Solar powered seaslugs (Opisthobranchia, Gastropoda, Mollusca): Incorporation of photosynthetic units: A key character enhancing radiation?

Abstract

Incorporation of photosynthetic units has been investigated under the assumption of them representing a key character that enhanced speciation and led to an adaptive radiation. Two independent opisthobranch systems were chosen to test this hypothesis: the nudibranch genus Phyllodesmium living in mutualistic symbiotic relationship with the dinoflagellate Symbiodinium, and the taxon Sacoglossa (herbivorous seaslugs) sequestering and partly incorporating healthy chloroplasts from chlorophytes. Photosynthetic activity of various members of both taxa was measured under starving conditions and efficiency of incorporation studied by analysing yield values of photosynthesis. For both systems, certain trends were observed: clades exhibiting a functional incorporation of photosynthetic units (i.e., these units were not digested after sequestration) are about five times more species rich than their sister taxon. When comparing the species, observed efficiency differed. Furthermore, morphological adaptations to higher efficiencies could be identified in long-term retention forms (e.g., Phyllodesmium longicirrum with a highly branched digestive gland in the dorsal appendages, and sacoglossan Plakobranchus ocellatus with special ridges on the dorsal side, housing most of the incorporated chloroplasts). Nevertheless, other factors driving evolution of these clades cannot be excluded; they might even be linked with the investigated key character. These are, e.g., food switch and/or incorporation of secondary metabolites for defense.