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Although the crabs are most commonly associated with the red mangrove,
Rhizophora mangle, populations
also reside among the branches of the black mangrove, Avicennia
germinans; the mangrove, Avicennia schaueriana;
the white mangrove, Laguncularia
racemosa and the tea mangrove, Pelliceria rhizophorae
(Conde et al. 2000).
IRL Distribution:
The mangrove tree crab is a common inhabitant of red mangrove, R.
mangle, forests throughout the lagoon (Rader & Reed 2005),
although crabs can also be found in and around the white mangrove,
L. racemosa and the black mangrove, A. germinans.
III. LIFE HISTORY & POPULATION
BIOLOGY
Age, Size, Lifespan:
The maximum carapace width for A. pisonii is approximately
2.7 cm (Díaz & Conde 1989). However, average size varies
among the sexes, measuring 2.0 cm and 1.8 cm for males and females,
respectively (Díaz & Conde 1989). The size at the onset
of maturity (SOM), when individuals are sexually reproductive, occurs
at approximately 0.9 to 1.6 cm, depending on salinity (Conde &
Díaz 1992). For more information, see “Salinity” below. Size
also appears to vary with habitat type, with larger crabs often
found in more mature mangrove forests and smaller individuals among
stunted mangroves (Conde & Díaz 2000). Although lifespan
and growth is highly variable, depending on food availability and
environmental conditions, Warner (1967) reported that male crabs
reach full size after one to five years.
Abundance:
Mangrove tree crabs are a common inhabitant of mangrove ecosystems.
Although absolute abundance measurements for the species are scarce,
studies on Venezuelan populations of A. pisonii have yielded
20 to 170 individuals per study site (Conde & Díaz 1989,
Díaz & Conde 1989). In Belize mangrove habitats, low
crab abundances were seen, ranging between 0.03 to 0.10 crabs per
cubic meter (Feller & Chamberlain 2007). Díaz and Conde
(1989) observed that crab abundance may be related to species richness
of nearby mangrove fouling communities and the presence of macroalgal
food sources. In adult populations, sex ratios are often skewed
toward females. This trend may develop because females exhibit slower
growth rates than males on average, allowing them longer periods
between risky and dangerous molting events (Díaz & Conde
1989).
Reproduction:
Like other brachyuran crabs, sex can be
determined in A. pisonii by examining the abdomen. In females,
it is broader and can be tightly flexed to hold the egg mass, called
a sponge (eg. Ruppert et al. 2004). For A.
pisonii, Warner (1967) found that females from 1.5 to 1.7 cm
were the most common size class to breed. As with most decapod crustaceans,
fertilization occurs during copulation shortly after the female
molts. The male transfers sperm-filled cases, called spermatophores,
to the female. After the eggs are fertilized, the female broods
them on her abdomen until hatching. Although this species reproduces
continuously, the peak of egg hatching may occur during the rainy
season (Conde & Díaz 1989) or may be synchronized with
lunar rhythm (Warner 1967, Warner 1977). At this time, the female
climbs down from the tree into the water and rapidly vibrates her
abdomen to release a cloud of larvae (Warner 1977). Each reproductive
female may repeat this process up to six times annually (Warner
1967).
Embryology
/ Larval Development:
Depending on carapace width, each
female may hold between 5,000 and 35,000 eggs (Conde & Díaz
1989, Warner 1977). After fertilization occurs, eggs hatch in approximately
16 days and begin the larval cycle in the water column (Warner 1977).
Larvae pass through four zoeal stages and one megalopa, measuring
between 0.6 and 0.9 mm (Cuesta et al. 2006), before settling
to the benthos as juvenile crabs. Factors such as salinity, temperature
and diet may affect growth and the duration of the larval period
(Anger 2001). At 25°C and 34ppt, the entire settlement process
spans about 20 days, and field observations have led to the estimation
that newly hatched larvae can reach a juvenile size of 10mm in four
to five months (Warner 1967).
IV. PHYSICAL TOLERANCES
Temperature:
The mangrove tree crab is a tropical to subtropical species, inhabiting
warm coastal areas of the Atlantic and Pacific Oceans. Given this
range, it is likely that the thermal tolerance of the crab is narrow.
Although published reports of temperature preferences in adults
are limited, culture of larvae is successful between 24 and 28 °C
(Cuesta et al. 2006, Schwamborn et al. 2006).
In the field, A. pisonii are found in regions where they
experience air temperature ranges spanning from at least 6 to 39
°C (Conde et al. 2000).
Salinity:
Díaz and Bevilacqua (1986, 1987) have documented a salinity
range for A. pisonii of 15 to 35 ppt. In the field, populations
occur in a wide variety of locations, from river mouths to hypersaline
lagoons. These areas can fluctuate in salinity by 35 ppt or greater
(Conde et al. 2000, Conde & Díaz 1989). Although
tolerances seem to be high, some differences in growth and maturation
size have been seen among populations at different salinities. Crabs
found in hypersaline lagoons appear to mature at a smaller size
than those in fresher riverine areas (Conde & Díaz 1992).
