A blue crab found on the east end of Sanibel.

A blue crab found on the east end of Sanibel.

by Johnny Rader

Recently many crabs have been washing up on our beaches, so today we want to tell you a little bit more about the species we often find at our feet.

Many of the crabs you might see in the wrack line (the line of debris left on the beach by the high tide) are swimming crabs. Although some people refer to all of them as “blue crabs”, this is a misconception. There are eight types of swimming crabs found in the Gulf of Mexico.

All swimming crabs are members of the family Portunidae, and all have a pair of modified legs that are flattened, paddle-like swimmerets – these help the crabs move through the water. Many also have diamond-shaped bodies and large, powerful claws.

An iridescent swimming crab.

An iridescent swimming crab.

Swimming crabs grow quickly, which has helped them gain status as one of the top benthic predators. As opportunistic animals, they will actively hunt live animals if scavenging for dead things is unsuccessful.

As swimming crabs grow, they molt their exoskeleton periodically, then re-calcify the exterior of their body to create a new shell. This is where the term ‘soft shell crab’ comes from – it is the stage directly after molting occurs, when the body is still soft and unprotected. Swimming crabs in this stage are gathered and enjoyed as a delicacy in many parts of the world, including the east coast of the United States.

There are many reasons why swimming crabs might wash up on our beaches. During the summer of 2018, many swimming crabs were observed crawling out of the water or swimming at the surface.

The most widely accepted explanation is that the rotting carcasses of animals killed by red tide house bacteria that consume much of the available oxygen, creating anoxic water conditions. When no oxygen is available in the ocean, the crabs seek oxygen on land, where they eventually dry up and die.

During normal times (when there is little or no red tide present), crabs are often found dead on our beaches due to cold water temperatures. Thermoregulation is difficult for invertebrates, and in cold weather they become lethargic and are often washed ashore.  

Here is a list of the known species of swimming crabs found in the Gulf of Mexico:
speckled swimming crab, blue crab, lesser blue crab, ornate blue crab, Florida lady crab, iridescent swimming crab (pass crab), sargassum crab, blotched swimming crab