Thelenota

Abstract: The ecology of the tropical Indo-Pacific aspidochirote holothurian Stichopus variegatus was studied from quadrat samplings carried throughout the lagoon and reef systems of New Caledonia. S. variegatus is a common species, characteristic of inner lagoon holothurian community, where its mean population density is around SO individuals-ha-'. Size distribution appears unimodal at a single station, but modes differ between stations, suggesting a downwards migration to deeper water during life. This iteroparous species spawns during the warm season. Average body length at first sexual maturity is 270 mm; the diameter of ripe oocytes is 180 !tm and the absolute fecundity is around lOx 103 oocytes. External tagging is inadequate for study of growth and mortality of this species. Compared with other tropical aspidochirotes of larger and smaller size, S. variegatus is a medium-size species showing intermediate values of the demographic parameters. Holothurians are common and large invertebrates that influence the structure and functioning of coral reef communities (Bakus, 1973; Birkeland, 1989). Among the aspidochirotes, two families, Stichopodidae and Holothuridae, inhabit shallow waters. The Stichopodidae, although particularly common in reef habitats, have been studied mostly in temperate regions, where their diversity is generally low and where a few species, such as Stichopus japonicus and S. californicus, are fished for human consumption (Conand, 1990). In the shallow waters of the tropical Indo-Pacific the diversity is higher. The Stichopodidae are represented by two genera, Thelenota and Stichopus. Thelenota comprises three species, T. ananas, T. anax and the recently described T. rubralineata (Massin and Lane, 1991). Stichopus presents taxonomic difficulties; it comprises approximately 10 species of which five have been recorded from New Caledonia: S. chloronotus, S. horrens, S. noctivagus, S. pseudhorrens, S. variegatus (Cherbonnier, 1980; Cherbonnier and Feral, 1984; Feral and Cherbonnier, 1986; Conand, 1989a). Two other species have been observed and are illustrated in Conand, 1989a, but they are undescribed. S. variegatus is widely distributed in the Indo Pacific (Clark and Rowe, 1971). It has often been described in taxonomic studies, but surprisingly little is known about its biology, apart from feeding rhythm and habits (Yamanouchi, 1939, 1956) and tentacular mechanisms (Roberts, 1979). In the course of the study conducted at the Centre ORSTOM ofNoumea on the ecology and the resources of the lagoon, S. variegatus was one of the commonest holothurians. Results have been obtained on its ecology at the scale of an entire lagoon and reef system, and aspects of its biology are described here for the first time. MATERIAL AND METHODS The distribution, abundance and ecology of S. variegatus were determined by sampling the different biotopes from the entire lagoon of New Caledonia. Seven regions were therefore delimited according to the general topography. West and East lagoons, parallel to the coasts, extend far away into North and South lagoons. Subdivisions are based on coastal morphology (Fig. IA). Based upon reef geomorphology (barrier, islet, fringing reefs) and structural organisation of the reef(outer and inner slopes, outer and inner reef flats, passage), II main reef biotopes are defined. The lagoon floor includes four biotopes, the outer zone (or white sand bottoms), the inner zone (or grey sand bottoms), the bay (or muddy bottoms) which correspond to the three major communities identified by Chardy et al. (1988),