Reproductive biology and ecology of the sea cucumber Holothuria tubulosa Gmelin, 1788

Sea cucumbers play a key role on the benthic ecosystem functioning and marine biodiversity. They bioturbate the seafloor sediment reducing the organic load, recycling nutrients and redistributing surface sediments assuming the role of keystone species in the bioremediation process in marine ecosystems. Many sea cucumbers have also a remarkable food interest particularly in Asia as traditional food product and object of traditional folk medicine, a food market resulting in an international demand. The increasing of uncontrolled international harvesting led to the overexploitation of many holothurians species, including the Mediterranean sea cucumber Holothuria tubulosa, the species I focused on in the present PhD thesis. The Italian government in 2018, adopting a precautionary approach, declared a moratorium on all fishing of sea cucumbers with the intention to understand the biological characteristics and the available stock, although this latter objective remained only theoretical. The Mediterranean sea cucumber H. tubulosa, although one of the most common sea cucumber species, showed a knowledge gap in many research areas starting from its life history traits, those characteristics that play a crucial role in the species management and assessment. A careful preliminary review allowed me to focus on some important questions to face the gaps in the life history, biology and ecology of H. tubulosa that then resulted in the structuring of the doctoral project. My key questions were: When and How does this species spawn in the central Mediterranean? Which are the environmental cues that lead the spawning? Can the early larval stages be affected by density-dependent factors? Moreover, what are the stimuli related to food availability affecting settlement and metamorphosis of larvae of H. tubulosa? Finally, I dealt with the important issue of the feeding behavior of adults of this deposit feeder species. My last question regarded the ecological role of H. tubulosa and its effect on the sedimentary organic matter, in particular: Can H. tubulosa modify the trophic status of the marine sediment? Can H. tubulosa select the sediment particles during the food ingestion and can it concentrate the protein-rich particles in the digestive tract? I sought to answer all these issues with different ecological approaches, by using experimental manipulations that have been coupled with documentation of natural processes, and field work, that required the development of complex sampling and experimental designs and evaluation in the field of experimental and statistical procedures. Four developments have followed from these research lines. First, H. tubulosa showed a unique reproductive event during the year in the central Mediterranean, corresponding to a relatively short time interval during the end of the Summer season. Second, the most effective trigger for spawning of H. tubulosa was the bubbling, which acted in the increasing of the water turbulence. The third development is that the algal biofilm that allowed the maximum settlement success was the green algae U. lens, also ensuring high survival of the post-settlers, a crucial issue that can help the feasibility of sea cucumbers aquaculture. The fourth development shed light on the trophodinamyc of H. tubulosa, revealing that this sea cucumber can select the protein-rich particles during the feeding action and can modify, locally and temporally, the trophic status of the sediment by producing feces rich in the most labile components of the organic matter.