Migrating crabs take their eggs to the sea | Science News for Students

Migrating crabs take their eggs to the sea

This yearly spectacle of marching crabs could help Cuba preserve a critical swamp and coast
Mar 15, 2018 — 6:15 am EST
crab march
Invasion! A highway along the Bay of Pigs is covered with skittering land crabs on their way to lay eggs in the sea.
Allan Hopkins

PLAYA LARGA, Cuba — When Cuba’s dry season ends and the spring rains start, strange creatures begin stirring within the soggy forests of Zapata Swamp. Rain here, along the country’s southern coast, means romance for land crabs. After they mate in underground burrows, the red, yellow and black females emerge by the millions. Then they scuttle toward the ocean to deposit their fertilized eggs in the water.

Some observers have compared the waves of skittering crabs to scenes from a horror movie. The bizarre mass migrations, though, form an important link in the coastal ecosystem here. The crabs, after all, are a welcome source of food for other animals, both on land and at sea.  

So many of the ten-legged creatures appear at dawn and dusk that they can turn roads and beaches red. They also can puncture the car tires of unlucky drivers. A few weeks after the yearly invasion, broken bits of shell and crab legs still litter the main highway by Playa Larga. The crabmeat is toxic to people. But scientists are finding that other animals love it.

Cuban land crab
On guard! A close-up of a feisty red land crab on its way from the Zapata Swamp to the Bay of Pigs in Cuba.
Charlie Jackson (CC BY 2.0)

This crunchy land crab is sometimes on the menu of the critically endangered Cuban crocodile. Orestes Martínez García, a local bird watching guide and researcher, points out another important predator. Two Cuban black hawks have built a nest in a tree next to a coastal highway. Like the crocodile, the hawks are unique to this island country. A male stands guard on a branch while his female mate incubates eggs in the nest. It’s the perfect perch from which to swoop down and feast on the crabmeat. Even better, many of the flattened crabs have already been shelled.

Once they’ve carefully released their eggs into the ocean, mother crabs turn around and skitter back to the swamp. In the sea, a feeding frenzy now ensues. Mullet and other fish in the shallow reefs gorge on the tiny crabs that hatch from the eggs. The baby crabs that survive their first few weeks adrift will clamber out and join adults in the nearby forest. Eventually, some of them will make the same journey back to the ocean.

Despite being pounded into crab cakes by the thousands, Cuba’s population doesn’t seem to be in immediate danger. Officials close the highway and other streets to protect the crabs (and car tires!) during peak crossing times.

Even so, scientists warn that building too many homes and businesses nearby could reduce the crabs’ habitat. Hotels or other barriers could prevent the adults from reaching the ocean or keep their babies from returning home. Scientists have documented this threat on other Caribbean islands. They warn that more development also could increase the harmful pollution flowing into the swamp and ocean.

Some tourists come to see the odd spectacle of the crabs’ march to the sea. Others come to view the local crocodiles, birds and corals. These visitors have been good for Playa Larga, Martínez García says. The popular attractions mean that area residents have incentives to help preserve the swamp and sea around them. In doing so, they may help ensure that the weird and wondrous land crabs will feed other creatures far into the future.

Land crabs invade the Bay of Pigs on their journey to the sea.
Reuters/YouTube

Power Words

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Caribbean     The name of a sea that runs from the Atlantic Ocean in the East to Mexico and Central American nations in the West, and from the southern coasts of Cuba, the Dominican Republic and Puerto Rico down to the northern coasts of Venezuela and Brazil. The term is also used to refer to the culture of nations that border on or are islands in the sea.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

development    (in economics and social sciences) The conversion of land from its natural state into another so that it can be used for housing, agriculture, or resource development.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

egg     The unfertilized reproductive cell made by females.

endangered     An adjective used to describe species at risk of going extinct.

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

incubate     Sitting on eggs to keep them at the right temperature until they hatch.

litter     Material that lies around in the open, having been discarded or left to fall where it may. (in biology) Decaying leaves and other plant matter on the surface of a forest floor.

migration     (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.

playa     A flat-bottomed desert area that periodically becomes a shallow lake.

population     (in biology) A group of individuals from the same species that lives in the same area.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

reef     A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

resident     Some member of a community of organisms that lives in a particular place. (Antonym: visitor)

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

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