Microhabitat differentiation and trophic specialisation in coastal deposit-feeding sea cucumbers

The coexistence of deposit-feeding sea cucumber species with similar ecological and functional roles is common in many coastal ecosystems and may lead to niche overlap. This study investigates the behaviour and feeding ecology of four cohabiting coastal sea cucumbers: Holothuria mammata, H. polii, H. sanctori and H. tubulosa, focusing on the mechanisms that allow their niche segregation in the Mediterranean and Atlantic. Laboratory observations were conducted to examine substrate and microhabitat preferences, circadian activity and sediment intake dynamics. Findings revealed significant interspecific segregation. Holothuria sanctori was associated with hard substrates, while the other species preferred soft substrates, differing further in sediment layer use: H. tubulosa and H. mammata foraged superficially, whereas H. polii also utilised sub-surface layers. Species also differed in their sheltering strategies: H. mammata and H. sanctori sheltered in hard substrate refuges, while H. polii burrowed into soft sediments. Finally, sea cucumbers exhibited distinct feeding strategies, revealing two contrasting patterns: high sediment intake with low organic matter selectivity (H. polii), or high organic matter selectivity with lower sediment ingestion (H. sanctori), with intermediate behaviours (H. tubulosa and H. mammata). This study evidenced that both microhabitat and resource partitioning enable the coexistence of sea cucumber species in coastal ecosystems.