324
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
abdomen at middle, approximately one-third width of sternum,
gradually narrowing from third to seventh segment, sides of third
convex, sides of seventh obtusely rounded ; sutures between segments
of abdomen and sternum with narrow lines of dark pubescence.
Strikingly light dorsal color pattern of bare spots on a background of
dark pubescence consisting of a median stripe constricted on middle and
behind, a subtriangular spot on each side in front of middle, and a
linear spot on each side behind; chelipeds with dark pubescence on
inner and upper surface of carpus, a bit on upper surface of merus and
inner side of palm proximally, otherwise scattered flecks on hands and
walking legs.
Some males resemble mature females in coloration and structure of
legs, ranging in length from about 4 mm upwards. Such males are
symbiotic.
Life cycle. Accounts of the life cycle of P. maculatus have been
contributed by Pearce (1964) and Costlow and Bookhout (1966). As in
the case of P. ostreum, the zoeal and megalopal stages are planktonic.
Myman (1924) has described the first zoeal stage which, unlike that of P.
ostreum, has well-developed spines in the carapace. According to
Costlow and Bookhout, who have reared the larval stages from hatching
to the crab stage under laboratory conditions there are five zoeal stages
followed by the megalops stage. Thus, in addition to morphological
differences, which have been given in detail by Costlow and Bookhout,
P. maculatus differs from P. ostreum in its number of zoeal stages, the
former with five and the latter with four. It may be noted that thenumber of zoeal stages is known to vary among the Pinnotheridae. For
example, Lebour (1928b), working with material from the plankton, has
reported two zoeal stages in the life cycle of P. veterum; Hart (1935),
who has reared P. taylori, has also reported two zoeal stages in this
species ; and Irvine and Coffin (1960) have reported four zoeal stages
in the life cycle of Pabia subquadrata.
According to Pearce (1964), ovigerous females of P. maculatus occur
from late May to mid-June in the Woods Hole, Massachusetts, area. By
early August the eggs are hatched and the zoeal and megalopal stages
are planktonic. The megalopal stage molts during mid-September to
give rise to the first true crab stage. It is at this stage that P. maculatus
invades the mantle cavity of Mytilus edulis. Within the host, the crab
undergoes several molts with each succeeding instar being somewhat
larger but morphologically similar to the preceding. By mid-October,
the crabs measure approximately 3.3 mm in carapace width. A molt
follows, resulting in an anomalous juvenile with a well calcified exoskeleton (thus compamble to Christensen and McBermott’s hard stage). It is
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
abdomen at middle, approximately one-third width of sternum,
gradually narrowing from third to seventh segment, sides of third
convex, sides of seventh obtusely rounded ; sutures between segments
of abdomen and sternum with narrow lines of dark pubescence.
Strikingly light dorsal color pattern of bare spots on a background of
dark pubescence consisting of a median stripe constricted on middle and
behind, a subtriangular spot on each side in front of middle, and a
linear spot on each side behind; chelipeds with dark pubescence on
inner and upper surface of carpus, a bit on upper surface of merus and
inner side of palm proximally, otherwise scattered flecks on hands and
walking legs.
Some males resemble mature females in coloration and structure of
legs, ranging in length from about 4 mm upwards. Such males are
symbiotic.
Life cycle. Accounts of the life cycle of P. maculatus have been
contributed by Pearce (1964) and Costlow and Bookhout (1966). As in
the case of P. ostreum, the zoeal and megalopal stages are planktonic.
Myman (1924) has described the first zoeal stage which, unlike that of P.
ostreum, has well-developed spines in the carapace. According to
Costlow and Bookhout, who have reared the larval stages from hatching
to the crab stage under laboratory conditions there are five zoeal stages
followed by the megalops stage. Thus, in addition to morphological
differences, which have been given in detail by Costlow and Bookhout,
P. maculatus differs from P. ostreum in its number of zoeal stages, the
former with five and the latter with four. It may be noted that thenumber of zoeal stages is known to vary among the Pinnotheridae. For
example, Lebour (1928b), working with material from the plankton, has
reported two zoeal stages in the life cycle of P. veterum; Hart (1935),
who has reared P. taylori, has also reported two zoeal stages in this
species ; and Irvine and Coffin (1960) have reported four zoeal stages
in the life cycle of Pabia subquadrata.
According to Pearce (1964), ovigerous females of P. maculatus occur
from late May to mid-June in the Woods Hole, Massachusetts, area. By
early August the eggs are hatched and the zoeal and megalopal stages
are planktonic. The megalopal stage molts during mid-September to
give rise to the first true crab stage. It is at this stage that P. maculatus
invades the mantle cavity of Mytilus edulis. Within the host, the crab
undergoes several molts with each succeeding instar being somewhat
larger but morphologically similar to the preceding. By mid-October,
the crabs measure approximately 3.3 mm in carapace width. A molt
follows, resulting in an anomalous juvenile with a well calcified exoskeleton (thus compamble to Christensen and McBermott’s hard stage). It is