4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
117
toxicity, coupled with intense studies of population fluctuations in
nature, must be forthcoming before we can fully understand this
aspect of host-parasite relationship.
The only detailed study to date on the histopathoIogy of a marine
pelecypod caused by larval trematodes is that of Cheng and Burton
(1965a). We have studied the effects of the dendritic sporocysts of
Bucephalus sp. on Crassostrea virginica in Rhode Island. Our findings
are summarized below.
In lightly infected oysters, the sporocyst branches of the parasite
are confined primarily to the intertubular spaces in the digestive gland,
in the space normally occupied by Leydig cells. These sporocysts
appear merely to exert mechanical pressure on the digestive diverticula,
pushing them towards the periphery, against the gonadal tissues. As
the result, many of the diverticula are collapsed and their characteristically T- or X-shaped lumina are no longer apparent. In addition to
invading the interdiverticular spaces, a number of sporocyst branches
have also been observed among the Leydig cells surrounding the gut.
The presence of sporocysts in this region causes the disruption and
destruction of the normally loosely arranged Leydig cells and connective tissue fibers.
In moderately infected oysters, the spaces normally occupied by the
digestive diverticula and surrounding Leydig cells are almost completely
taken over by sporocyst branches. The branches are tightly packed
and what remains of the diverticula are in the form of isolated tubules
and fragments. We have indicated that their appearance suggests that
their destruction is the result of both mechanical and physiological
pressures, comparable to those that have been described by F. G. Rees
(1934) and James (1965) in marine gastropods. Again, as in lightly
infected oysters, sporocysts in moderately infected ones are not confined
to the region of the digestive gland. Branches infiltrate the connective
tissue tunica surrounding the digestive tract also. This tunica is reduced
to a thin compressed layer enveloping the gut, apparently resulting
from physical pressure exerted by sporocysts extending from the area
of the digestive gland. On the external periphery of the digestive gland
a few sporocyst branches have been observed to infiltrate the interlobular spaces of the gonads.
In heavily parasitized oysters the digestive gland is completely
obliterated, and it is of interest to note that in spite of this the oysters
are not killed. Furthermore, the sporocyst branches completely
infiltrate the surrounding gonadal tissues. The parasitized oysters
studied were all in the female reproductive phase, hence the ovaries
were destroyed. The Leydig cells normally occurring between the
117
toxicity, coupled with intense studies of population fluctuations in
nature, must be forthcoming before we can fully understand this
aspect of host-parasite relationship.
The only detailed study to date on the histopathoIogy of a marine
pelecypod caused by larval trematodes is that of Cheng and Burton
(1965a). We have studied the effects of the dendritic sporocysts of
Bucephalus sp. on Crassostrea virginica in Rhode Island. Our findings
are summarized below.
In lightly infected oysters, the sporocyst branches of the parasite
are confined primarily to the intertubular spaces in the digestive gland,
in the space normally occupied by Leydig cells. These sporocysts
appear merely to exert mechanical pressure on the digestive diverticula,
pushing them towards the periphery, against the gonadal tissues. As
the result, many of the diverticula are collapsed and their characteristically T- or X-shaped lumina are no longer apparent. In addition to
invading the interdiverticular spaces, a number of sporocyst branches
have also been observed among the Leydig cells surrounding the gut.
The presence of sporocysts in this region causes the disruption and
destruction of the normally loosely arranged Leydig cells and connective tissue fibers.
In moderately infected oysters, the spaces normally occupied by the
digestive diverticula and surrounding Leydig cells are almost completely
taken over by sporocyst branches. The branches are tightly packed
and what remains of the diverticula are in the form of isolated tubules
and fragments. We have indicated that their appearance suggests that
their destruction is the result of both mechanical and physiological
pressures, comparable to those that have been described by F. G. Rees
(1934) and James (1965) in marine gastropods. Again, as in lightly
infected oysters, sporocysts in moderately infected ones are not confined
to the region of the digestive gland. Branches infiltrate the connective
tissue tunica surrounding the digestive tract also. This tunica is reduced
to a thin compressed layer enveloping the gut, apparently resulting
from physical pressure exerted by sporocysts extending from the area
of the digestive gland. On the external periphery of the digestive gland
a few sporocyst branches have been observed to infiltrate the interlobular spaces of the gonads.
In heavily parasitized oysters the digestive gland is completely
obliterated, and it is of interest to note that in spite of this the oysters
are not killed. Furthermore, the sporocyst branches completely
infiltrate the surrounding gonadal tissues. The parasitized oysters
studied were all in the female reproductive phase, hence the ovaries
were destroyed. The Leydig cells normally occurring between the