REITZEL, A.M.*; SULLIVAN, J.C.; FINNERTY, J.R.; Boston Univ.: Metamorphosis in the Life History of a Parasitic Sea Anemone, Edwardsiella lineata

Larvae of the sea anemone Edwardsiella lineata selectively parasitize the ctenophore Mnemiopsis leidyi. Our observations have shown that a parasite excised from the host will differentiate into a larva that can undergo either of two developmental trajectories: re-infect a ctenophore or settle as a benthic juvenile. From a morphological and an ecological perspective of metamorphosis, we suggest that E. lineata undergoes two metamorphic events, one from larva to parasite and a second from larva to benthic juvenile. We support these as independent metamorphic processes by 1) differences between the larva (planula) and both parasite and juvenile and 2) morphological and ecological differences in parasite and juvenile stages. The planula is a non-feeding, swimming pelagic stage in the life history and is mostly undifferentiated with at most two mesenteries and an oral-aboral axis. Morphological comparisons of parasites and juveniles showed that juveniles have a typical sea anemone morphology (tentacles, mesenteries with muscles, oral opening with pharynx) while the parasitic stage lacks tentacles, muscles and all or some mesenteries. Feeding mode differs where parasites are suspension feeders on predigested food from the ctenophore and juveniles are tentacular feeders on zooplankton via nematocysts. Metamorphosis from larva to parasite is reversible, a feature uncommon in metamorphic taxa. Since larvae have two trajectories that can be manipulated in the laboratory, E. lineata represents an attractive species to study how environmental cues and developmental changes are integrated in metamorphosis. We hypothesize how changes in expression of developmental regulatory genes may underlie the two metamorphic events with comparisons to a closely related anemone, Nematostella vectensis.