Introduction
Integrated multi-trophic aquaculture can improve the sustainability of marine farming by converting dissolved and particulate wastes from fed species into new biomass through extractive and low-trophic organisms (Chopin et al., 2012). In this framework, sea cucumbers are highly promising candidates because of their deposit-feeding behaviour and ability to process organic matter accumulated in sediments (Zamora et al., 2018). Among Mediterranean species, Holothuria tubulosa is particularly relevant for aquaculture diversification and bioremediation-oriented systems (Grosso et al., 2021). However, its successful integration into circular aquaculture requires suitable feeds that mimic natural detrital resources while enhancing the nutritional role of associated microbial communities. Biofloc-based diets offer a valuable approach because they rely on heterotrophic microbial biomass growing on organic carbon sources and nitrogen inputs (Avnimelech, 2015). In integrated systems, nitrogen-rich aquaculture waste can therefore be reused to upgrade low-value agricultural by-products into nutritionally enriched feeds. The carbon-to-nitrogen ratio is a key factor in this process, as it regulates microbial growth, nutrient immobilization and the final quality of the biofloc available to deposit feeders. This study tested the growth performance of H. tubulosa fed biofloc-based diets produced from agricultural by-products and enriched with nitrogen derived from aquaculture waste. Three plant-based diets were compared with a seagrass-detritus control to assess whether carbon-to-nitrogen optimization and microbial conditioning can support sustainable sea cucumber production within integrated multi-trophic aquaculture systems.
Materials and Methods
Specimens of Holothuria tubulosa were collected by scuba diving from infralittoral habitats from the central Tyrrhenian Sea and transported to the Laboratory of Experimental Ecology and Aquaculture, University of Rome Tro Vergata in aerated seawater under controlled temperature. After acclimation in quarantine tanks, specimens were fasted for 15 days to allow gut evacuation, then weighed and selected for the feeding trial. The experiment was carried out in independent recirculating aquaculture tanks adapted for sea cucumber rearing, each containing a fine silica sand substrate suitable for deposit-feeding activity and bacterial colonization. Four biofloc-based diets were tested: seagrass detritus as natural control, corn stalks with nitrogen addition, wheat husks with nitrogen addition, and corn stalks without nitrogen addition. Agricultural by-products were dried, ground, sieved and matured in aerated bioreactors with probiotic supplementation. Nitrogen, derived from aquaculture waste, was added to selected diets to optimize the carbon-to-nitrogen ratio and promote heterotrophic microbial development. Sea cucumbers were stocked at 500 g m-2 and fed twice weekly. Water quality was monitored regularly, while survival and somatic growth were assessed monthly through drained weight measurements.
Results
The second feeding trial showed that all upcycled plant-based biofloc diets were suitable for Holothuria tubulosa, with 100% survival and positive somatic growth under all conditions. However, growth differed significantly among diets. Sea cucumbers fed Posidonia oceanica detritus showed the highest performance, with mean growth rates above 35%. Wheat husks with nitrogen addition supported similar growth, around 27%, and did not differ significantly from the natural control. Corn stalks with nitrogen addition also promoted high growth, around 25%, while corn stalks without nitrogen addition showed the lowest growth, around 19%, confirming the importance of nitrogen enrichment and carbon-to-nitrogen optimization.
Discussion
The second trial showed that Holothuria tubulosa can efficiently exploit sustainable plant-based diets derived from agricultural by-products, confirming its trophic plasticity and suitability for low-trophic aquaculture. All diets supported positive somatic growth, but clear differences emerged according to substrate type and nitrogen availability. The highest growth was obtained with Posidonia oceanica detritus, confirming the ecological relevance of seagrass debris as a natural food source for Mediterranean sea cucumbers and providing a suitable reference diet (Boncagni et al., 2019). Notably, wheat husks and corn stalks enriched with nitrogen produced growth performances close to the seagrass control, despite their lignocellulosic and intrinsically low-protein composition. This suggests that microbial conditioning in biofloc systems can upgrade low-value agricultural residues into nutritionally effective feeds. The poorer performance of corn stalks without nitrogen addition confirmed that nitrogen availability and carbon-to-nitrogen optimization are key factors for biofloc development. A balanced carbon-to-nitrogen ratio likely stimulated heterotrophic microbial growth, improving substrate digestibility and generating protein-rich microbial biomass available to deposit feeders. Overall, these findings indicate that nitrogen-enriched agricultural by-products can support sustainable H. tubulosa aquaculture, while future metagenomic and transcriptomic analyses may clarify the microbial mechanisms driving nutrient upgrading.
References
Chopin, T., Cooper, J.A., Reid, G., Cross, S., & Moore, C. (2012). Open-water integrated multi-trophic aquaculture: environmental biomitigation and economic diversification of fed aquaculture by extractive aquaculture. Reviews in Aquaculture, 4(4), 209–220. https://doi.org/10.1111/j.1753-5131.2012.01074.x
Avnimelech, Y. (2015). Biofloc Technology: A Practical Guide Book — 3rd edition. The World Aquaculture Society, Baton Rouge, Louisiana.
Boncagni, P., Chemello, G., Stabili, L., & Piraino, S. (2019). Preferential assimilation of seagrass detritus by two coexisting Mediterranean sea cucumbers: Holothuria polii and Holothuria tubulosa. Estuarine, Coastal and Shelf Science, 231, 106464. https://doi.org/10.1016/j.ecss.2019.106464
Zamora, L.N., Yuan, X., Carton, A.G., & Slater, M.J. (2018). Role of deposit-feeding sea cucumbers in integrated multi-trophic aquaculture: progress, problems, potential and future challenges. Reviews in Aquaculture, 10(1), 57–74. https://doi.org/10.1111/raq.12147
Grosso, L., Rakaj, A., Fianchini, A., Morroni, L., Cataudella, S., & Scardi, M. (2021). Integrated Multi-Trophic Aquaculture (IMTA) system combining the sea urchin Paracentrotus lividus, as primary species, and the sea cucumber Holothuria tubulosa as extractive species. Aquaculture, 534, 736268. https://doi.org/10.1016/j.aquaculture.2020.736268