Four colour morphs exist within the species: black, red, yellow and green. The carapace of G. ruricola grows in width at a rate of about per year, with the crabs reaching maturity after 5 years, and living for up to 10 years in total. G. ruricola have a number of adaptations to terrestrial life, mostly regarding water conservation. They are nocturnal, to prevent the hot sun from drying them out. They also have a "nephritic pad", onto which urine is released, in order to be cleaned by microbes before the water is then reabsorbed.
The eggs of G. ruricola hatch in the sea, where the larvae live as plankton. The return of the larvae to land seems to be infrequent, but when they do return, they return as megalopa larvae, in sufficient numbers to turn roads red. They move at a speed of, or faster if startled. For the next three years, the young crabs live in burrows inhabited by other crabs and eat food brought back to the burrow by the older crab. After mating, mass migrations occur with the females returning to the sea to release their fertilised eggs. An average female carries around 85,000 eggs.
Ecology
G. ruricola is an omnivorousscavenger, feeding mostly on nitrogen-poor plant matter. The meat of G. ruricola is rich in protein, and has often been harvested by local people. Other predators may include birds, although information is scarce. When confronted, they rear up and hold their open claws outwards in a defensive posture.
''Drosophila''
G. ruricola is the host organism for two species of commensalflies in the genus Drosophila. They were first observed on Montserrat by Henry Guernsey Hubbard in 1894, and presented at a scientific meeting later that year, but no further research was conducted until 1955, when specimens were again collected, this time from Mona Island, and named as Drosophila carcinophila by M. R. Wheeler. In 1967, a second species of fly, Drosophila endobranchia was discovered on Gecarcinus ruricola, although it also inhabits the closely related species Gecarcinus lateralis. Drosohila endrobranchia is not closely related to D. carcinophila, and this trait appears therefore to have evolved convergently. D. endobranchia evolved from a group of species which breed on fungi or bark, while D. carcinophila evolved from a group which breed on cacti. The flies spend most of their lives on the crab, and are reluctant to leave. They do not need to flee predators, because the crabs they inhabit are fast animals and will flee:
The flies … hardly move at all, are extremely reluctant in leaving their host crabs and are hard pushed to take flight. Although the flies are sluggish, the crabs on which they reside are anything but. Chasing after crabs through a pitch-black jungle, while trying to aspirate flies from their carapaces is not trivial. Obtaining large amounts of flies in this way is simply a nightmare.
The eggs are laid around the crab's compound eyes; the first instar larvae migrate to the crab's nephritic pad, and live there, feeding on the microbes that cleanse the crab's urine. The second instar is spent in the crab's gill chamber. The third instar sees the larvae return to the mouthparts before falling to the ground to pupate.
Linnaeus
described the species in 1758, noting the species' annual migrations from the forests to the coast.
Philately
G. ruricola appeared on two African postage stamps for the International Year of the Ocean in 1998, under the name "mountain crab". These were a Tanzanian stamp worth 500 shillingi and a Ugandan stamp worth 250 shillings.
