Rainy Season Brings the Mexican Land Crabs

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Rainy Season Brings the Mexican Land Crabs       

Tara A. Spears

Hundreds of low slung Mexican land crabs are side-stepping on their journey from the mountain jungle to the ocean to mate. They prefer to begin their annual migration in the rain. These brightly colored crustaceans flaunt shell colors ranging from deep purple to bright blue to orange to grey with orangey-yellow legs, hence their common name of Harlequin Land crab. The coloration varies by region and diet, with the Jaltemba Bay land crabs a more muted blue shell with light orange legs. These nocturnal, burrowing crabs are important to the coastal neotropical ecosystem.crab 2

The Mexican Land crab-simply known in Spanish as cangrejos azules- can be found along both the Pacific and Atlantic coasts south to Peru. Adult males can usually be recognized by their lopsided appearance, as they have one claw much larger than the other, although often older females have this too. The overdeveloped claw indicates sexual maturity and the ability to breed.

The Mexican Land crab’s spawning season (female migration to ocean) lasts from June to December, peaking in October and November. Of course, where the ladies go the males will follow. The early life of the land crab takes place in the ocean with the baby crabs leaving the water only during a full moon to migrate inland: This phenomenon sometimes seems as if an invasion of young land crabs has occurred. I’ve seen super tiny MLC that are no bigger than a tick and I have also seen MLC (inside my house) as large as a floor tile!

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The photo on the left show a swarm of Mexican Land crabs scaling a concrete wall. The young crab’s journey is trial and error: sometimes they fall into swimming pools or dog water dishes; sometimes they erroneously get trapped by a house that blocks the path that their instinct sets them on. Many of these one inch (2.5 cm) critters become the prey of other shore predators, such as herons and egrets, which will stab blue crabs with their sharp bills before picking out the meat. Anhingas, diving ducks and other birds that fish underwater are also predators. Raccoons will snag a blue crab if one is unlucky enough to wander by on land.  Adult MLC will remain on land throughout their lives, which can be up to ten years.

The blue MLC is primarily vegetarian, preferring tender leaves, fruits, berries, flowers and some vegetables but occasionally they will eat beetles or other large insects. Mature female crabs are called a “Sook” and have a dome or bell shaped abdomen.  Male crabs are called a “Jimmy” and have a T-shaped abdomen- study to photo to note body differences.

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The Mexican Land crab has sensitive eyes. During the day they avoid light and at night they identify the brightest part of the horizon for orientation. They also use vibrations, landmarks and prevailing winds to guide them when they migrate to the sea. The MLC does not depend on its eyes alone, for it senses motion and sound. Surprisingly, the crabs can move in any direction, quickly when they feel threatened.  Their hard pinchers, used for tearing food and protection, will regenerate if torn from its body. 

It always delights me to see these alien-like terrestrial crustaceans scurrying through the moonlight; another summer is here.

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