Peculiar digestion patterns of sponge-associated zoochlorellae in the caddisfly Ceraclea fulva

The caddisfly Ceraclea fulva feeds exclusively on the freshwater sponge Eph ydatia fluviatilis. Sponge spicules are accumulated in the insect midgut an d arranged perpendicularly to the longitudinal axis of its gut. The peritro phic membrane of the midgut is so thick that it prevents spicules from dama ging the epithelium during their transit. The digestion process of the endo cellular zoochlorellae, which are vehiculated by the sponge cells, was exam ined by transmission electron microscopy (TEM). Zoochlorellae were seen in the midgut lumen, close to the peritrophic membrane and in the underlying s pace. Discrete algal cells became evident in tight apposition to the brush border of the midgut cells and were enveloped by the microvilli. Digestion progressed to the final transformation of the organism into membrane-delimi ted vacuoles. (C) 2001 Harcourt Publishers Ltd.