Larvae of A. pisonii have been successfully cultured in
the laboratory at salinities of 25 and 34 °C (Schwamborn et
al. 2006, Cuesta et al. 2006).
Oxygen:
Although A. pisonii
releases its larvae in estuarine and marine waters, juveniles and
adults are more terrestrial, found mostly above the water line in
tree canopies and exposed surfaces. It is not an air-breathing species
like some crabs. Instead, it gains oxygen by flowing a thin film
of water over the branchiostegites, a reticulated expansion of the
carapace covering the gills (Warner 1977). When desiccation occurs
and the branchial water begins to dry up, the crab must descend
from the tree to wet its gills again (Warner 1967). Given the proximity
of a saline water source, this behavior allows the crabs to spend
substantial amounts of time in the terrestrial environment.
V. COMMUNITY ECOLOGY
Trophic Mode:
The
mangrove tree crab has been reported in some literature as an herbivore
(de Lacerda 1981, Warner 1967), although it is more likely an opportunistic
omnivore (Díaz & Cuesta 1989) in many instances. Observations
in the field and examinations of gut contents have determined that
A. pisonii consumes a variety of plant and animal tissue.
Plant matter found in the guts of adult crabs consists of: several
species of macroalgae; seagrasses including shoal grass, Halodule
beaudettei and turtle grass, Thalassia
testudinum; and the leaves of the white mangrove, L.
racemosa, the red mangrove, R. mangle and the black
mangrove A. germinans. Of these, R. mangle was
the most abundant food, comprising approximately 84% of gut contents
(Erickson et al. 2003). Crabs feed on living mangrove leaf
tissue, leaving behind distinctive scraping marks (Beever et
al. 1979, Erickson et al. 2003, Feller 1995). Even
in areas of low crab abundance, this behavior can account for up
to 96% of the herbivory in the mangrove forests (Feller & Chamberlain
2007), focused mainly on the older leaves in fringing zones (Erickson
et al. 2003, Feller & Chamberlain 2007). In addition
to plant material, crab guts have included animal matter such as
nematodes, crustacean appendages, fish scales, foraminiferans and
polychaetes (Erickson et al. 2003). The larvae of A.
pisonii appear to be somewhat omnivorous as well. The majority
of the diet in larvae studied consisted of diatoms, mangrove detritus,
tintinnids and copepods were also consumed (Schwamborn et al.
2006).
Predators:
As with most organisms that reproduce
via planktonic larvae, the mortality of A. pisonii is high.
On average, it is estimated that only 0.04% of larvae live to become
newly settled juveniles. Of those, about 17% reach an adult size
of 1.8 cm (Warner 1967). One reason for mass mortality is the high
level of predation. While in the water column, the larvae of A.
pisonii may be preyed upon by a variety of other zooplankton,
small fishes and benthic filter feeders like barnacles, hydroids
and anemones. Once the mangrove tree crab reaches adulthood, it
has the potential to be preyed upon by birds, mammals and larger
crabs such as Goniopsis cruentata (Warner 1967). However,
terrestrial predator avoidance in this species appears to be highly
developed. Adults have the ability to cling tightly to tree branches
and bark, may reach climbing speeds of 1 m/s and have the ability
to leap from tree canopies onto mud banks or into the water below
(Warner 1977). This jumping behavior may occasionally prove fatal
as fishes can also consume A. pisonii when the crabs leap
into the water (Díaz & Conde 1989).
Social Behavior & Territoriality:
Like many decapod crustaceans, mangrove tree crabs have developed
social behaviors and territoriality. Females are most always subordinate,
relegating physical disputes to the males in the population. Males
have larger, more developed chelae, or claws, which they use to
push each other during aggressive encounters concerning territory
or mate choice (Warner 1970). Once a home range has been established,
individuals may inhabit the same area for periods of several weeks
to months (Warner 1970).
Associated Species:
As common inhabitants of mangrove canopies, A. pisonii
are associated with other fauna dwelling in these habitats, including:
birds, snakes, lizards, insects, snails, small mammals and other
crabs. For a more extensive list of species found in the mangrove
forests of the Indian River Lagoon, please refer to the Mangrove
Habitats page of this inventory.
VI. SPECIAL STATUS
Special Status:
None
Benefit
to the IRL:
In addition to contributing
the majority of herbivory in many mangrove habitats, A. pisonii
impacts lagoon ecosystems as an important producer of zooplankton
to the surrounding water column. The production of thousands of
crab zoeae is thought to be one of the main pathways of energy transfer
between benthic and pelagic food webs (Schwamborn et al. 1999, Schwamborn
et al. 2002, Schwamborn & Saint-Paul 1996). These zoeae join
other invertebrate larvae that are consumed by a variety of organisms
in and around mangrove communities.
VII.
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Report by: LH
Sweat, Smithsonian Marine Station at Fort Pierce
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