The blue crab Callinectes similis is a decapod crustacean in the
Portunidae family. It is a strong and agile swimmer powered by a pair of
flat, oar shaped rear legs called swimmerets giving it freedom of motion to
rapidly swim backwards, sideways, and sometimes forward (Barnes 1980). It
is an offshore congener in the genus of Callinectes. The lesser
blue crab has a smoother, more uniform granulated carapace when compared to
C. ornatus and C. sapidus (Williams 1984). Adult males have
a green carapace with irregular areas of iridescence at the base of the
teeth. The swimming legs are mottled with white. The females are similar
in color except that the inner surfaces of the chelae are more violet blue.
The juveniles are not as brilliantly colored appearing olive-yellow to
greenish. The length and curvature of the reproductive organs (gonopods)
are distinctive in mature males for each of the Callinectes species
(Gore 1977, Barnes 1980) See illustrations from Williams (1974) below.
Potentially Misidentified Species
It can be difficult to distinguish the juveniles, immature males, and adult
females of C. ornatus from C. danae and C. similis (Williams 1984).
C. ornatus is very similar to C. similis and the two were confused
until the 1960's (Gore 1977).
C. similis can be distinguished from
C. ornatus by the pale translucent blue dactyls of the swimming legs and
the propodi that are olive on the ends and banded with translucent blue
Comparative detail of the reproductive structures of mature male Callinectes. Modified from Williams 1974.
HABITAT AND DISTRIBUTION
Adult Callinectes similis occur in the oceanic littoral zone in
salinities above 15‰ at a depth of 100 meters along the eastern seaboard
from Delaware Bay to Key West, Florida and southern coasts of the United
States, to the Yucatan, Colombia and northern Jamaica (Williams 1984,
Piller et al. 1995). They are widely distributed along the northern Gulf
of Mexico in open bay habitats (Hsueh et al. 1992a). Juveniles are found
in estuaries in these same regions.
Juveniles of the lesser blue crab are common in seagrass beds in the Indian River Lagoon (Gore 1977).
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
The Callinectes similis male grows to a maximum width of 122 mm and the female grows to a maximum width of 95 mm. A study of populations in Mobile Bay in the Mississippi Sound reported a larger female to male ratio (Hsueh et al. 1993).
The lesser blue crab is common in marshes and estuaries. In the barrier reef marsh habitats of Mobile Bay, Alabama, Callinectes similis is reported to be the dominant crab in open bays (Hsueh et al. 1993).
Callinectes similis has spring and fall spawning seasons (Williams 1984). The egg carrying females migrate to nearshore waters with higher salinities to release their larvae (Hsueh et al. 1993).
In Callinectes similis, there are generally 8 zoeal stages and 1 postlarval, or megalopal stage (Bookhout and Costlow 1977). Larval release usually occurs at high tide assuring larval abundance is at its peak during the ebbing tide. Crab larvae are advected offshore, and complete development in coastal shelf waters. The typical time for development through the 7 zoeal stages is between 30-50 days before metamorphosis to the megalopal stage. The megalop persists between 6-58 days. It is widely believed that the megalopal stage subsequently returns to the estuaries for settlement, and recruitment to adult populations (Barnes 1980, Hsueh et al. 1993).
Temperature may be a factor in the reproductive cycles of the lesser blue crab (Williams 1984).
Adult Callinectes similis are not usually encountered in estuaries
where salinities are below 15 ppt. Like its congener C. sapidus,
the lesser blue crab is a hyperosmoregulator able to respond to changes in
salinity by maintaining a high hemolymph osmolarity by active ion pumping
in the gills (Piller et al. 1995).
Juveniles of the lesser blue crab exhibit a higher tolerance for salinity
change than adults allowing them to inhabit estuaries with salinities as
low as 10 ppt (Guerin and Stickle 1997). In Mobile Bay, Alabama, juvenile
C. similis were encountered in the barrier reef marsh habitats with
salinities ranging from 8-28 5ppt (Hsueh et al. 1992a). In laboratory
experiments, C. similis juveniles survived in near freshwater (as
low as 5 ppt) to hypersaline conditions, but were reported to be best
adapted to 35 ppt (Hsueh et al. 1993).
Juveniles of the lesser blue crab tolerate high levels of hypoxia but will die under anoxic conditions. In laboratory studies, there was a noticeable decrease observed in feeding, growth and molting rates below normoxia. This adaptation may allow populations of Callinectes similis to survive seasonal occurrences of hypoxic water masses on continental shelves (Das and Stickle 1993).
Callinectes similis is an opportunistic predator feeding primarily on bivalves and other benthic macroinvertebrates crushing their prey with their claws. Stomach content studies also show that the lesser blue crab diet includes fish, crustaceans, squid, detritus, and plants (Hsueh et al. 1992a).
The lesser blue crab is usually associated with Callinectes sapidis where they compete for food and other resources (Williams 1984). Typically, adult C. similis inhibits higher-salinity waters ( 315ä) than C. sapidis (Guerin and Stickle 1997). However, juvenile C. similis and C. sapidis overlap considerably in estuaries where they can find refuge and abundant food sources.
Because Callinectes ornatus occupies the same habitats as the commercial species, C. sapidus, it is impacted by the crab fishery along western Atlantic coast (Hsueh et al. 1992).
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haemastoma to hypoxia and anoxia. Marine Ecology Progress Series
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bioenergetics of juvenile lesser blue crabs, Callinectes similis.
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Williams) in barrier island salt marsh habitats of the Gulf of Mexico.
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the Eastern United States, Maine to Florida. Smithsonian Institution
Press, Washington, D. C., pp 373-376.