FEEDING AND DIGESTION IN THREE ENTOSYMBIOTIC GRAFFILLID RHABDOCOELS FROM BIVALVE AND GASTROPOD MOLLUSCS

¹ Department of Pure and Applied Zoology, University of Leeds, Leeds LS2 9JT, England, U.K.

1. Diet, gut structure, digestive physiology and food reserves have been studied in three entosymbiotic graffillid rhabdocoels.

2. In all three species these aspects of their nutritional physiology are much modified relative to those characteristic of free-living flatworms; they show adaptive features related to the entosymbiotic habit and, in particular, to the feeding and digestive processes of the respective hosts.

3. The three species feed on their hosts' partially digested food and the cellular debris released at the end of the digestive cycle occurring within the hosts' digestive gland. In one species this is extended to include removal of intact cells from the digestive epithelium.

4. The ingested food contains enzymes of host origin which continue to act within the rhabdocoels' alimentary systems and play a dominant role in the digestive physiology. There is concomitant reduction in the amounts of enzymes produced by the symbiotes.

5. Food reserves are of the type and relative amounts characteristic of other entosymbiotic Platyhelminthes and consist mainly of glycogen.

6. Two of the species studied are viviparous and there is evidence that advanced embryos receive nutrients from the parental gut.

7. One species, which migrates from the host's intestine into the digestive gland to feed during the mid-ebb to mid-flood tidal period, possesses an endogenous haemoglobin. It is suggested that this helps overcome oxygen deficiencies, during feeding, by facilitated diffusion. The other two species, which live and feed in conditions of more constant oxygen availability, lack haemoglobin.