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Species Description:
Cardisoma guanhumi, the blue
land crab, is a large burrowing crab whose distribution on land is generally
limited to within 5 km of the ocean. Large individuals may grow to over 11 cm
and weigh over 500 grams.
This species is similar in shape
to Uca species; its eyes are stalked and fairly widespread, and males have one
enlarged cheliped. Adults of both sexes have carapaces which range in color from
dark blue to various shades of brown to gray/white. The adult color pattern is
well developed in most crabs by the time they reach 80-90 grams; however, some
may attain180 grams before they show an adult pattern. Juveniles generally have
brown carapaces and orange colored legs. Ovigerous females frequently appear
light gray or white.
II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
Cardisoma guanhumi is a
circumequatorial species found throughout estuarine regions of the Caribbean,
Central and South America including Columbia, Venezuela, the Bahamas, and Puerto
Rico. Within the U.S. it is limited to the Gulf of Mexico and coastal Florida
and is rarely found more than 8 km from the ocean. It is found in greatest
concentration on low lying ground, and burrow concentrations in these optimum
habitat areas may exceed 7500 per acre. In Central and South America, as well as
in the Bahamas, C. guanhumi is exploited for food; however, in the U.S.
and in much of Puerto Rico it is considered a pest.
The population distribution of this species is heavily influenced by water temperature. In areas where water temperatures fall below 20 C in winter, larval survival is affected. Within the U.S.,
Cardisoma guanhumi has been observed as far north as Vero Beach, Florida; however, north of this area, water temperatures in the winter limit the growth of large populations.
IRL Distribution:
C. guanhumi is found throughout the Indian River Lagoon.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Cardisoma guanhumi
reaches sexual maturity in approximately 4 years, when it attains a mass of 40
g; however, it is not uncommon for adult crabs to measure 10-11 cm and weigh up
to 500 g.
Cardisoma guanhumi is a slow
growing species compared to most other crabs. While most crabs may require
approximately 20 molts to reach maximum size, Cardisoma requires more than 60
molts. Premolt crabs seal the entrances to their burrows with mud and will
remain sealed inside the burrow until molting takes place, generally within 6-10
days. Molt frequency decreases with age.
Abundance:
This species is abundant throughout much of the Indian River Lagoon and its
adjacent areas.
Locomotion:
Cardisoma are typically
able to walk at a rate of 100 m/hr. and may move over 500 m or more per evening.
However, if actively foraging, this distance is decreased to an average of less
than 100 m.
Land crabs orient themselves by
using polarized light during the day, and by identifying the brightest sector of
the horizon at night. However, other cues such as geotaxis, substrate vibration,
landmarks and prevailing winds may also aid in orientation, especially during
migratory periods.
Reproduction:
The reproductive cycle is
closely linked to seasonal weather patterns and lunar phase. Heavy rains in the
spring initiate migrations. At this time, Cardisoma gains weight rapidly as
foraging intensity is increased for the first few weeks of the migratory period.
Males actively court ripe females during this period. Fertilization is internal,
and throughout July and August most females carry external egg masses. Eggs are
carried for approximately 2 weeks prior to hatching, and must be released into
salt water for larvae to survive. Females typically complete spawning migrations
within 1-2 days and generally spawn within 1-2 days of a full moon. Thus, though
Cardisoma and other terrestrial crabs have been successful invaders of the land,
they are still heavily dependent on the ocean for at least part of the life
cycle.
Several spawns per year may
occur, with spawning season varying with location within the range. In Florida,
spawning season lasts from June to December, peaking in October and November. In
the Bahamas the season extends from July - September, while in Venezuela
spawning lasts from July - November.
Eggs hatch into free swimming
larvae with 5 zoeal stages and 1 postlarval, or megalopal stage. Typical
development time from hatching to the first crabs stage is 42 days under
laboratory conditions; however, this time may be much shorter in nature.
Fecundity in Cardisoma is related
to body mass. A 300 g female may produce 300,000 - 700,000 eggs per spawn.
IV. PHYSICAL TOLERANCES
Temperature:
As a tropical/subtropical
species, Cardisoma adults are generally not heavily impacted by low winter
temperatures. However, Cardisoma larvae are highly affected by water
temperatures below 20 C. Under laboratory conditions, zoea larvae held at 20 C
required twice as long to complete development as those held at higher
temperatures. Further, less than 10% of the zoea held at 20 C molted to the
megalopal stage. Optimum larval development in the laboratory occurred when
water temperature was 25-30 C.
Salinity:
Adult Cardisoma utilize a
range of habitats and tolerate salinities from zero to hypersaline. Because
these crabs are usually surrounded by air rather than water, they are
essentially closed systems and not subject to salt gain on or through the body
surface. Further, they are able to osmoregulate down to fresh water levels.
Larval development was shown to
be optimal at salinities of 20 - 40 ppt, though a wide range of salinity was
tolerated. Larval survival fell off dramatically at salinities below 15 ppt and
above 45 ppt. At 10 ppt, zoea developed only to the second stage; and at 45 ppt
less than 1% of larvae completed development to the megalopa.
