Sally Lightfoot Crab(Grapsus grapsus)

Sally Lightfoot Crab (Grapsus grapsus)

By Christopher Davis

Taxonomy Habitat Ecology Recent Research Commercial Importance Bermuda Laws Personal Interest References Links

Abstract

Grapsus grapsus is a crustacean found frequently along the coasts of subtropical and tropical America, Africa, and Chile on the intertidal rocks (Baird 1978). It belongs to the Grapsidae family and gets its common name, Sally Lightfoot crab from its quick mobility. The carapace is bright-red in coloration in adults (Baird 1978) and can range anywhere from 5-7 cm in width. (Lubchenco et al. 1984). It predominantly feeds on algae using their spoon shaped tipped pinchers (Baird 1978). G. grapsus runs with extraordinary speed and agility along the upright sides of rocks and has the uncanny ability to hide away in crevices to escape predation by rapacious birds (Rathbun 1918). G. grapsus are mainly solitary but can be seen in large congregations competing for food and mates (Baird 1978). Males can mate with multiple females by depositing sperm in the female’s spermthecae while the female carries the eggs underneath her abdomen, caring for the embryos until they hatch as zoea larvae. This paper explains the habitat, ecology, recent research, and commercial importance of the crab Grapsus grapsus.

Taxononmy

Phylum:Arthropoda
  Class:Crustacea
    Order:Decapoda
      Family:Grapsidae

Habitat

Grapsus grapsus is a crustacean found frequently along the coasts of subtropical and tropical America, Africa, and Chile on the intertidal rocks (Baird 1978). It belongs to the Grapsidae family and gets its common name, Sally Lightfoot crab from its quick mobility. The carapace is bright-red in coloration in adults (Baird 1978) and can range anywhere from 5-7 cm in width. (Lubchenco et al. 1984). It predominantly feeds on algae using their spoon shaped tipped pinchers (Baird 1978). G. grapsus runs with extraordinary speed and agility along the upright sides of rocks and has the uncanny ability to hide away in crevices to escape predation by rapacious birds (Rathbun 1918). G. grapsus are mainly solitary but can be seen in large congregations competing for food and mates (Baird 1978). Males can mate with multiple females by depositing sperm in the female’s spermthecae while the female carries the eggs underneath her abdomen, caring for the embryos until they hatch as zoea larvae. This paper explains the habitat, ecology, recent research, and commercial importance of the crab Grapsus grapsus.

Grapsus grapsus are commonly found along the coasts of subtropical and tropical America, Africa, mid-Gulf of California, and Chile on the intertidal rocks (Baird 1978). Grapsus grapsus shows it greatest affinity to the inshore benthic marine habitat of the Galapagos which shows a strong similarity to the environment of the Caribbean and West Indian region (Baird 1978).

Ecology

Grapsus grapsus have been known to reach widths of 73 mm in the Galapagos (Baird 1978) while those living in the Caribbean can range anywhere from 5 to 7 cm in width (Lubchenco et al. 1984). The carapace is nearly circular due to the convex sides of the carapace (Sterrer 1986). Armed with dimorphic claws or chelae to help them in feeding on mollusks (Vermeij 1977) and algal growths along the steep, barren, rocky exposed cliffs (Sterrer 1986). The large claw situated on the right is generally used for crushing while the more slender claw on the left is used for cutting (Vermeij 1977). The chelipad wrists are equipped with broad flattened sharp tipped spines (Sterrer 1986). The chelipeds in males are about as long as the carapace while they are shorter in females (Rathbun 1918).

The adult G. grapsus have a bright-red carapace in contrast to the juvenile’s black or greenish carapace which occasionally may be speckled with cream-colored dots (Baird 1978). The carapace is made up of a thin/lipid protein waterproof epicuticle layer and a thick procuticle of protein and chiton (Wood 2004). G. grapsus also have yellow or orange banded markings on their legs, particularly about the joints (Sterrer 1986). These joints are connected by flexible membranes to the hard exoskeleton which create moveable joints (Wood 2004). This allows for leverage and strength for complex movements (Wood 2004). They primarily feed on the algae throughout the intertidal region but they will eat meat and pursue any smaller animals in their immediate vicinity (Baird 1978). In Panama, they forage throughout the intertidal zone due to their mobility and exceptional vision and are capable of locating any animal or algal prey that went unnoticed by the sluggish, slow moving consumers (Menge & Lubchenco 1981). G. grapsus becomes frequently submerged by breakers washing over it during feeding (Warner 1977). During this encounter the crabs press their flattened bodies to the rock and cling on until the waves recedes (Warner 1977). They emerge from the surf and continue feeding as if nothing had happened (Warner 1977).

G. grapsus runs with extraordinary speed and agility along the upright side of rock and even under rocks that hang horizontally over the sea (Rathbun 1918). Their ability to hide away in crevices is often necessitated for escaping predation by rapacious birds which pursue them (Rathbun 1918). Previous studies on the rocky intertidal shores of the Bay of Panama indicate that for many mobile organisms such as G. grapsus holes and crevices are important refuges from consumers (Menge et al. 1983). Previous studies (Menge & Lubchenco 1981, unpublished data) indicated that these habitat irregularities are depended upon by 88% of the macroinvertebrate such as G. grapsus for their survival in the community (Menge et al. 1983).

Growth is an important mechanism in the life of Grapsus grapsus and is characterized by change in size with time and change in shape during growth (Warner 1977). Growth progresses in steps by a series of molts as a consequence of possessing an exoskeleton (Warner 1977). The frequency of molting and the increment at each molt are depended upon by the number of molts that G. grapsus undergoes before becoming fully grown (Warner 1977). G. grapsus continues to grow throughout its life but with increasingly long intervals between molts (Warner 1977). Like other crabs, G. grapsus has the ability to regenerate limbs through the voluntary shedding of a limb by snapping it off at the base as an escape tactic (Warner 1977).

