Location: Intertidal marshes of Jacksonville and Fernindina, Florida
Oceanic Region: Intertidal barrier islands near Jacksonville

The Illusive Great Land Crab

Figure 1 The great land crab, Cardisoma guanhumi.

The lumbering land crab (Cardisoma guanhumi) is not easy to find at the limit of its range in north Florida, where the water temperature can dive consistently below 20^oC in the winter months. The crab is nocturnal and can be found poking and digging around in the glass worts at night or on a very cloudy, blustery day at ebb tide. Its habitat usually includes glass worts (Salicornia sp.), salt meadow hay (Spartina alterniflora), sweet bay, (Magnolia virginiana), youpon (Ilex vomitoria), fiddler crabs (Uca sp), and an occasional raccoon.

Procyan lotor
This is a beautiful 13-cm crab, as crabs go. Its color is blue to violet in adults; the young are tan. The carapace is smooth except for a fine ridge or line along the sides. The legs are long and somewhat covered with tactile hairs (Figure 1).

I have happily photographed this crab as far as 2 km from the ocean as "the pelican flies," but never far from the salt marsh. One usually hears this beast walking around before ever seeing it.

The great crab, which weighs over 1.1 lb (500 grams) at 11 cm, looks very much like a giant form of the fiddler uca. Its eyes are on stalks. Male crabs have one enlarged claw (cheliped). The "great one"; gathers and eats leaves and fruit, which are taken to its burrow, but the land crab is omnivorous, feeding also on carrion. Burrows are large, up to 18 cm wide, and usually intersect the water table at low tide, at a depth of up to 2 m. A pool of water is found at the bottom of each burrow. The great crabs may migrate at a rate of 500 m or more in a single evening using polarized light during the day and the illuminated horizons at night. An adult crab reaches sexual maturity in 4 years, which requires as many as 60 molts [ecdysis periods]. During each molt, the crab seals itself inside its burrow with mud from its habitat for a week or more. During this molting season, the male and female crabs rendezvous in the male's burrow after a courtship vigil. Females carry egg masses for about 15 or more days until the larva are ready to hatch. The female then migrates to the ocean to spawn her eggs. Lunar periodicity controls spawning, and the entire process occurs close to a full moon throughout the late summer in the tropics. In Florida, spawning may continue through Christmas day.

The amphibious crabs have become terrestrial, but have to return to the ocean to complete their life cycle. The mother crab carries the young larva [protozoea] until she spawns in the ocean waves by shaking her babies off in the surf. Each female may produce around three-quarters of a million eggs per spawn.

Larval development tolerates a wide salinity range, but only a limited temperature change. The adults are more tolerant than the young, and temperature may play a very important bruiting role in north Florida. In the Jacksonville salt marshes, these crabs can be seen in the climax oak forests that border the salt marshes to the west. Fernandina, at the south end; Ft. George Island, near the federal park; and Kingsly Plantation are havens for these crabs.

When the land crab was tested in the laboratory, it was discovered that this amazing crab was able to maintain its oxygen concentrations at a constant rate both in and out of the water. Apparently this is accomplished by changing the ventilation rate.

The next time you are in the salt marsh late in the afternoon or evening at low tide, keep your eyes open (and bring your favorite mosquito repellant). Look around the sweet bays and youpons for holes. Foraging raccoons or busy armadillos may not have dug all of these holes. (Dasypus nineringus) At the bottoms of some of these personally engineered holes, you may find the "great one"; with a claw full of muddy leaves sitting beside its below-ground swimming pool of brackish marsh water.

Unfortunately for the great land crab, it is considered a highly prized food in the Caribbean, where it is often thrown in stew pots. Sometimes the crab has the last word, however. When it feeds on the poisonous fruit of the Manchineel tree, it can cause ulcers and lesions to those who eat it.

Key Principles

1. Filling marshes
2. State mitigation of marsh and wetlands is a good policy and is being enforced in most areas.

Author
Roger M. Lloyd
Department of Natural Sciences
Florida Community College at Jacksonville
South Campus
Jacksonville, Florida 32246