The latest article in our series about native Cozumel wildlife looks at the life of the island's Blue Land Crabs.
These alien looking creatures spend much of their life in holes around Cozumel's mangroves, so they are often only seen when they make their spectacular mass migrations.
Cozumel Blue Land Crab.
As the migrations demonstrate, they are clearly land crabs, but their name can be confusing as they are not always blue, and there is also another type of blue crab that lives in the sea.
Dr. Bjorn Tunberg, Marine Ecologist at the Smithsonian Marine Station in Fort Pierce, Florida, says they should really be called "Great Land Crabs", noting that they're often given the wrong name in the United States too.
To locals in Cozumel, they're simply known in Spanish as cangrejos azules - blue crabs - but perhaps to avoid mistaken identity we should also use their scientific name, Cardisoma guanhumi.
Spectacular mass crab migrations.
More fascinating than the disagreement over the crab's name is their reproductive life cycle.
Although they are adapted to living on land, the females still have to deposit their eggs in the sea, where they develop into larvae and then later into miniature baby crabs.
The need to reach the sea causes the amazing sight of their mass migrations in late summer in Cozumel, when hundreds - or even thousands - of females head for the ocean with their undersides covered in eggs.
There is also another big crab exodus earlier in the year, which coincides with the island's first summer rains, usually in June, but biologists are in less agreement as to why this one takes place.
The crabs prefer to make long journeys when there is rain.
Dr. Luis Mejía, at the local state University of Quintana Roo, thinks the earlier annual population movement happens when males look for females to mate with and young crabs come of age and leave the sea to move onto the land.
However, while Dr. Tunberg agrees it may be related to mating behaviour, he is not so sure about the movement of the young crabs.
"We see juveniles very far inland during these migrations, and it is impossible that the have migrated this far from the sea so fast," he says.
What is certain is that the crabs prefer to make long journeys when there is rain, this is because they still have gills - a little like fish - and need to keep them moist.
Adults tend to be blue or gray and younger crabs are a reddish color.
When the females are carrying eggs, wet weather also helps stop them drying out.
The same species of crab can be found in coastal areas all around the Caribbean, as well as the Gulf of Mexico and parts of South America.
Colors vary depending on regions and diets, but the adults in Cozumel tend to be blue or gray.
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.
Adult males, and often older females, have one claw larger than the other.
The younger crabs are smaller and usually a copper-reddish color, sometimes quite bright in tone.
In other parts of Mexico, the Caribbean and the Americas they are considered a pest or caught for food, leading to their scarcity in some regions.
However, in Cozumel they are rarely eaten and it's more common to see them in great groups or scurrying in and out of mud holes, rather than on a diner's plate.
Look for Blue Land Crabs on our Lagoon Nature and Birdwatching Tour.
Photos courtesy of Casa Viento guest house.
Island Wildlife: Pygmy Raccoons.
Island Wildlife: Crocodiles.
Island Wildlife: Iguanas.
Lagoon Nature and Birdwatching Tour.
Northern Lagoons Photo Album on Facebook.