Adaptive advantages of the "symbiosis" between algal chloroplasts and Sacoglossan molluscs

All available information is reviewed on the characteristics and the way the "symbiosis" between algal chromoplasts and sacoglossan molluscs works, using the existing bibliography and the authors' own studies. Emphasis is placed on the importance of the food subsidy received by the mollusc, which is one of the various benefits derived from the association, and on the possible benefit obtained from this consortium by the algal species involved and not by the retained chloroplasts, which are, after all, non-autonomic cell organelles. After making various theoretical considerations, and on the basis of the population dynamics of some pairs of alga-sacoglossan species, especially an Acetabularia acetabulum and Elysia timida population thoroughly studied by the authors, the following conclusions are drawn: a) The establishment of a symbiotic relationship between sacoglossans and chromoplasts and the different levels of mutual adaptation and efficiency reached, are a direct function of the difficulty (seasonal or otherwise) experienced by the mollusc in obtaining food. b) Depending on the moment of the biological cycle in which the animal receives it, the energy subsidy provided by the chloroplasts to the host mollusc acts either as a dietary complement, a dietary supplement or a partiaJ substitute of the normalIy obtained algal food. c) This subsidy is invested, successively and/or altenatively, in the mollusc's growth (Pg), in the shifting of its reproductive pattem from a lecithotrophic larval development to a direct one (Pr), or in increased exploratory and foraging activity (∆R). All these entries represent a benefit for the mollusc, in allowing it to be more active, live longer, reproduce more efficiently and/or explore a greater area of the sea bed. d) When the subsidy involves an energy complement for the sacoglossan species, it is plausible that the moIlusc reduces its grazing rate on the alga species, in the same proportion at which it receives the subsidy. In this case the alga species also benefits, which would aHow the relation between the alga and the sacoglossan to be classified as mutualistic. In all the other cases the benefit is exclusively for the mollusc.