Mutualistic relationship between the amphipod Stenula nordmanni (Stephensen, 1931) and the nephtheid coral Gersemia rubiformis (Ehrenberg, 1834)

Gersemia rubiformis is a soft coral belonging to the family Nephtheidae, known to provide habitat for many organisms and to play an essential role in benthic ecosystems. The present study examined the relationship between G. rubiformis and the epibiotic amphipod Stenula nordmanni in the coastal waters of eastern Canada. The prevalence of the association was 100% with an average burden of 35 amphipods (mostly juveniles) per coral host. The sex ratio in the amphipods was female-biased and there was a distinct sexual dimorphism, males being significantly larger than females. In a single coral colony, about 38% of female amphipods were found to brood propagules at various stages of development up to juveniles. Upon release, juvenile amphipods were transparent and acquired the pink color characteristic of the adult by feeding on the coral tissues, thereby mimicking the coloration of the host. Ingestion of coral tissues was confirmed by the presence of host sclerites in the digestive tract of amphipods. In turn, amphipods were seen to protect their host by repulsing predatory nudibranchs approaching the coral colony. Together, findings suggest that the amphipods display a specific and obligate mutualistic association with their coral host, considering that they benefit from shelter and food, and the host benefits from active protection against potential predators. The apparent trophic relationship does not appear to generate any detriment to the host. In addition, some of the symbiotic amphipods examined were found to be hosts themselves, to chytrid fungi attached to their exoskeleton.