Behavioural adaptations and diet specificity of sacoglossan opisthobranchs

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Abstract
Sacoglossan opisthobranchs are suctorial feeders and the majority of species feed on the cytoplasm of siphonalean green algae. All shelled sacoglossans feed on the morphologically variable, but anatomically strictly monosiphonaceous algal genus Caulerpa. Thus stenophagy is plesiomorphic within the group. After losing the shell, an adaptive radiation of diets has taken place. Most shellless species are stenophagous, but some, in particular species with temperatt distributions, are relatively euryphagous. A number of behavioural adaptations have been involved in the evolution of the Sacoglossa. Those most important for diet radiation are: food handling methods, such as grasping the algal filament with oral and pedal lobes, and “cutting” the algal cell wall instead of “rasping” through it. Buccal regurgitation, which probably mixes the algal cytoplasm with saliva is also important. The evolution of diet specificity within the Sacoglossa involves morphological as well as physiological and behavioural adaptations. Behavioural responses may change in a regular pattern, and thus be useful for phylogenetic analyses. Conversely, morphology shows some degree of plasticity, changing in response to external stimuli.