Species Description: The mangrove tree crab, Aratus pisonii, is one of several species of crabs belonging to the family Sesarmidae. The carapace is a mottled brown to olive-green (Kaplan 1988) and is widest at the front, tapering posteriorly (Abele 1986). Eyes are widespread at the front corners of the carapace. Legs are brown to mottled, and the claws bear tufts of black hair. Sharp tips at the end of the legs allow A. pisonii to climb mangrove trees and other vertical surfaces (Kaplan 1988).
Potentially Misidentified Species: In many locations, the mangrove tree crab shares its habitat with the spotted mangrove, or mangrove root crab, Goniopsis cruentata. However, this species can reach carapace lengths up to 6.3 cm, much larger than A. pisonii (Kaplan, 1988). Coloration also distinguishes the two crabs. The body of G. cruentata is dark brown, with red legs bearing a pattern of yellow spots. The palms are white and claws lack the black hairs characteristic of A. pisonii.
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.
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).
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.
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).
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.
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