Physiological adaptations to entosymbiosis in three species of graffillid rhabdocoels

Abstract

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.

Paravortex panopea n. sp. (Platyhelminthes: Rhabdocoela) on clams from the northern Patagonian coast, Argentina: pathogeny and specificity

“…This high degree of metabolic dependence may be responsible for the high specificity observed in the species of Paravortex. Regarding the turbellarian pathogenicity, no evidence of direct physical damage was observed in our study, agreeing with previous reports on bivalves infected with Paravortex (Jennings & Phillips, 1978;Jennings & Le Flore, 1979;Jennings, 1981). Although it seems to be innocuous to the host, some gravid specimens occupying a great part of the intestine lumen were observed, therefore they may reduce the absorption of the nutrients by obstruction.…”

“…Since the graffilids lack endogenous digestive enzymes, they use their host enzymes to feed on partially digested hosts food, and their food reserves consist mainly on glycogen (as it occurs in Digenea and Cestoda); these facts represent physiological adaptations to a parasite mode of life (Jennings, 1981;Jennings & Phillips, 1978;Jennings & Le Flore, 1979). This high degree of metabolic dependence may be responsible for the high specificity observed in the species of Paravortex.…”

An observation of the turbellarianParavortexsp. in the New Zealand scallopPecten novaezelandiae(Bivalvia: Pectinidae)

“…Given the low intensity of infestation observed, and that not all gaping or dead scallops contained flatworms, we cannot conclude that the flatworms are the cause of the change in scallop behaviour, particularly if the flatworms are only feeding on the scallop's digested food as has been demonstrated for other Paravortex (Jennings & Phillips 1978;Jennings 1981). Some other factor may have caused the change in scallop condition and the presence of the flatworms may be simply opportunistic.…”

“…Paravortex there appears to be no evidence of direct physical damage caused to the host (Jennings & Phillips 1978;Wardle 1980;Fleming et al 1981;Jennings 1981), although the rhabdocoel Graffilla buccinicola has been found to supplement its diet of the host's partially digested food by removing intact cells from the digestive epithelium (Jennings 1981). Consequently, many of these associations have been termed as entosymbiotic.…”

“…The occurrence of turbellarian rhabdocoels in a variety of marine bivalves such as cockles, mussels, and oysters (Atkins 1934;Freeman 1957;Jennings & Phillips 1978;Wardle 1980;Jennings 1981;Bower et al 1994) has been well documented although the effects that these infestations may have on the host are not well understood. The occurrence of Paravortex sp.…”