Effect of Different Algal Biofilms on the Larval Settlement of the Holothuria tubulosa Sea Cucumber (Gmelin, 1788)

The increasing exploitation of sea cucumbers has driven widespread population declines, highlighting the need to improve knowledge and understanding of the early life history stages of exploited species such as Holothuria tubulosa, one of the most common holothurians along Mediterranean coasts. This study investigated larval settlement success and juvenile early survival of H. tubulosa larvae, considering two algal biofilms as settlement cues: the diatom Amphora sp. and the green alga Ulvella lens. Larvae were reared under controlled hatchery conditions, and, once reaching the doliolaria stage, larvae were individually exposed to biofilm-conditioned substrates vs. a control without biofilm. Settlement dynamics and larval development were monitored over 35 days and analysed using generalised linear mixed models, while the biochemical composition of the biofilms was assessed through protein, carbohydrate, and lipid quantifications. Larvae exposed to algal biofilms successfully settled and metamorphosed, whereas no settlement occurred in the control. U. lens induced the highest settlement success (54%) and supported subsequent juvenile development, while Amphora sp. resulted in lower settlement rates (21%) and higher post-settlement mortality. Although Amphora sp. showed higher protein and carbohydrate content, settlement and survival were enhanced on U. lens, suggesting that biofilm structure and biochemical cues play a primary role in regulating settlement processes. These findings improve the understanding of settlement mechanisms in H. tubulosa and provide valuable insights for hatchery production, conservation strategies, and the sustainable aquaculture of Mediterranean sea cucumbers.