Physiological adaptations to entosymbiosis in three species of graffillid rhabdocoels

Three entosymbiotic graffillid rhabdocoels (Paravortex scrobiculariae, P. cardii and Graffilla buccinicola) from marine bivalve and gastropod molluscs show several physiological adaptations to their life-style which are intimately related to the nutritional physiology and ecology of their respective hosts.All three species feed on their hosts' partially digested food plus the cellular debris released at the end of the hosts' own digestive cycle. G. buccinicola supplements this diet by actively removing intact cells from the host's digestive epithelium. Host enzymes, ingested with the food, are utilized for digestion within the flatworms' gut; there is concomitant reduction in the types and amounts of endogenous enzymes and the gastrodermal gland cells characteristically found in free-living species are absent.Food reserves in the three species consist mainly of glycogen, following the pattern seen in other entosymbiotic flatworms (Turbellaria, Digenea, Cestoda); in P. scrobiculariae this primary adaptation, believed to be linked in all entosymbiotes to the ready availability of food and to high fecundity, probably has a secondary function, in relation to anaerobic respiration, of the type found in cestodes.Other adaptive features, closely correlated with host ecology, are the occurrence of a physiologically active haemoglobin in the brain and pharynx of P. scrobiculariae and, in this species and P. cardii, the differential occurrence and distribution of dehydrogenase systems concerned with aerobic respiration (Krebs cycle), the pentose phosphate shunt and anaerobic respiration (glycolysis). P. scrobiculariae and P. cardii are viviparous and the normal provision of yolk for embryonic nutrition is supplemented by direct passage of materials to the later larvae from the parental gut, thus facilitating extended development of the larva until its birth as an immature miniature adult.