Gut ultrastructure of the appendicularian Oikopleura dioica (Tunicata)

The appendicularians, planktonic tunicates, possess a specialized, external filtering system that captures food particles <1 mum in size. In this work the alimentary canal of Oikopleura dioica has been studied by serial secti ons of whole animals and ultrastructure. The gut includes a dorsal esophagu s, a bilobed saccular stomach, and a curved intestine, divided into vertica l, mid-, and distal intestine (or rectum). No multicellular glands or cellu lar proliferative centers were found. Three main cell types were recognized , ciliated microvillar cells, globular cells and gastric band cells, with s pecializations reflecting different physiological roles in the various regi ons. Ciliated microvillar cells, the most diffuse, are considered to be inv olved in food propulsion, fecal pellet formation, absorption, and nutrient storage. Pinocytotic features and vacuoles suggest that absorption of macro molecules and intracellular digestion occur in the globular cells of the st omach and rectum. The large gastric band cells of the left lobe have typica l features of intense protein synthesis and probably produce enzymes for ex tracellular digestion. Diffuse interdigitations of many cells enormously in crease the plasmalemma surface and may be involved in liquid/ion exchange. Despite the apparent structural simplicity of' the gut epithelium, O. dioic a efficiently processes food to fulfill the energy requirements of its exce ptionally rapid life-cycle.