Summer Migration: Tropical Land Crabs
Tara A. Spears
With the summer rain comes the hoard of blue land crabs along the 
Riviera Nayarit coast. Since their main habitat is inland jungle forests within five miles of the ocean, most of the year these low-slung critters are not that visible. But, the life cycle of the land crab (terrestrial crabs of the family Ocypodidae) is influenced by the full moon and controlled by the sea.
Land crabs migrate from the jungle to the ocean to wash their eggs during the Mexican summer rainy season, then migrate back to their land territory. That’s why you see so many of the crabs in the summer. These alien-like terrestrial crustaceans have ecological importance as de-composers of dead organisms besides being a human food source.
Land Crab Life Cycle in photos: Adult female blue crab heading to sea; giving birth in the ocean.
Blue land crabs reach sexual maturity in approximately four years. Their peak reproductive activity occurs during full moons in the summer. After mating, an adult female lays her eggs but carries the egg mass beneath her body for approximately 2 weeks prior to 
migrating to the ocean and releasing the eggs into shallow inshore waters. A female may produce 300,000 – 700,000 eggs per spawn, but very few larvae survive to become small crabs. The larvae are eaten by fish and other aquatic animals.
The spawning season (female migration to ocean) lasts from June to December, peaking in October and November. Once the eggs hatch, the tiny larvae are carried out to sea. The development stage (from larvae to small crab) takes about a month out in the ocean; during that time the currents can carry a concentration of larvae quite far before depositing young crabs on another shore. This phenomenon sometimes seems as if an invasion of young land crabs has occurred in an area.
The adult male Blue crab (left) displays the asymmetric claws that indicate sexual maturity. In the juvenile phase, the male and female crabs look alike. Land crabs grow by moulting, a process of shedding their hard exoskeleton; the crab absorbs water internally to crack the old shell.
Crabs are usually aggressive towards each other, and males often fight over females. Social and mating behaviors are complex. Many make species – specific sounds by banging on the ground with their pincers or stamping with their feet, or make elaborate pincer movements. Mating occurs only when the female has just molted 
and her new shell is not yet hard; the males, however, are in their hard-shelled form. Thus males may protect a female just before she molts, so he can have first access when she is able to mate, and may continue to protect her till shell hardens. Some males even carry the female around. Usually the eggs are laid shortly after mating, but females can also store sperm for many months. The eggs are fertilized as they pass through the chamber holding the sperm. In most, the females carry their eggs cemented to their underside and protected by the tail, which is folded under the body to form a brood pouch.
This animal can measure up to 6 inches (15cm) across its carapace. In its juvenile form, the crab is a dark brown, purple, or orange in color. As an adult, it is a bluish-gray color. The adult female sometimes appears light gray or white. The reproducing, mature land crab has one claw that is larger than the other, and the walking legs are sparsely hairy.
As adults, the blue crab lives in burrows several feet deep or at least to a level that will allow water to seep in for moisture. The blue land crab is primarily vegetarian, preferring tender leaves, fruits, berries, flowers and some vegetables. Occasionally they will eat beetles or other large insects.
Land crabs have sensitive eyes. During the day they use light and at night they identify the brightest part of the horizon. They also use vibrations, landmarks and prevailing winds to guide them to the sea. It does not depend on its eyes alone, for it senses motion and sound. They can rapidly move in any direction. Their hard pinchers, used for tearing food and protection, will regenerate if torn from its body.
It’s an exotic sign that the tropical summer has arrived when the land crabs move en masse. Like the sea turtles, summer is the time for love and regenerating the species.