9. THE CLASS CRUSTACEA
32 1
seasonal. Few invasions occur before 1 August. It should be noted that
the peak of oyster setting occurs during July in Delaware Bay ; thus
by the time the f i s t crab stage commences to invade in large numbers,
the oyster spat has grown to sufficient size to harbor one or two crabs.
A single young oyster measuring 4.2 mm long may harbor two crabs
while larger ones may harbor up to seven crabs. The crab appears to
prefer spat (76.7 % infestation) over yearling oysters (54.6 yo infestation)
and older ones (21.5% infestation), but the survival rate of crabs is
better in yearlings and older oysters.
Although the first crab stage (invasive stage) of P. ostreum can be
found in oysters all winter, growth and development stop in Delaware
Bay a t the beginning of November when the ambient water temperature
begins to drop below 15°C.
Pathology. During the feeding of P. ostreum in oysters, two types of
gill erosions are secondarily effected. These pathological changes are
discussed in an earlier section (Chapter 4, Section 11, A, 6).
Resulting from a study designed to determine the effect of P.
ostreum on the gross physical condition of Crassostrea virginica, Haven
(1959) has demonstrated that oysters harboring crabs contain less soft
tissues (meat) per unit of shell cavity volume than those without crabs
but the water contents of oysters with and without crabs are not
significantly different.
Another aspect of crab-associated change in oysters has been postulated by Christensen and McDermott (1958). These investigators
believe that P. ostreum may influence the sex ratio among oysters
during their second spawning season if they retain their crab " parasites ''
from the first year. Their postulation is based on the earlier finding by
Awati and Rai (1931) who found that among 794 uninfested Ostrea
cucullata, 41.7% were males, 56.4% were females, and 2.9% werc
hermaphrodites. On the other hand, among eighty-six oysters harboring Pinnotheres s ~ . ,
82.6% were males, 10.4% were females, and 7.0%
were hermaphrodites. Since females can be induced to change sex in
the laboratory by simple starvation, Awati and Rai concluded that
the crab probably interferes sufficiently with food intake of the oyster
so that it produces sperm instead of the more " expensive " eggs. That
the change in sex is due to reduction in food intake and is not chemically
stimulated by some crab-secreted substance appears to be supported
by the findings of Amemiya (1935) and Egami (1953), who have shown
that the experimental removal of a part of the gill in Crassostrea gigas
will cause the number of males to far exceed that of females during
the breeding season, provided that the operation is performed no later
than the previous October.
A.P.B.-6
21
32 1
seasonal. Few invasions occur before 1 August. It should be noted that
the peak of oyster setting occurs during July in Delaware Bay ; thus
by the time the f i s t crab stage commences to invade in large numbers,
the oyster spat has grown to sufficient size to harbor one or two crabs.
A single young oyster measuring 4.2 mm long may harbor two crabs
while larger ones may harbor up to seven crabs. The crab appears to
prefer spat (76.7 % infestation) over yearling oysters (54.6 yo infestation)
and older ones (21.5% infestation), but the survival rate of crabs is
better in yearlings and older oysters.
Although the first crab stage (invasive stage) of P. ostreum can be
found in oysters all winter, growth and development stop in Delaware
Bay a t the beginning of November when the ambient water temperature
begins to drop below 15°C.
Pathology. During the feeding of P. ostreum in oysters, two types of
gill erosions are secondarily effected. These pathological changes are
discussed in an earlier section (Chapter 4, Section 11, A, 6).
Resulting from a study designed to determine the effect of P.
ostreum on the gross physical condition of Crassostrea virginica, Haven
(1959) has demonstrated that oysters harboring crabs contain less soft
tissues (meat) per unit of shell cavity volume than those without crabs
but the water contents of oysters with and without crabs are not
significantly different.
Another aspect of crab-associated change in oysters has been postulated by Christensen and McDermott (1958). These investigators
believe that P. ostreum may influence the sex ratio among oysters
during their second spawning season if they retain their crab " parasites ''
from the first year. Their postulation is based on the earlier finding by
Awati and Rai (1931) who found that among 794 uninfested Ostrea
cucullata, 41.7% were males, 56.4% were females, and 2.9% werc
hermaphrodites. On the other hand, among eighty-six oysters harboring Pinnotheres s ~ . ,
82.6% were males, 10.4% were females, and 7.0%
were hermaphrodites. Since females can be induced to change sex in
the laboratory by simple starvation, Awati and Rai concluded that
the crab probably interferes sufficiently with food intake of the oyster
so that it produces sperm instead of the more " expensive " eggs. That
the change in sex is due to reduction in food intake and is not chemically
stimulated by some crab-secreted substance appears to be supported
by the findings of Amemiya (1935) and Egami (1953), who have shown
that the experimental removal of a part of the gill in Crassostrea gigas
will cause the number of males to far exceed that of females during
the breeding season, provided that the operation is performed no later
than the previous October.
A.P.B.-6
21