area curves, intensity involves a parallel zooplankton and spatial variations in biomass, provide a depletion (RIEDL, 1966; M. Zabala, considerable amount of information on personal communication). community structure for ca1culating patch Passive organisms, such as hydroids and size. Thus, structure and minimal area are anthozoans, are particularly dependent on c10sely related and, even though minimal adequate degrees of water movement sampling area may vary according to the (RIEDL, 1971). Many benthic cnidarians criteria applied by different researchers are passive suspension feeders and depend (e.g., BOUDOURESQUE & BELSHER, upon water current to carry suspended food 1979; HAWKINS & HARTNOLL, 1980; across their food-capturing surfaces. The BALLESTEROS, 1986), a series of ability and distribution of benthic numerical parameters can be obtained for cnidarians is enhanced to maximize the comparing the structural complexity of volume of food-Iaden water they encounter communities. The interest in obtaining such per unit time (W AINWRIGHT & KOEHL, parameters is obvious since topics such as 1976). patch-size can be quantified and diffe rent The shape, size and spatial distribution commumt1es can easily be compared of benthic cnidarians affects the water flow (BALLESTEROS, 1986). around them. Individuals or colonies can The object of the present study is to survive in rapid or moderate flow if they examine differences in the structure of three are small, aggregate or if they adjust their benthic cnidarian populations occupying a body shape and size to surrounding objects short but sharp light-intensity and or organisms, (KOEHL, 1984). These water-flow gradient. The consequence of organisms, whose structures affect the such environmental gradients would be that water flow, operate like obstac1es that colonies and individuals would have a decrease fluid velocity and increase the different spatial pattern in each cornrnunity turbulence in their surface layer (VOGEL, which lead to different patch-sizes. 1981; LaBARBERA, 1984) and, therefore, Cnidarians make up a taxocoenosis that can enhance the possibilities of prey retention be quite representative of the community as and catch. Under fast flow conditions, a whole (WEINBERG, 1978; GILI et aL., feeding ability could be enhanced by the 1989a) and for this reason they may be a presence of neighbours with high body size good indicator of the structural pattero (SEBENS, 1984). However, under within each benthic community. moderate flow velocities, populations of CNIDARIAN POPULATION STRUCTURE IN THE MEDITERRAN EAN 245 MA TERIALS AND METHODS denudation (BOUDOURESQUE, 1971), scraping a surface consisting of 3 x 6 Samples were collected off the squares measuring 17 x 17 cm each. So, northeastem coast of the Medes Islands each sample consisted of 18 squares of 289 2 (Costa Brava, Northwestem Mediterranean, cm , making a total surface area gf 5,202 2 UTM 31TEG 15), where transition between cm (Fig. 1). The squares (subsamples) the coralligenous community and the dark were separated in the laboratory, and the cave community occurs in a reduced space hydrozoans and anthozoans were quantified (Fig. 1). in terms of: Three samples were obtained by - total biomass, measured as dry weight 102 cm 51 cm m . ... 5 10 15 Dark - cave 20 25 FIGURE l. Diagrammatic localization and configuration of the three communities studied. The reticulate rectangle abo ve the community diagrams shows the spatial distribution of samples in each community. The width and direction of arrows roughly indicates the intensity and direction of water movement in each community. 246 GILI & BALLESTEROS in grams, after drying each species at semilogarithmic coordinates. 110 Oc for 24 h; 4) The Molinier 20/5 point (NÉDÉLEC, - organic matter, i.e., ash-free dry 1979), corresponding to the point on the weight; ashes were prepared by buming species-area curve at which a 20 % increase samples for 6 h at 500 oC, in order to in surface area yields a 5 % increase in the minimize the importance of accreted number of speciés; this point can be skeletons. regarded as the minimum species patch size Two data matrices were thereby obtained . value (BALLESTEROS, 1986). for the samples, in each of which Xij was 5) Species diversity (A), the asymptotic' the biomass (or organic matter) for species value of diversity when the diversity-area i in subsample j. Species-area and curve is fitted to a Michaelis-Menten diversity-area curves were derived from function. these data matrices for total cnidarians, 6) Pattem diversity (S), the value of anthozoans, and hydrozoans. The surface area corresponding to the Calleja 3 species-area curves were fitted to a 1 . 10- point on the diversity-area curve semilogarithmic function of the form when fitted to a Michaelis-Menten function; this parameter also fumishes the area at y = a In x + b, which diversity practically stabilizes (minimal structural area; NIELL, 1974). where was the number of species and AH these parameters were used as y 2 x was the surface area in cm . The indicators in evaluating the structure of the diversity-area curves were fitted to