Aggregation behavior of suspension-feeding sea cucumbers in a tropical lagoon system of Rodrigues Island, Western Indian Ocean

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Abstract. Gerzer F, Kaullysing D, Ramah S, Bhagooli R. 2025. Aggregation behavior of suspension-feeding sea cucumbers in a tropical lagoon system of Rodrigues Island, Western Indian Ocean. Indo Pac J Ocean Life 9: 124-132. Suspension-feeding holothurians exhibit unique behavioral and ecological adaptations when compared to deposit-feeding sea cucumbers. Their population drivers, habitat preferences, and niche partitioning, however, remain poorly understood. In this study, we document a high-density aggregation of Holothuria cinerascens (Brandt, 1835) (16.9 ± 3.11 ind/m²), which was substantially higher than the coexisting deposit-feeders' density (Holothuria atra Jaeger, 1833: 0.8 ± 0.9 ind/m²; Stichopus horrens Selenka, 1867: 0.3 ± 0.5 ind/m²). The study site in Torrent, located on Rodrigues Island's east coast, is characterized by sandy flats and occasional limestone structures that clearly indicate habitat partitioning. The limestone structure along the transect showed 100% niche occupation by H. cinerascens, and the sandy flat had a composition of 72.7% H. atra and 27.3% S. horrens. The limestone's structural complexity provides a variety of crevices, access to water flow, predatory refuge, and attachment opportunities, ultimately creating a suitable habitat for suspension-feeding holothurians. Additionally, trade wind-generated hydrodynamics on Rodrigues Island likely enhance suspended nutrient flux on the east coast, favoring suspension-feeding plasticity of the sometimes trophically facultative H. cinerascens. These findings give insights into holothurian niche specialization, driven by microhabitat structure, hydrodynamic conditions, and trophic plasticity, delivering possible implications on conservation and mariculture strategies, as well as enhancing the overall understanding of holothurian ecology.

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