9. THE CLASS CRUSTACEA
325
at this stage that hard stage males and females leave their host and
engage in copulatory swimming in open water. After this, females
reinfest mussels and undergo four post-hard molts with each instar
possessing unique characteristics but all with soft and poorly calcified
exoskeletons. Male crabs in the hard stage are said to spend more time
in open water during and after copulatory swimming and hence are
more subject to predation; however, a few males are found in hosts
following swimming. It is of interest to point out that Pearse has
reported that juvenile (hard stage) females which enter hosts already
inhabited by mature females beome retarded in their development
and do not reach the sexually mature stage V instar. All crabs are
believed to over-winter in the first post-hard (stage 11) instar, but with
the advent of higher water temperatures in May the precociously
inseminated crabs undergo the remaining three post-hard molts and
attain the adult form (stage V).
Pathology. Detailed accounts of damage inflicted in molluscs by P.
maculatus are not yet available. It may or may not damage its host's
gills like P. ostreum and P. pisum although it is suspected that it does.
Ecology. The seasonal periodicity manifested in the appearance of
ovigerous females varies through the range. Rathbun (1918) has
indicated that such females occur in January at St. Thomas, Virgin
Islands, in March in Jamaica, from May to November in Florida, from
June to January in North Carolina, from July to September in Rhode
Island and Massachusetts (although Pearce has stated that they only
occur from late May to mid-June at Cape Cod), and in June in Brazil.
Relative to the swimming velocity of P. maculatus larvae, Welsh
(1932) has reported that it is greatly influenced by temperature and
light intensity. At temperatures between 20" and 25"C, the maximum
swimming velocity is attained at light intensities between 10 and 25
meter-candles. When maintained a t different constant temperatures,
it is known that there is a change in the relationship of velocity to
light intensity, with the maximum possible velocity occurring at each
constant temperature during the initial period of exposure to higher
temperatures. Furthermore, the light intensities will bring about a
marked effect on general activity.
3. Pinnotheres poisum Linnaeus, 1767. (Figs. 218-221)
(Order Decapoda; suborder Reptantia; section Brachyura; subsection
Brachygnatha ; superfamily Brachyrhyncha ; family Pinnotheridae)
The pea crab, Pinnotheres pisum, is the common symbiont of
pelecypods in northern European waters. It appears to be most
325
at this stage that hard stage males and females leave their host and
engage in copulatory swimming in open water. After this, females
reinfest mussels and undergo four post-hard molts with each instar
possessing unique characteristics but all with soft and poorly calcified
exoskeletons. Male crabs in the hard stage are said to spend more time
in open water during and after copulatory swimming and hence are
more subject to predation; however, a few males are found in hosts
following swimming. It is of interest to point out that Pearse has
reported that juvenile (hard stage) females which enter hosts already
inhabited by mature females beome retarded in their development
and do not reach the sexually mature stage V instar. All crabs are
believed to over-winter in the first post-hard (stage 11) instar, but with
the advent of higher water temperatures in May the precociously
inseminated crabs undergo the remaining three post-hard molts and
attain the adult form (stage V).
Pathology. Detailed accounts of damage inflicted in molluscs by P.
maculatus are not yet available. It may or may not damage its host's
gills like P. ostreum and P. pisum although it is suspected that it does.
Ecology. The seasonal periodicity manifested in the appearance of
ovigerous females varies through the range. Rathbun (1918) has
indicated that such females occur in January at St. Thomas, Virgin
Islands, in March in Jamaica, from May to November in Florida, from
June to January in North Carolina, from July to September in Rhode
Island and Massachusetts (although Pearce has stated that they only
occur from late May to mid-June at Cape Cod), and in June in Brazil.
Relative to the swimming velocity of P. maculatus larvae, Welsh
(1932) has reported that it is greatly influenced by temperature and
light intensity. At temperatures between 20" and 25"C, the maximum
swimming velocity is attained at light intensities between 10 and 25
meter-candles. When maintained a t different constant temperatures,
it is known that there is a change in the relationship of velocity to
light intensity, with the maximum possible velocity occurring at each
constant temperature during the initial period of exposure to higher
temperatures. Furthermore, the light intensities will bring about a
marked effect on general activity.
3. Pinnotheres poisum Linnaeus, 1767. (Figs. 218-221)
(Order Decapoda; suborder Reptantia; section Brachyura; subsection
Brachygnatha ; superfamily Brachyrhyncha ; family Pinnotheridae)
The pea crab, Pinnotheres pisum, is the common symbiont of
pelecypods in northern European waters. It appears to be most