Feeding ecology of the obligate urchin symbiont Dactylopleustes yoshimurai (Crustacea: Amphipoda: Pleustidae) revealed by DNA metabarcoding analysis.

Abstract Symbiosis includes commensalism, mutualism, and parasitism, however in many symbiotic relationships, it is difficult to determine which type they are in. Since trophic-linkage is a necessary factor to determine this, data on feeding ecology of symbionts is essential to understand symbiont-host relationships. Amphipods of the genus Dactylopleustes are known as obligate urchin symbionts. Though their ecology remains largely unknown, Dactylopleustes was recently reported to aggregate on diseased hosts, suggesting that Dactylopleustes feeds on diseased urchins’ tissues and uses urchins as both a habitat and prey. We investigated by DNA metabarcoding analyses, the feeding ecology of Dactylopleustes yoshimurai in relation to growth and whether host is disease or not. Contrary to our hypothesis, sequence reads from the gut contents were dominated by planktonic copepods regardless of body size or host disease. These results suggest that they mainly feed on copepod fecal pellets deposited on sediments, and do not have a strong trophic-linkage with their host. Large individuals on diseased urchins feed more on urchins than those on healthy urchins. However, their main prey still remains copepods, implying that host disease has a limited effect on the feeding behavior. In conclusion, our study supports this species is not a strong parasite but more of a commensal of urchins.