Biochemical signatures of Holothuria tubulosa and Holothuria forskali (Echinodermata) in co-culture with sea bass in the Frioul Islands (France)

Considerable knowledge in the reproductive biology, habitat and life-history traits of European sea cucumbers was acquired in recent years with the objective to use them as extractive species in multi-trophic aquaculture systems. Yet, the trophic link between aquaculture wastes and sea cucumbers remains insufficiently evidenced. The aim of this study was to assess the potential of Holothuria tubulosa and H. forskali to utilise fish waste as a food source within a co-culture system. The two species were placed below sea bass cages in the northwest Mediterranean Sea. After one year of caging, fatty acids and stable isotopes indicated that sea cucumbers located near the fish cages were in good physiological condition. Higher levels of the fatty acid 18:2ω6, a biomarker of terrestrial plant oils, in sea cucumber tissues clearly indicate the transfer of fish feed to holothurians. In the case of H. forskali , this signature was also linked to an increase in bacterial biomarkers, indicating that sea cucumbers likely do not feed directly on fish wastes. Feeding habits differed between the two species, with H. tubulosa poorly modifying its diet when placed in captivity but probably requiring a significant access to sediment to maintain growth, while H. forskali probably collecting increased secondary production stimulated by the inputs of fresh organic matter directly on the cage walls. Finally, fatty acid profiles of both the body wall and muscular bands of the sea cucumbers revealed a similar imprinting of fish feed, indicating that each tissue can be analysed individually for trophic purposes. • Both species survived well below sea bass cages after one year of captivity. • Levels of the fatty acid 18:2ω6 indicate the transfer of fish feed to holothurians. • Feeding habits and diet differ between the two species. • Body wall and muscular bands can be analysed individually for trophic purposes.