Sea urchin waste as a promising nutritional source for the polychaete Hediste diversicolor (OF Müller, 1776)

In aquaculture, species diversification emerges as an imperative sustainable strategy, essential for mitigating both biological and financial risks. In this context, the cultivation of sea urchins has gained prominence as an emerging trend. However, its commercial viability still faces several challenges, namely limited growth performance. Integrated multi-trophic aquaculture (IMTA) represents a potentially more profitable model by integrating extractive species to mitigate ecological impacts and promote circular economy principles. The use of polychaetes in IMTA has proven effective in recycling waste from fish aquaculture production, however, their potential should be further explored in low-trophic aquaculture solutions. This study evaluated the potential of reared sea urchin biodeposits as a nutritional feed source for the polychaete Hediste diversicolor (OF Müller, 1776). H. diversicolor juveniles were fed three diets, over 50 days: sea urchin waste (SUW), fish waste (FW) and fish feed (FF). Survival, growth, and protein and fatty acid (FA) contents were assessed. Total survival was observed in all treatments except for FW (98%). Polychaetes fed SUW and FF presented similar specific growth rates. The protein level in the polychaetes fed SUW was significantly lower, reflecting the diet's composition. The FA profile of H. diversicolor was also influenced by the respective diets. The absolute content (μg mg −1 DW) and FA profile of polychaetes were significantly different among treatments, reflecting the composition of the diets. Saturated FA (SFA) represented the dominant class in the polychaetes fed with waste diets, while in FF polychaetes polyunsaturated fatty acids (PUFA) were the major group. Nevertheless, polychaetes fed the waste diets demonstrated the PUFA biosynthetic capacity of this species, showing a considerably higher proportion of essential long-chain FA, compared to diets. Overall, these preliminary results suggest, for the first time, the potential for recycling sea urchin biodeposits into a polychaete biomass with high commercial value, presenting a promising opportunity for low-trophic integrated aquaculture models. • Hediste diversicolor was fed on three experimental diets (sea urchin waste, fish waste, and fish feed). • Sea urchin waste and fish feed promoted the same growth rate in polychaetes. • Polychaetes fed on sea urchin waste exhibited a reduced protein content. • Polychaetes fed the waste diets showed a higher proportion of long-chain fatty acids, compared to diets. • This study is the first to test sea urchin waste for polychaetes, suggesting potential for a novel low-trophic IMTA design