Abstract :
[en] Crinoids of the Order Comatulida are renowned for harboring a remarkable diversity of symbiotic organisms within echinoderms,
including polychaetes, myzostomids, gastropods, crustaceans, brittle stars, or fish. Crinoids provide essential services
to their symbionts, such as shelter, access to food resources, mating areas, nesting grounds, and nurseries. Symbionts within
crinoids developed a variety of strategies, including foraging in the arm ambulacral grooves, preying upon other symbionts,
living within galls, or accessing suspended food particles from the water column. In this work, we focused on the Great
Reef of Toliara, where we collected specimens from seven crinoid species. Among the 84 crinoids examined, a total of
285 symbiotic organisms were retrieved. These symbionts were either moving freely on their host or found within cysts.
Stable isotope analyses of carbon and nitrogen for both hosts and symbionts have shown that (a) all crinoids shared a common
trophic niche; (b) a community-based approach indicated that crinoids initiated trophic networks primarily based on
suspended particulate organic matter; (c) non-specific symbionts exhibited consistent dietary preferences regardless of their
host; (d) myzostomids inhabiting cysts were found to feed on their host tissues; and (e) free-moving symbionts displayed
divergent trophic niches linked to their predatory, kleptoparasitic, or filter-feeding behaviors. This research underscores the
role of crinoids, particularly comatulid species, as key components of tropical ecosystems in the Western Indian Ocean,
inhabited by a hidden biodiversity with complex trophic networks. Their intricate morphology accommodates a range of
feeding strategies, supporting a diverse associated fauna.
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