Cardisoma guanhumi
blue land crab

Cardisoma guanhumi is found along the Gulf of Mexico and Caribbean Sea throughout the southeastern United States, Central America, the northern tip of South America, and parts of the Caribbean Islands. (anonymous, 2003; Hill, 2001)

Cardisoma guanhumi lives within several hundred meters of the shore, particularly along estuaries and river banks. It burrows in dense shrubbery, mud, or coastal sand above the tide line. Burrows can extend up to six feet underground, and contain small pools of water at the bottom. Cardisoma guanhumi shares its burrow with insects and other small arthropods. The atmospheres within such burrows typically have very high carbon dioxide concentrations because they are often sealed with mud. (Burggren and McMahon, 1988; Hill, 2001; Pinder and Smits, 1993)

Cardisoma guanhumi grows roughly 127 mm wide and 102 mm long, weighing 400 g on average, though males are generally larger than females. As a juvenile, it is typically tan or brown in color with orange legs. As an adult, its coloring ranges from blue to violet, though some females are white or ashy gray. Its body, protected by a smooth carapace, consists of a cephalothorax and abdomen. Connected to its cephalothorax are five pairs of biramous appendages covered with tactile setae, each of which are sturdy and longer than the width of the cephalothorax. At its front are two pairs of antennae and two pincers, one larger than the other. The larger pincer in males can reach 300 mm in length. The difference in pincer size notwithstanding, C. guanhumi is bilaterally symmetric. (Burggren and McMahon, 1988; Hill, 2001; Lloyd, 2001)

Cardisoma guanhumi has a complex life cycle that begins during the rainy season (which varies by latitude) when females spawn their eggs into the ocean, strictly coinciding with the lunar cycle. Once hatched C. guanhumi molts through five larval periods before it undergoes metamorphosis into a megalops, the stage when the appendages first appear. The nest metamorphosis produces the juvenile crab, which closely resembles the adult form. Each metamorphosis is accompanied by a molting of the cuticle. The following molt, named the puberty molt, precedes full sexual maturation. Molting regulates the life cycle of C. guanhumi. Generally, C.guanhumi molts 60 times during its lifespan, compared to 20 times, which is typical of other crab species. After achieving sexual maturation, C. guanhumi becomes primarily terrestrial, returning to the ocean only to spawn and begin the life cycle once again. (Burggren and McMahon, 1988)

The reproductive cycle of C. guanhumi is heavily dependent on weather and the lunar cycle. At the beginning of the rainy season (typically late spring, but varies by latitude), females begin to actively search for a mate while migrating from their burrow to the shore. Major migrations take place prior to full moons; minor ones precede new moons. While males can copulate as much as they wish between molts, females can only copulate several times. (Burggren and McMahon, 1988; Lloyd, 2001)

Cardisoma guanhumi reaches sexual maturity at roughly four years of age. Mating begins during the rainy season when females migrate to the shore--particularly before a full moon and (to a far lesser extent) before a new moon. In preparation for their migrations, females gain substantial weight. Males actively court females and in response, females emit pheromones as well as tactile and auditory signals. Copulation typically takes place outside of burrows, and fertilization is internal. Following copulation, females carry their eggs (which can number anywhere between 20,000 and 1,200,000) for approximately two weeks before spawning them into the ocean. (Burggren and McMahon, 1988; Hill, 2001; Lloyd, 2001)

After fertilization, females carry their eggs on their backs for approximately two weeks. At this point, the eggs begin to hatch, and she shakes them off into the ocean. In spite of its evolution toward terrestriality, C. guanhumi is still heavily dependent on the ocean for at least part of the life. (Burggren and McMahon, 1988; Hill, 2001)

Many giant land crabs do not survive the larval stage. Those who reach adulthood achieve sexual maturity in approximately four years. Unfortunately, there is not much further data available regarding the lifespan of C. guanhumi. However, biologists hypothesize that the lifespan of a species of land crab is inversely proportional to its growth rate. In other words, the faster they grow, the shorter they live, and vice versa. From this, it is probable that C. guanhumi has a relatively longer lifespan than other land crabs, as it grows more slowly and molts three times as much (averaging 60 molts per lifetime as opposed to 20). In fact, the largest female C. guanhumi kept in captivity lived 13 years. Albeit biologists admonish against extrapolating laboratory findings to a species' actual environment, I have given some rough estimates of lifespan below based on given data regarding the lifespans of C. guanhumi and other land crabs. (Burggren and McMahon, 1988)

