Aspects of the ecology of the digenetic trematode Proctoeces subtenuis (Linton)

Abstract

Proctoeces subtenuis (Linton) a digenetic trematode that had previously only been found as a parasite in the hind gut of sparid and labrid fish, was first described as a parasite of the lamellibranch Scrobioularia (da Costa), in the Thaws Estuary, by Freeman and Llewellyn (1958). During the present investigation Proctoeces was only found within S. glana collected from localities along the north coast of the Thames Estuary, although the lamellibranch host was found to be common in neighbouring areas. An investigation of the S. Plana from eight locations along the north coast revealed that the abundance of the parasite was far from uniform; the S. plena collected from certain localities being heavily infected whereas those collected from localities a short distance away (1 mile or less) were often only rarely infected. The pattern was repeated in each of the three years of study. The investigation of a heavily infected population of E. Plana over the period of study demonstrated that the parasite was very successful. From a level of infection of 2-3 Proctoeces per host, in 1967 an increase occurred to a level of infection in 1969/70 at which over 95% of all S. plana collected were infected and with with an average of 4-5 Proctoeces per host. As many as 14 Proctoeces were recovered from a single host and the number of Proctoeces per Scrobicularia increased pari passu with the size of the host. The discovery that the pattern of distribution of Mytilus edulis (L) was very similar to that of Proctoeces suggested that this lamellibranch could be involvod in the life cycle of Proctoeces in the Thames Estuary. This was further suggested by the discovery of sporocysts from M. edulis, collected from two areas where Proctoeces was common that closely resembled those described for members of the genus by American Workers. On the basis of the evidence a life cycle has been suggested and some general topics considered. The availability of large numbers of Proctoeces allowed for an investigation of some aspects of the physiology of this animal. In order to do this satisfactorily the environment of Proctoeces, the kidney of S. Plana, was also investigated. Proctoeces and the kidney fluid of S. Plana were found to be isosmotic with respect to the external medium from a depression of freezing-point of about 0.5°C to 2.0°C. Proctoeces was found to be able to survive for significant periods in sea water dilutions of between 20% and 100: the greatest duration of survival being in 30% and 50% sea water. The significance of this latter observation is uncertain although it has been suggested that this osmotic concentration could be similar to that oocurring in the hind gut of a fish. The oxygen tensions within the kidney of S. plana have been shown to fall, during a period of emersion, to levels approaching zero, but no evidence has been obtained that the haemoglobin of Proctoeces would dissociate its oxygen under such conditions. Observations of the behaviour of the pigment in vivo and its possible functions have been discussed.