A comparative study of digestion in a raptorial ciliate and in the facultative carnivorous form of a filter feeding ciliate.

Digestion in the carnivorous form (giant) of the filter feeding ciliateOxytricha bifaria and in the obligatory carnivorous, raptorial feeding ciliateLitonotus lamella were studied and compared at the ultrastructural level. It was found that whenO. bifaria, shifts from the normal, bacterivorous to the gigantic carnivorous form, it modifies its morphology, acquiring new and more effective feeding devices but maintaining unaltered the digestion pattern and mode of food absorption. This takes place through pinocytotic activity at the food vacuolar membrane. The digestive process inLitonotus is far more rapid: within 10 min the vacuolar membrane disappears; in this way the cytoplasm of the prey, not yet completely digested, is mixed with that of the predator and a pinocytotic mechanism does not seem to he necessary for nutrient assimilation. In general, results demonstrate that filter feeders and raptorial ciliates differ not only in their food intake mechanism, but also in the pattern and the timing of their digestive process, even when they ingest similar kinds of food.