Résumé
In South African kelp forests, juvenile abalone (Haliotis midae) have a symbiotic relationship with adult Cape sea urchin (Parechinus angulosus), often aggregating beneath them, thereby gaining shelter, protection, food, and potentially beneficial microorganisms from their faecal matter. To assess whether sea urchin faeces enhance the performance of hatchery-produced abalone, an 8-week feeding trial was conducted at a commercial abalone farm. The trial included two sets of three tanks, where juvenile abalone were fed a mixed diet of diatoms, Ulva lacinulata, Gracilaria gracilis, and a formulated feed, with one set of tanks supplemented with sea urchin faecal matter and the other serving as a control. Abalone receiving faecal supplementation exhibited greater growth in shell length over the study period, particularly in the first four weeks of the study, which was coupled with an increased gut fullness index. Bacterial microbiome analyses indicated similarity between sea urchin faeces and abalone bacterial communities, with 62 shared bacterial genera. Faeces-supplemented abalone showed higher abundances of the sea urchin symbionts Psychromonas and Propionigenium, which are known for their involvement in fermentation, digestion, fatty acid synthesis, energy and nitrogen metabolism, and antagonism to pathogenic bacteria. These findings demonstrate that dietary supplementation with sea urchin faecal matter benefits juvenile abalone growth, likely through a combination of improved access to nutrients and abalone gut microbiome enrichment. This intra-trophic approach represents a novel integrated multi-trophic aquaculture (IMTA) strategy that can enhance abalone production, promote species diversification, and improve circularity in commercial farms.