Symbiosis between the Zooxanthella Symbiodinium (= Gymnodinium) microadriaticum (Freudenthal) and Four Species of Nudibranchs

The dinoflagellate Symbiodinium (= Gymnodinum) microadriaticum (Freudenthal) occurs in a symbiotic association with the nudibranchs Melibe pilosa, an undescribed Melibe sp., Pteraeolidea ianthina, and Berghia major. The algal symbionts reside in host-derived "carrier" cells associated with the host's digestive gland. Longer survival of starved M. pilosa, P. ianthina, and B. major in constant light than in constant dark indicates that photosynthetically fixed carbon is translocated from symbiont to host. Large lipid deposits, present in the same animal cells that contain zooxanthellae in Melibe sp. and P. ianthina, suggest that lipid or lipid precursors may comprise part of the translocated nutrients in these species. A large proportion of the fecal material in Melibe sp., P. ianthina, and B. major is composed of degenerate algal cells. It is possible that these species obtain part or all of their translocated nutrients by digestion of some of their algal symbionts. An organ that appears to function as the site of zooxanthella digestion is present in the cerata of P. ianthina. Zygotes and larvae of all four nudibranch species are devoid of symbionts, thus each new host generation must be re-infected with S. microadriaticum. Adults of B. major feed on prey that contain symbiotic zooxanthellae while the other three nudibranch species examined do not. These facts suggest that while the historic inception of the symbiosis in B. major was probably secondary, the symbionts being derived from the prey; in M. pilosa, Melibe sp., and P. ianthina the inception may have been primary, the symbionts being obtained by inadvertant ingestion by the host.