Trophic ecology of crustacean endoparasites Petrarca and Baccalaureus (Ascothoracida, Thecostraca) in scleractinian and zoantharian corals

Abstract

Coral reefs house a great variety of symbiotic associations, including parasitic ones. One of the crucial issues in host-symbiont interactions is the mode of parasite feeding. Most of the parasitism in scleractinian and zoantharian corals is endosymbiotic (endoparasitic). Their trophic interactions are difficult to identify since they only occur in intact associations. Petrarca and Baccaulareus are endosymbiotic crustaceans. In scleractinian corals, Petrarca forms galls in the coral skeleton. Baccaulareus lives inside zoantharians’ gastric cavities. δ13C and δ15N stable isotope analysis (SIA) of Petrarca and its host (holobiont tissue) Turbinaria revealed similar isotopic signatures of the parasite and host (δ13C, -11.77 to -12.69‰; δ15N, 3.66 to 5.52‰), indicating that Petrarca feeds on its host coral. Petrarca has developed mandibles with a row of sharp and dense teeth to cut coral (holobiont) tissue, while maxillules with compound teeth and massive maxillae are involved in further chewing the food. SIA of Baccaulaureus and its host (holobiont) Palythoa revealed that the parasite and its host differ by >12‰ in δ13C, indicating that Baccalaureus does not feed on its host. The analyzed specimens of Baccalaureus revealed 2 groups with different δ15N values; these specimens may obtain food from different trophic levels. The mouth parts of Baccalaureus have reduced mandibles and maxillules with weak armament. Baccalaureus seems to be an opportunist feeder that steals food directly from the host gastric cavity, resulting in significant variations in δ13C and δ15N values.