Sea urchin and polychaete integration in IMTA as a model for sustainable waste valorisation

Integrated Multi-Trophic Aquaculture (IMTA) is a sustainable aquaculture approach that recycles nutrients by co-culturing species from different trophic levels. This study assessed the biological feasibility of co -culturing the sea urchin Paracentrotus lividus with the polychaete Hediste diversicolor in a novel low-trophic IMTA configuration, using sea urchin faecal waste as a food source. Three experimental sea urchin diets (D1, D2, D3), varying in marine and plant-based ingredients, were tested over 40 days, including a novel fishmeal- and fish-oil-free diet (D1). Sea urchins showed no significant differences in somatic or gonadal growth between diets. In contrast, polychaetes exhibited substantial growth across all treatments, with the highest performance observed in those fed waste from the energy-rich, animal-based diet (D3). No significant dietary effects were detected on polychaete gametogenesis, although a maturation trend aligned with energy availability. Notably, polychaetes consuming waste from the diet free of fishmeal and fish oil (D1) displayed a significantly improved fatty acid profile, particularly in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) content. These results highlight both the influence of feed composition and the capacity of H. diversicolor to enhance the nutritional value of waste through differential assimilation and biosynthesis. By converting waste into high-value biomass, this co-culture model improves sustainability, contributes to reducing reliance on fish-oil inputs, and adds economic value to sea urchin aquaculture. • Sea urchins and polychaetes co-cultured in a novel low-trophic aquaculture design. • Polychaetes grew fastest on waste from the energy-rich, animal-based sea urchin diet. • Polychaete EPA and DHA levels differed among experimental diets. • Sea urchin waste ensured survival, nutrient recycling and fatty acid assimilation. • IMTA model supports sustainable, circular and high-value echinoderm aquaculture.