330
MARWE MOLLUSCS AS HOSTS FOR SYMBIOSES
communication) later found an ovigerous female in S. solida and informed Houghton (cited in Houghton, 1963) that he had also found
what is possibly a pre-hard stage of P. pisum in a mussel at Roscoff,
he considers these to be exceptions. Thus it would appear that although
the symbiotic (parasitic) stages of P. pisum are comparable to those of
P. ostreum (see Table XVII), its life cycle differs in that two hosts are
involved, with the second being invaded by the hard stage.
It is of interest to note that Christensen (personal communication)
has reported that the ovigerous female found in Spisula solida was only
about 4 mm in carapace width and was obviously retarded in its growth
but not its development.
Ecology. As the result of field studies in Langstone Harbour, near
Portsmouth, England, and at Conway, Wales, Houghton (1963) has
reported that the larger crabs are found only in Iarger mussels ; furthermore, this crab is found more frequently in the larger mussels which
occur in greater numbers lower along the shore. Houghton has also
reported that “ the percentage of infestation differs from one locality
to another a t the same tidal level and that it increases from the middle
shore to the sublittoral zone for any given size-group.” Since Christensen
(cited in Houghton, 1963) has found that the first crab stage, which is
the invasive stage, is photonegative, it is unlikely that it will invade a,
primary host high up on the shore line or near the water surface. It
thus follows that the hard stage escaping from the primary host is
more likely to enter Mytilus edulis living from the middle shore to the
sublittoral zone where the primary host is found.
Other information. Orton (1920) has observed the feeding habits of
P. pisum in Mytilus edulis through a window cut in one of the host’s
valves. He has reported a feeding habit similar to that observed by
Stauber (1945) in P. ostreum (see Chapter 4, Section 11, A, 6). It should
be noted, however, that Orton did not observe the pea crab to inflict
any injury to Mytilus edulis comparable to that observed by Stauber
in Crassostrea virginica invaded by P. ostreum. Nevertheless, the
presence of P. pisum in mussels is known to cause certain physiological
changes. Specifically, Berner (1952) has noted that there is a partial
or even complete cessation in the production of gametes in those
mussels infected with P. pisum measuring 10 mm or more in carapace
width. Mussels harboring smaller crabs are very seldom affected in
this manner. This change in sex in Mytilus edulis is most probably the
result of starvation as is the case of Ostrea ceccullata infested by Pinnotheres sp. (Awati and Rai, 1931) and not due to some crab-secreted
substance.
MARWE MOLLUSCS AS HOSTS FOR SYMBIOSES
communication) later found an ovigerous female in S. solida and informed Houghton (cited in Houghton, 1963) that he had also found
what is possibly a pre-hard stage of P. pisum in a mussel at Roscoff,
he considers these to be exceptions. Thus it would appear that although
the symbiotic (parasitic) stages of P. pisum are comparable to those of
P. ostreum (see Table XVII), its life cycle differs in that two hosts are
involved, with the second being invaded by the hard stage.
It is of interest to note that Christensen (personal communication)
has reported that the ovigerous female found in Spisula solida was only
about 4 mm in carapace width and was obviously retarded in its growth
but not its development.
Ecology. As the result of field studies in Langstone Harbour, near
Portsmouth, England, and at Conway, Wales, Houghton (1963) has
reported that the larger crabs are found only in Iarger mussels ; furthermore, this crab is found more frequently in the larger mussels which
occur in greater numbers lower along the shore. Houghton has also
reported that “ the percentage of infestation differs from one locality
to another a t the same tidal level and that it increases from the middle
shore to the sublittoral zone for any given size-group.” Since Christensen
(cited in Houghton, 1963) has found that the first crab stage, which is
the invasive stage, is photonegative, it is unlikely that it will invade a,
primary host high up on the shore line or near the water surface. It
thus follows that the hard stage escaping from the primary host is
more likely to enter Mytilus edulis living from the middle shore to the
sublittoral zone where the primary host is found.
Other information. Orton (1920) has observed the feeding habits of
P. pisum in Mytilus edulis through a window cut in one of the host’s
valves. He has reported a feeding habit similar to that observed by
Stauber (1945) in P. ostreum (see Chapter 4, Section 11, A, 6). It should
be noted, however, that Orton did not observe the pea crab to inflict
any injury to Mytilus edulis comparable to that observed by Stauber
in Crassostrea virginica invaded by P. ostreum. Nevertheless, the
presence of P. pisum in mussels is known to cause certain physiological
changes. Specifically, Berner (1952) has noted that there is a partial
or even complete cessation in the production of gametes in those
mussels infected with P. pisum measuring 10 mm or more in carapace
width. Mussels harboring smaller crabs are very seldom affected in
this manner. This change in sex in Mytilus edulis is most probably the
result of starvation as is the case of Ostrea ceccullata infested by Pinnotheres sp. (Awati and Rai, 1931) and not due to some crab-secreted
substance.