|dc.description.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
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