Holothuria tubulosa Gmelin 1791 (Holothuroidea, Echinodermata) enhances organic matter recycling in Posidonia oceanica meadows

The role of Holothuria tubulosa in organic matter recycling of Posidonia oceanica detritus was estimated in a laboratory feeding experiment. The individual mean ingestion rate of seagrass detritus by holothurians was evaluated and carbon and nitrogen stable isotopes, carbon and nitrogen content and C:N ratio analyzed in H. tubulosa, P. oceanica detritus and surface sediment. Results showed that H. tubulosa is capable of ingesting from 30% to 100% of the detritus produced by P. oceanica meadows, depending on the holothurian density. Stable isotope analysis provided evidence for the assimilation of seagrass detritus by H. tubulosa as their signatures were compatible with 1 trophic level-step, and for the accumulation of organic material of seagrass origin into sediments, being their δ13C closer to that of P. oceanica only in aquaria with holothurians. As a result, holothurians affected the transfer of seagrass-derived organic matter to surface sediments, enhancing the sediment uptake of organic carbon and total nitrogen by up to 30 × and 3 ×, respectively. Quantitative information on the role of H. tubulosa in seagrass meadows highlights the importance of this species in accelerating the degradation of seagrass detritus, affecting its recycling and availability for seagrass benthic communities.