Preferential assimilation of seagrass detritus by two coexisting Mediterranean sea cucumbers: Holothuria polii and Holothuria tubulosa

Holothuria polii and Holothuria tubulosa are two of the most commercially exploited sea cucumbers of the Mediterranean Sea. As deposit-feeders, they represent an important component of the benthic community. Hence, knowledge of their feeding behaviour is crucial for understanding their function in terms of benthic ecology and sediment dynamics. Using information obtained from stable isotope analysis, the food selectivity/assimilation, temporal variations in diet and trophic niche of H. polii and H. tubulosa were investigated. Analysis of carbon and nitrogen isotopic ratios in body wall tissue showed a preferential assimilation of seagrass detritus among multiple food sources, with a total contribution to the diet ranging from 63% to 74%. The temporal changes in δ13C and δ1⁵N values followed the seagrass isotopic dynamics with depleted values in winter and enriched values in summer. Additionally, δ13C and δ1⁵N measurements were used to outline the species' trophic niche width. The two species coexist, showing an overlap on the δ13C axis and a partial degree of niche segregation on the δ1⁵N axis. Our results provide an overview on the feeding activity of H. polii and H. tubulosa, providing evidence of their crucial role on the seagrass detrital pathways for Mediterranean coastal ecosystems.