Introduction
Integrated multi-trophic aquaculture represents one of the most promising approaches to improve the sustainability of marine farming by coupling fed and extractive species within the same production system. In this framework, organic by-products generated by one species can be reused as a trophic resource by another, reducing waste outputs while producing additional biomass of commercial value (Ridler et al., 2007). Low-trophic marine invertebrates, such as sea urchins and sea cucumbers, are particularly suitable for this approach because of their complementary feeding ecology and high market value (Grosso et al., 2021). Paracentrotus lividus is a valuable echinoid mainly fed on macroalgal resources, whereas Holothuria tubulosa is a deposit-feeder able to ingest and assimilate organic biodeposits, potentially acting as an extractive species in benthic integrated systems. However, the development of echinoderm co-culture still requires sustainable diets able to support growth while maintaining the circularity of the system. In this context, circular aquafeed ingredients such as probiotics and single-cell proteins may represent promising tools to improve nutrient utilization and reduce dependence on conventional marine-derived protein sources (Eroldo��an et al 2023). Therefore, this study aimed to investigate, for the first time, the growth performance and survival of P. lividus and H. tubulosa in co-culture using sustainable dietary supplements. Three macroalgal-based diets were tested as the sole food source to sustain both species rearing: a control diet (macroalgae), a probiotic-enriched diet and a diet enriched with methanotrophic bacterial single-cell protein.
Materials and Methods
Sub-adults of Paracentrotus lividus (N = 90) were collected from the central Tyrrhenian Sea, while sub-adults of Holothuria tubulosa (N = 90) were obtained through artificial reproduction at the Laboratory of Experimental Ecology and Aquaculture, University of Rome Tor Vergata. After acclimation at 20 ± 0.5 °C and a two-week fasting period, specimens were allocated to recirculating aquaculture systems for a three-month feeding trial. Three macroalgal-based diets, composed of Ulva sp. and Laminaria sp. in equal proportions, 1:1 dry weight, were tested: a control diet containing only macroalgae (CTL); a probiotic-enriched diet supplemented with Bacillus subtilis, Lactobacillus casei and Lactobacillus plantarum (PRO); and a single-cell protein-enriched diet supplemented with the methanotroph Methylococcus capsulatus (META). Sea urchins were reared and fed in floating baskets, while sea cucumbers were placed on the sediment-enriched tank bottoms. Sea urchin feces passing through the basket mesh represented the only food source for sea cucumbers. Growth was assessed through somatic growth rate in both species, while the gonado-somatic index was calculated for sea urchins. Absorption efficiency was estimated from the organic matter content of food and feces.
Results
Survival was high throughout the experiment, with only two P. lividus specimens dying during the first days and no mortality recorded for H. tubulosa. All diets supported significant somatic and gonadal growth in sea urchins and positive somatic growth in sea cucumbers. However, the methanotrophic-enriched diet produced the highest overall performance. In P. lividus, META resulted in the highest somatic growth rate and gonado-somatic index, reaching 0.61%, and 13.76%, respectively. Sea cucumbers also showed the highest growth under META, followed by PRO and CTL, with final biomass increases of 50.17%, 41.90% and 33.80%. Absorption efficiency was high in both species, especially under META, where sea urchins absorbed 58% of dietary organic matter and sea cucumbers absorbed 60% of sea urchin biodeposits. Overall, the META diet reduced residual organic waste to 17%, compared with 30% in PRO and 36% in CTL.
Discussion
This study demonstrates for the first time that sustainable dietary supplements, particularly methanotrophic bacterial single-cell protein, can support echinoderm integrated multi-trophic aquaculture. The high survival of both Paracentrotus lividus and Holothuria tubulosa confirmed the compatibility of these species in co-culture and the suitability of the tested diets. The single-cell protein diet was the most effective treatment, significantly improving sea urchin somatic growth, gonadal development and absorption efficiency, with performances comparable to those reported for fishmeal-enriched diets (Fernandez and Boudouresque 2000). It also enhanced sea cucumber growth, producing higher somatic growth rates than previous integrated aquaculture trials (Tolon et al., 2017; Grosso et al., 2021). These results suggest that methanotrophic bacterial meal can represent a sustainable alternative to fishmeal protein in low-trophic aquaculture. The probiotic-enriched diet had species-specific effects. It did not significantly improve sea urchin growth compared with the control, suggesting that the selected strains and dose were not effective for P. lividus. Conversely, it improved sea cucumber growth and absorption efficiency, supporting the key role of bacteria in deposit-feeder nutrition. Overall, both supplemented diets enhanced the circularity of the system, but the single-cell protein diet provided the best production and bioremediation outcomes. Under this treatment, only 17% of the ingested organic matter remained as final waste, highlighting the potential of this co-culture system for circular and regenerative echinoderm aquaculture.
References
Fernandez, C., & Boudouresque, C. F. (2000). Nutrition of the sea urchin Paracentrotus lividus (Echinodermata: Echinoidea) fed different artificial food. Marine Ecology Progress Series, 204, 131-141.
Tolon, M. T., Emiroglu, D., Gunay, D., & Ozgul, A. (2017). Sea cucumber (Holothuria tubulosa Gmelin, 1790) culture under marine fish net cages for potential use in integrated multi-trophic aquaculture (IMTA). Indian J. Geo-Mar. Sci, 46, 749-756.
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.
Eroldo��an, O. T., Glencross, B., Novoveska, L., Gaud��ncio, S. P., Rinkevich, B., Varese, G. C., ... & Rotter, A. (2023). From the sea to aquafeed: A perspective overview. Reviews in aquaculture, 15(3), 1028-1057.
Ridler, N., Wowchuk, M., Robinson, B., Barrington, K., Chopin, T., Robinson, S., ... & Boyne-Travis, S. (2007). Integrated multi��� trophic aquaculture (IMTA): a potential strategic choice for farmers. Aquaculture Economics & Management, 11(1), 99-110.