Wildlife and Science

Poetry is an Eight-Legged Shuffle


PUBLICATION: The Beach Times, Costa Rica, print edition


The migration of the land crab appears quite poetic.

Each year thousands of magenta crustaceans descend to the streets, shiny backs glistening, where in tap-dance-like shuffles they cross towards the sea.

They often journey in early morning to avoid heat and cash in on the moisture, a trend that casts complimentary light on their backs. Adding to the enchantment, scientists confirm it is the moon that calls them seaward—a functional utility of the tides that triggers a primitive instinct.

With all this verse, one would never guess this magnificent decapodal exodus is in fact a swarm of pregnant (or rather fertilized) females racing to liberate their young.

And no small race at that. The females, who carry between 300,000 and 700,000 eggs, generally journey for two weeks, often for 12 hours per day. In other global regions have been known to travel five kilometers in five days.

While this 5000-meter-stretch may seem insignificant to a six-foot-tall human being, the trip for a ground-dwelling creature of ten to 20 centimeters in length, is nothing short of a Homeric odyssey. Many females die along the way.

According to University of Costa Rica biologist and crustacean expert Dr. Ingo Wehrtmann, Costa Rica’s Pacific coast has three species of land crab: the blue mangrove crab (Cardisoma crassum), the red land crab (Gecarcinus planatus,) and the Halloween crab (Gecarcinus lateralis or Gecarcinus quadratus), also known as the moon crab.

There are actually two migrations, he says, one beginning in late April (when females head to sea) and another when juveniles return to the forest. The first marks the beginning of the rainy season and the second sees it out.

According to experts, hatching cycles mimic the moon because of its affect on the tide. When it is full, for example, tidal amplitudes—how much lower or higher they are than normal—are especially wide.

“The hatching larvae can quickly be dispersed by a strong current away from the dangerous inter-tidal area with all the predators,” says Wehrtmann.

According to a 1998 study of Christmas Island crabs near Australia, the moon also affects the speed of migration.

“Red crabs must walk to the coast in time to synchronize their breeding with the lunar cycle and, consequently, must walk a pre-determined distance in a fixed period,” reads the article in the Journal of Experimental Biology.

According to the study, the crabs will move very fast and then rest for intervals.

Radio-tracking studies have shown their average speed is almost their maximum aerobic speed (MAS) and field observations indicate they pause for minutes or even hours between periods of “higher-speed walking.”

The creatures, Wehrtmann says, are not considered marine (though they do have gills) but rather terrestrial species. They head to the ocean only for the purpose of reproduction.

“Only through the larvae are they connected to the marine ecosystem,” says Wehrtmann.

This is one of the things that makes the crabs unique, the doctor says, noting that less than five percent of all decapods ever spend significant time outside the ocean. Even less, he adds, are exclusively terrestrial.

Unlike the ocean-dwelling ghost crab—which burrows into the sand to hibernate six months a year, holding its breath via stored oxygen—land crabs spend the rest of the year digging holes in nearby forests and mangroves.

Wehrtmann says this plays a critical role in those ecosystems in two ways: First, they are mainly plant-eaters and distribute seeds throughout the forest, adding to its composition. Secondly, the holes they burrow bring oxygen deeper into the soil and improve sediment quality.

Other biologists agree.

“Burrowing animals can also be important ecosystem engineers,” says Peter Green in a 2004 article in the Journal of Crustacean Biology. “On Christmas Island, red land crabs may cause local nutrient enrichment in the soil by gathering leaves around their burrow entrances.”

Yet despite their contributions, the crawly critters remain highly ignored in the field of research, where marine and freshwater counterparts take center stage.

A Google search on Costa Rica’s ocean crab Ocypode quadrata, for example, yields 17,600 hits, yet the same search for the land crab Cardisoma crassum sees a mere 502.

“Nobody knows exactly what their life cycle is, nobody has seen the larvae,” says Wehrtmann.

There are several theories, he says, one of which is lack of a solid industry. Even though similar species are caught and eaten in Brazil, in Costa Rica they are of little interest.

Likewise, they are not endangered, which limits research funding. Their terrestrial nature makes them difficult to study and most research now available, he says, has been conducted by foreigners.

The biggest threat they face is traffic, which kills hundreds every year. Wehrtmann says in some countries like Cuba, for instance, tunnels have been set up to protect them.

“The idea is to facilitate the migration,” he said. “In Germany there are a lot of tunnels for frogs who are also migrating.”

Like all crustaceans, the crabs grow through a process called molting which Wehrtmann likens to “changing clothes.” In this, the entire mid-section, or carapace is shed. They remain soft for about 24 hours while a new center grows.

This is why, according to Wehrtmann, in the United States and other places “soft crab” is found on some menus.

In many crab species, one of the front pincers (called chelipeds) is larger than the other. The phenomenon is sometimes seen in Costa Rican land crabs too, but not uniformly, and not enough information is available to know why.

Wehrtmann says in other species it can be for cracking tough mollusk shells or used for defense. In the case of the famous fiddler crab of West Africa and the Atlantic, the feature is found only in males and linked to courtship.

“Usually the big claw is for manipulating the food or for impressing the female,” he says, reiterating that in Costa Rica there is “no clear pattern.”

At present, perhaps the most valuable industry for the crabs is their consistency in predicting the rains. From Panama to Nicaragua, they are known as messengers of the changing season.

Sarah Pfieffer of Hotel Sugar Beach near Playa Potrero says the land-crawlers announce the beginning of the rainy season each year like clockwork.

“They seem to get into spaces where you can’t find them,” she says. “They find their way into the rooms. It’s not all that funny when you have 14 of them dancing on the tile floor.

“When they are here it’s like the walkways are vibrating.”


This article was originally published in The Beach Times. Guanacaste, Costa Rica. Photo with permission from Pixabay.