Getting to know three … Malacostracan edition

Ever know instinctively that some animals are ‘related’ and just can’t pinpoint their similarities? On the third day of every month I explain three features that are common among three animals of a certain group. Of course, generally each group has more than three representatives and even  many more similarities and then even more differences, but I am going to choose three similarities that link threes to keep it simplified. This month is focused on the shrimp, the lobster, and the crab – all crustaceans, but more specifically all are members of the largest of the six classes of crustaceans known as the Malacostraca class. (Need a refresher on the trusty mnemonic device for classification? Click here.)

Check out ‘Getting to know three … Echinoderm edition’.

Holy, mole(y) crab!

Mole crabs are recognizable decapods with their barrel-shaped bodies and a grey colored exoskeleton and known to many of us as ‘sand fleas’. We often only see them for a wee bit when a wave (or curious hand playing in the sand) dislodges them and they quickly scurry back under the sand. These clawless crabs are not biting fleas and are fairly harmless member of the crustacean group.

Let me tell you what makes the mole crab a critter I can get excited chatting up during a day  at the beach. First, is that as the tideline goes in and out during the day, so do the mole crabs. Their preferred home is to be buried under the sand right at the tideline. Also, rather fascinatingly, mole crabs only move backward! They use their back legs to bury themselves under the sand as their head goes later.

They depend on the action of the ocean to filter them loose plankton and organic debris. They grab the particles by keeping their antennae at the sand surface (which is why their head goes in last as they bury themselves). They also use gills to take oxygen out of the water to breather, just like other crustaceans. During the summer you may see a female carrying one with eggs on her underside or near the legs. The orange eggs have just been fertilized while the darker grey eggs are ready to hatch. Male mole crabs are smaller than females, only reaching about half an inch in length, while females are typically an inch long.

Mole crabs are also a very popular form of bait for surf fishermen.  There are ten species recognized throughout the world, but two are most common on the eastern Atlantic and Gulf coasts, Emerita talpoida or Emerita benedicti.

Image (c) NJ SeaGrant

Why do I always see so many dead crabs along the shoreline?

Rest assure those crab skeletons are not all dead crabs. They are the molts from the animals. Crabs, lobsters, horseshoe crabs, and many other crustaceans go through a molting phase and the old shell is basically washed up in the wrack line.

The wrack line is the deposits from the ocean after the tide has gone back out to sea. It’s often defined by seaweed that entangles lots of fun ocean treasures such as sea beans, old leathery sea turtle eggs, and sometimes marine debris. It’s my favorite spot to explore!

Do you have another great question? Email and let me know what you always ponder while digging your toes in the sand!