Observing the behavior of Grapsus grapsus can help in determining the maturity of the crab (Baird 1978). Adult crabs are solitary for the most part in contrast to the small or medium sized crabs which often form groups with components of the same size (Baird 1978). It has been suggested that the crabs can perceive mechanical or chemical signals through touching of one another’s walking legs (Baird 1978). Hairy ridges on the upper surfaces of the walking legs may serve as the receptors in any mutual tactile interaction such as the battle for a mate (Baird 1978).

The growth of secondary characteristics marks the attainment of sexual maturity (Warner 1977). The development of gonads corresponds with these anatomical changes (Warner 1977). The growth of sexual characters is under hormonal control (Lockwood 1968 in Warner 1977). Females lay their eggs shortly after copulation with the male who stores sperm in the female in the spermathecae (Warner 1977). Males will mate frequently but have to wait 10-20 days to recharge their sperm (Warner 1977). In tropical crabs such a G. grapsus breeding is continuous throughout the year and may show lunar rhythm in which egg hatching coincides with full or new moon (Warner 1977). The number of eggs in a batch varies with the size of the crab (Warner 1977). The female assists in the release of the larvae from the egg by wafting the abdomen back and forth and by digging into the egg mass with the chelae (Warner 1977). “The zoea larva hatches in embryonic form as a prezoea but quickly molts to the first zoea” (Warner 1977). G. grapsus have a typical zoea larva with long rostral, dorsal, and lateral spines on the carapace, slender curved abdomen, and forked telson (Gurney 1960). Growth and development of the zoea larva occurs with each molt by adding setae to the existing appendages and adding appendages to the posterior half of the body (Warner 1977).

Recent Research

Beninger and Larocque (1998) examined the gonopod tegumental glands (GTG) to determine their prevalence and role in the Brachyura crabs. They examined the first six gonopods of Grapsus grapsus as the representative from the Grapsidae family. They reported that the prime, if not exclusive role of the gonopod tegumental glands is in reproduction, and that the GTG may therefore be considered accessory sex glands. They also uncovered that the nature of the gland secretions are neutral mucopolysaccharide (NMPS) layers. “The NMPS secretions may function as a lubricant to reduce mechanical wear of the ejaculatory canal by the second gonopod during copulation, and to reduce the viscosity of the ejaculate from the vas deferens as it enters the narrow ejaculatory canal.”

Commercial Importance

Grapsus grapsus is an edible species and is often used as food (Tinker 1965). Crab fisheries play an important role in the United States export of seafood. Crabs currently represent the greatest proportion of crustacean captures in the world’s fisheries at 1.7 million tons declared in 1993 (Anonymous 1995 in Beninger & Larocque 1998).

Bermuda Laws

No Bermuda laws were found pertaining to Grapsus grapsus.

Personal Interest

I first became interested in crabs during our snorkel at Bailey’s Bay. It was a challenge to find crabs and actually catch them. I became interested in the morphological and histological differences between each species we identified. My interest for crabs grew even more after our discovery of Myrtle, the blue crab at Spittal Pond. During that field trip we also collected the exoskeleton of a Sally Lightfoot crab. Since Grapsus grapsus is one of the quickest and most intelligent crabs in the world it was fascinating to see body structure and coloration of this amazing animal. I was also enthralled by first hand accounts of people trying to catch Grapsus grapsus in the field but having minimal to no luck at doing so. Since my research project is on the diversity of crabs in rocky intertidal habitats which is the domain of the Sally Lightfoot crab I am excited at having the opportunity of possibly finding and collecting one. I know that it is a challenge I am up for.

References

Baird, T. 1978. Sally Lightfoot. Sea Frontiers. 24:2.

Beninger, P.G., Larocque, R. 1998. Gonopod Tegumental Glands: A New Accessory Sex Gland in the Braychura. Mar. Biol. 132:435.

Gurney, R. 1960. Larvae of Decapod Crustacea. J. Cramer Publisher.

Lubchenco, J., Menge, B.A., Garrity, S.D., Lubchenco, P.J., Ashkenas, L.R., Gaines, S.D., Emlet, R., Lucas, J., Strauss, S. 1984. Structure, Persistence, and Role of Consumers in a Tropical Rocky Intertidal Community (Taboguilla Island, Bay of Panama). J. Exp. Mar. Biol. & Ecol. 78:23.

Menge, B.A., Ashkenas L.R., Matson, A. 1983. Use of artificial holes in studying community development in cryptic marine habitats in a tropical rocky intertidal region. Mar. Biol. 77:129.

Menge, B.A., Lubchenco J. Community organization in temperate and tropical rocky intertidal habitats: prey refuges in relation to consumer pressure gradients. Ecological Monographs. 51:429.

Rathbun, M.J. 1918. The Grapsoid Crabs of America. Bulletin of the United States National Musuem.

Sterrer, W. 1986. Marine Fauna and Flora of Bermuda: A Systematic Guide to the Identification of Marine Organisms. Wiley-Interscience Publication.

Tinker, S.W. 1965. Pacific Crustacea. Charles E. Tuttle Company.

Vermeij, G.J. 1977. Patterns in crab claw size: the geography of crushing. Systematic Zoology. 26:138.

Warner, G.F. 1977. The Biology of Crabs. Elek Science London.

Wood, J. 2004. MIZ Lecture Notes.

Links

Sally Lightfoot Crab on rock
Fighting Sally Lightfoot Crabs
Hanging out with friends
Upside down Sally Lightfoot Crab