Annual survey of hydroids (Cnidaria, Hydrozoa) cohabiting in shrimp-crevices on a reef flat of La Réunion (Indian Ocean)
Nicole Gravier-Bonnet, Dominique Mioche

On La Réunion Island reef flats exposed to rough conditions, coral colonies are cut by crevices of decicentimetric size and regular shape inhabited by a crustacean borer. These crevices serve as a habitat for three species of hydroids: a thecate, Nemalecium lighti, and two athecates, Rhizogeton sp. and Sarsia nipponica. Fortnightly for one year, the development of colonies was followed in situ by mapping and countings of hydranths in six crevices and by samples taken to the laboratory. Rearings were also made to observe the reproduction and the diet of the species. On the vertical walls of the crevices, a three-leve1 zonation of the microflora (mainly filamentous red and brown algae and diatoms) was found to dominate in the two upper levels. The hydroids cohabited on the upper levels but Rhizogeton sp. was also present at the deeper level where cryptic microfauna was predominant. The spatial and temporal distribution of the species was dependent on: 1/ their tolerance to abiotic factors, light and water movement (N. lighti and S. nipponica were strictly photophilous and rheophilous, Rhizogeton sp. tended to be sciaphilous); 2/ the dominance of N. lighti, which was linked to the great density of polyps in the colonies and to the uprights which lead to a continuous distribution (belt) around the crevices whereas the stolona1 species Rhizogeton sp. and S. nipponica developed discontinuous clusters of polyps; 3/ the opportunistic behaviour of S. nipponica which is able to occupy free space quickly. Located on a reef flat subject to contrasting environmental conditions the crevices formed a particularly stable shelter in respect to their size, morphology and constitution. This shelter served as a refuge for the small colonies of the three hydroid species which, moreover, met few competitors and predators (either overgrowing or grazing). The destruction of the habitat and consequently of its occupants had only been noted after climatic (long emergence) or biological (shrimp disparition followed by sea urchin grazing) disturbances. The presence of the species was noticed throughout the year, with sometimes a low density of hydranths but without important changes in the development of the colonies according to the low seasonal variations. Sexual reproduction took place in the summer in N. lighti after the release of free-swimming gonophores, al1 year round in Rhizogeton sp. which produced fixed gonophores intermittently, and was never found in S. nipponica which reproduced by frustules and mainly in the winter (medusa production occurred only in the laboratory). The morphology of the species was well adapted to the environment (small size, reptant colonies, flexibility). There was no competition for food since the main prey was diatoms in N. lighti, vagile microfauna (ostracods and nematodes) in Rhizogeton sp. and large polychaetes in S. nipponica. Competition for space was not very obvious between Rhizogeton sp. and S. nipponica; though they regressed in the presence of N. lighti, the ability of S. nipponica to remain on the extreme edge of the crevice and wether mixed with the microflora in the intermediate zone or in the deeper level in Rhizogeton sp., provided other adequate conditions for the cohabitation of the three species in the microhabitat.

Keywords: Cnidaria, Hydroids, Indian Ocean, coral reef, habitat, competition, distribution, reproduction.
Contents of this volume Sci. Mar. 60(1) : 165-181 Back PDF
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