Blue Crab Spawning
General Information
Unlike most marine organisms, blue crabs mate and spawn at different
times. During mating the male crab transfers his sperm into special sac-like
receptacles in the female crab.  These
receptacles store the male's sperm so that it can be used for egg fertilization
at a later time. Viable sperm can live in the female's seminal receptacles
for well over a year and will be used for two or more spawnings. See Mating for
more information on this subject.
After the female matures and mates, the newly molted "sook" must
regain strength and build muscle mass in order to prepare for her migration
to high-salinity spawning grounds where she will produce a "sponge" and
then release eggs. To prepare for this journey she will remain and forage
in the same general area where she mated for a period of weeks to months
(Turner, Wolcott, Wolcott, and Hines 2003). It isn't until late September
or October that she will begin her migration to the lower Chesapeake
Bay to spawn. The "pregnant" females that survive the gauntlet
of crabpots, scrapes, trotlines and dredges will usually spawn for the
first time the following season since they reach the spawning grounds
late in the season. In other words sooks won't release any eggs until
approximately 2-9 months after mating, usually May-August the next season.
Egg Development
When the female is ready to spawn, egg cells (oocytes) are forced from
the ovaries through the seminal receptacles where they are fertilized.  The
fertilized eggs, which are about 0.25 mm in diameter, are then extruded
into a large, cohesive mass or "sponge" that remains attached
to fine hairs beneath her abdomen until they hatch (see female
anatomy for more information.) The average sponge contains about
two million eggs, but may contain anywhere from 750,000 to 8 million
eggs, depending on the size of the crab. The sponge is formed in about
two hours and is roughly one third the size of the female crab's body.
Note that unmated females may produce sponges of unfertilized eggs but
this is very rare, as there always seems to be an abundance of males,
each capable of mating with many different females.
The eggs take about two weeks (14 days) to fully develop and hatch.
Initially the egg mass is orange-yellow in color and gradually darkens
to black. The color change is caused by absorption of the yellow yolk
and development of dark pigment in the eyes and on the body of the embryos. Click
here to see full color range of egg mass development (numbers correspond
to days of development.)
Hatching
 Hatching
of blue crab eggs occurs at salinities of 23-33 ppt and temperatures
of 66-84º F. (19-29º C.). In the Chesapeake Bay, larval release appears
to be concentrated at the extreme lower Bay between the Virginia capes
and at the mouths of the Bay's southern rivers (spawning rarely occurs
in Maryland and occasionally occurs between the mouth of the Potomac
River and Wolf Trap Light where salinities are higher, 15-20 ppt.) Once
released, blue crab larvae drift like orphans from the lower Bay toward
the open sea to the continental shelf which is where most larval development
occurs.
On the average, only one out of every million eggs survives to become
a mature adult. Mortality of eggs has been attributed to fungal infection,
predation, suffocation in stagnant water, and exposure to extreme temperatures.
Spawning peaks in the blue crabs are closely associated with the region
inhabited. In Chesapeake Bay, for example, spawning is initiated in May
and June, with a second spawning in August. In North and South Carolina,
spawning occurs from March through October, with peaks from April to
August. Around the St. John's River in Florida, spawning occurs from
February to October, with peak spawning occurring from March through
September. In the Gulf of Mexico, two spawning periods are common: one
in February and March, and one in August and September.
Although a female will mate only once, she will produce many fertilized
egg masses during her lifetime from this single mating. Studies in Florida
found that some female crabs produce as many as seven broods (sponges)
in one year from a single mating, and up to 18 broods over 2-2½ years.
Research at the University of Maryland Biotechnology Institute (UMBI)
Center of Marine Biology (COMB) has shown that Chesapeake Bay females
were very capable of spawning five or more times, though most will not
produce more than one or two broods due to their short average life span,
typically 1-2 years (their life span is short because most are harvested
before they can grow old.) In all cases, successive spawns may occur
during the same season or females may overwinter before spawning again
the following spring.
The presence of empty egg cases on swimmerets or the occurrence of
large, bright-red adult nemertean worms (Carcinonemertes carcinophila)
on the gills of a mature female indicates that she has spawned at least
once (Churchill 1919; Hopkins 1947). After reaching sexual maturity,
these worms feed on the egg masses carried by the female crabs and live
in the gills of the crab after the eggs hatch (Hopkins 1947). In lower
Chesapeake Bay, mature red nemertean worms occurred in the gills of more
than 95% of the female crabs that had spawned; immature crabs supported
only immature, light-colored worms (Hopkins 1947).
References & Citations
Turner, H., Wolcott, D., Wolcott, T., Hines, A. 2003.
Post-mating behavior, intramolt growth, and onset of migration to Chesapeake
Bay spawning grounds by adult female blue crabs, Callinectes sapidus
Rathbun. Journal of Experimental Marine Biology and Ecology 295:107-130.
Churchill, E.P., Jr. 1919. Life history of the blue
crab. Bulletin of the Bureau of Fisheries 36:95-128.
Hopkins, S.H. 1947. The nemertean Carcinonemertes as
an indicator of the spawning history of the host, Callinectes sapidus.
J. Parasitol. 33(2):146-150 |