Other Physical Tolerances:
Terrestialization in
crabs is accompanied by a decrease in the number, volumes and area occupied by
the gills. This adaptation perhaps serves to decrease the amount of evaporative
surface area, thus minimizing the effects of desiccation. The gill structure in
land crabs also shows increased sclerotization to support platelets which may
result in decreased permeability of gases. One result of this structural change
is that oxygen availability is lowered in terrestrial crabs. A possible
compensation for this potential problem is to increase the folds of the vascular
epithelium and vascular tufts associated with the gills.
The gills of Cardisoma are
relatively small, less than 10% of the volume of the branchial cavity. However,
gill surface area is increased due to extensive folding and high vascularization
of the epithelial sheet lining the branchial cavity.
Cardisoma, when tested against
other crabs, both terrestrial and aquatic, maintained a nearly constant rate of
oxygen consumption in both air and water. This was accomplished by its
manipulating the rates of oxygen extraction and ventilation over the gills. In
water, where oxygen concentrations are lower, and extraction rates tend to fall
off, Cardisoma increases its ventilation rate. In air, where both oxygen
concentration and extraction rate are higher, Cardisoma decreases its
ventilation rate.
Invasion of land by Cardisoma
guanhumi and other terrestrial crabs is also limited by mechanisms of water
conservation. Land crabs are unable to conserve water; thus, though they are
able to function extremely well in moist air, they die when faced with
desiccating conditions for over 2 days.
V. COMMUNITY ECOLOGY
Trophic Mode:
For the most part,
Cardisoma guanhumi is a vegetarian crab which collects and eats leaves fruits
and grasses collected near the vicinity of its burrow. However, these crabs will
also eat insects, carrion, feces and is sometimes cannibalistic; thus, it is
functionally an omnivore. The preferential foods of C. guanhumi are the leaves
of red and white mangroves, and the buttonwood tree. They feed throughout the
day in shaded areas; however, if exposed to direct sunlight for prolonged
periods, they prefer to feed at night.
Cardisoma guanhumi uses light and
sound detectors to locate food items. Field experiments using falling fruit and
leaves showed that crabs can be drawn from their burrows to investigate the
sound and initiate a search for food. Predatory behavior is released in these
crabs by detection of small moving objects. Cardisoma is able to detect small
substrate vibrations in the range of 10-1500 Hz and 70 dB. Visual acuity
increases with body size due to an increase in both the number and diameter of
ommatidia.
Habitat:
Cardisoma guanhumi
colonizes habitats of various ecological conditions. Thus, in areas where it
occurs, its distribution tends to be patchy and uneven owing to differences in
the type and availability of food resources, ground water levels and the
aggressiveness or gregariousness of conspecifics.
Terrestrial crabs are limited to
areas where they can burrow to intersect the water table and maintain a 1-2
liter pool in the bottom of the burrow. Thus they are functionally limited to
areas where the water table is within approximately 2 m of the surface. In south
Florida, burrow densities have been found to be highest in firm, muddy
substrates, with as many as 7500 burrows per acre.
Burrow openings lie flush with
ground level and may be 1-18 cm in width. Burrows descend, first gradually, then
more steeply, to the water table where a small pool of 1-2 liters of water is
maintained. This water may be either fresh or salt water, depending upon
location.
Generally only a single
individual occupies a burrow. However, small juveniles (less than 10 mm CW) do
not dig their own burrows, and will often share a burrow with an adult crab.
Males tend to be more territorial than females and will defend burrows and small
surrounding territories. Agonistic behavior accounts for the spacing of burrows
in populations. Burrow disputes, along with intense competition for optimum
shoreline burrows, may be the force which drives terrestrial crabs to migrate
further inland.
Activity Time:
Peak activity time is at
dawn and dusk, though activity in C. guanhumi tends to increase under low light
levels and with reduced food availability. Activity in this species is decreased
by high daytime temperatures.
Associated Species:
In swampy habitats,
Cardisoma guanhumi commonly occurs with 2 Grapsoid crabs: Ucides cordatus and
Goniosis cruentata. In shoreline habitats, it most commonly is observed with
fiddler crab species.
VI. SPECIAL STATUS
Special Status:
Economic
Notes on Special Status:
Throughout the Bahamas
and the Caribbean, Cardisoma guanhumi is intensively exploited as a food
resource. Harvesters of wild populations in Venezuela have reported that as many
as 400 crabs per harvester per night can be collected even during the months of
lowest catch. This figure increases as migration periods approach.
Economic Importance:
Cardisoma gunahumi is economically important in the Caribbean and Bahamas. In the U.S., it is not generally
exploited for food. Some people consider it a common garden pest in
Florida, because it regularly chooses to dig its burrows in lawns.
Report by: K. Hill,
Smithsonian Marine Station
Submit additional information, photos or comments to:
irl_webmaster@si.edu
Page last updated: July 25, 2001
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