How sediment granulometry affects feeding behaviour in sea cucumbers: a case study on Holothuria sanctori

The sea cucumber Holothuria sanctori is an Atlanto-Mediterranean deposit feeder that contributes to sediment processing in coastal areas. However, the feeding ecology of this species, including its sediment processing capacity, remains largely unexplored under controlled conditions. This study investigates the effect of sediment granulometry on the feeding behaviour of H. sanctori, focusing on organic matter (OM) selectivity and sediment ingestion. Specimens were tested under five experimental treatments, each containing sediments with particle sizes ranging from 0 to 1000 μm, with standardized OM availability across treatments. A natural simulation with a hard, sediment-free substrate was also included. Data on defecation rate and faecal OM content was collected daily to evaluate sediment processing capacity. Results indicate that H. sanctori optimally selects OM (average OM: 4.2%) and processes sediment (average defecation rate: 1.5 g/day per 100 g biomass WW) in fine particle treatments (< 250 μm). Additionally, the species exhibits an extended sediment processing capacity across a wide range of sediment sizes, including particles up to 1000 μm. The study further revealed that the sea cucumber increases sediment ingestion (by one order, on average) to compensate for reduced OM availability (by 0.15 order, on average), maintaining a constant OM intake. The results suggest that sediment granulometry might not be the primary factor in determining the species' microhabitat preferences in the natural environment due to its extended capacity to process different sediment granulometries. These findings enhance our understanding of H. sanctori's feeding behaviour and highlight its potential in Integrated Multi-Trophic Aquaculture applications.