Parasitism in coral reefs: Trophic ecology of crustacean ascothoracidan parasites and their coral hosts from Malaysia

Coral reefs house a great variety of symbiotic associations, including parasitism. One of the crucial issues in the host-symbiont interactions is the parasites' feeding mode. Does the parasite/symbiont use the host's tissues for nutrition, steal food from the host's digestive system, or take food directly from the environment? However, most of the parasitism in corals is endosymbiotic (endoparasitic). Their trophic interactions are difficult to identify since they only occur in intact associations. This work uses stable isotope analysis (SIA) of carbon and nitrogen and morphological analysis to study the trophic relationship between the crustacean endoparasites, the Ascothoracida (genera Baccalaureus, Sessilogoga, and Zibrowia) and their various coral hosts ranging from Zoantharia (Palythoa) to Antipatharia (Antipathes), and Scleractinia (Dendrophyllia). The hosts belong to different coral taxa and obtain food from different sources, reflected in their stable isotope values. The SIA, supported by the morphological analysis, suggests that the Zibrowia parasite feeds directly on its Dendrophyllia host. Sessilogoga retains vagility within and around the black coral colony. It has typical generalized piercing mouth parts with numerous teeth and denticles. Sessilogoga may use antipatharian tissues for food directly as well as sucks food fluids from the host's gastrovascular system. There is no clear trophic shift trend between Palythoa and its parasite Baccalaureus. Such differences exclude the possibility of the parasite feeding predominantly on its host's tissues and suggest a broad spectrum of food sources. Thus, SIA reveals that endosymbiotic ascothoracidans may not always be true parasitic but also opportunistic feeders, which steal food directly from the host gastric cavity.