Cardisoma guanhumi is not a social species, spending most of its time in its burrow when not foraging or migrating to mate and spawn. Burrows range from one to eighteen centimeters in width, extending up to six feet into the ground. At the bottom of each burrow are one to two liters of water. In particularly abundant habitats, there can be up to 7,500 burrows per acre. Cardisoma guanhumi fiercely defends its burrow, and competition for the best burrow location often results in migration away from the shore for the losing competitor. Cardisoma guanhumi typically forages at dawn and dusk, its peak activity time. Cardisoma guanhumi stays within several square meters of its burrow, except when females migrate to the ocean to release their offspring. Midday temperatures keep C. guanhumi burrowed throughout the day. (Hill, 2001)

Cardisoma guanhumi communicates in various ways, but mainly with visual, auditory, and chemical signals. To attract mates, females release pheromones. To orient itself, C. guanhumi depends on polarized light or the light from the horizon at dawn or dusk. It also uses the setae on its appendages for tactile purposes. It is heavily sensitive to vibrations. (Burggren and McMahon, 1988; Hill, 2001)

Cardisoma guanhumi is an omnivore. Although it prefers leaves, fruits, and grasses, C. guanhumi also feeds on insects, carrion, and feces. To forage it typically does not stray far from its burrow and uses light and sound to find food. After foraging, it carries its food in its claws back to its burrow, eats, and saves whatever it does not finish for later. (Hill, 2001)

Due to their size, C. guanhumi is not heavily preyed on. However, it sometimes falls prey to large birds, mammals, and other C. guanhumi. Humans are the largest threat with respect to predation, harvesting giant land crabs in massive quantities for food. Fortunately, it is fairly safe in its burrow from predation. Hence, burrowing not only provides C. guanhumi with shelter, but also protects it from predation. In fact, the absence of a burrow, in addition to physical vulnerability and other factors, explains the high mortality rates of C. guanhumi during the larval stage. (Burggren and McMahon, 1988)

Due to the moist atmosphere within its burrow, C. guanhumi provides a myriad of arthropods with habitats. These arthropods live on its body, but do not seem to harm it, primarily feeding off of left over debris from previous C. guanhumi meals. Because the food habits of C. guanhumi are so eclectic, its effect on its prey populations are, if anything, trivial. However, through preferential feeding, C. guanhumi can alter the compositions of various species of plants by choosing to eat certain plants and seeds over others. (Burggren and McMahon, 1988)

Cardisoma guanhumi is a significant source of food in various parts of the Caribbean, particularly in Puerto Rico and the Bahamas. Harvesters have been known in Venezuela to catch as many as 400 giant land crabs per night. Sold at $30.00 (US) per dozen, this amounts to $1,000.00 per day per harvester in giant land crab revenues. ("Blue Land Crabs Draft Rule Review", 2002; Hill, 2001)

While the adverse affects on humans of C. guanhumi are minimal, it is sometimes deemed a garden pest for digging burrows in lawns and crop fields. In addition, it can occasionally cause ulcers and lesions to those who eat it when it has previously consumed poisonous fruit. Lastly, the pool at the bottom of the burrow provides a breeding ground for mosquitos that may potentially vector diseases such as malaria and subperiodic filariasis. (Burggren and McMahon, 1988; Hill, 2001; Lloyd, 2001)

Although C. guanhumi is not endangered, there is concern regarding its harvesting. It has been exploited excessively in the Caribbean for food. To combat this, Puerto Rico enacted strict regulations and Florida is currently considering similar legislation. ("Blue Land Crabs Draft Rule Review", 2002)

Cardisoma guanhumi is fundamental to the cuisine of many Caribbean nations as well as Florida and Venezuela; however, there is much concern regarding its excessive harvesting. ("Blue Land Crabs Draft Rule Review", 2002)