Nature Connection: Feeling Crabby?

The heat and humidity of midsummer can make anyone feel a little crabby. But what about the crabs that live here?

Crabs are crustaceans and cannot regulate their own temperature, being exothermic. We used to call them cold-blooded, but that wasn’t an accurate description. It’s not that they have cold blood, but that they can’t maintain an internal temperature independent of their environment. 

Exo, meaning outside, describes animals that can only absorb heat or cold through their bodies. We humans are endothermic, meaning we are able to maintain a constant body heat in spite of outside temperatures, hot or cold. 

This means that crabs and other exothermic animals must find ways of staying cool or warm in their environments or they will die of extreme cold or heat.

Although there are land crabs in other places, the crabs we find on Cape Cod are marine crabs, living in water, and in our case, salt water. Many crabs burrow into the sand to stay cool while others hide under rocks, seaweed, and other shaded spots.

Depending on where you are on the Cape, you will see different species of crabs. Each has a preferred habitat and food. Some prefer the colder water of the bay while others like the warmer waters of Nantucket Sound. The Outer Cape offers some real challenges for crabs, and the species there tend to stay farther out, away from the tidal movement during tide changes.

On the south side of the Cape that fronts Nantucket Sound there is a wide variety of crabs, including the blue crab, calico crab, spider crab, mole crab, fiddler crab, and hermit crab. Some of these can also be found on the north side, especially as the water in the bay warms. The purple marsh crab is a native that is causing a stir in area marshes due to its plant-eating tendencies. Once controlled by predators such as bass and other crabs, its population is growing, devastating some marsh areas by stripping them of the grasses that stabilize them.

Invasive crabs such as the Asian shore crab, which is found all over now but especially on the bay side, are disrupting age-old patterns. The green crab, another Asian invasive, has become so common here that many think it is native. Both these crabs are causing a ruckus in our fisheries, and figuring out how to control their populations is an ongoing, often frustrating, task.

All our crabs are decapods, having ten legs. These include two claws and in some cases, two swimmerets in the back. If you compare the back legs of calico and blue crabs with green crabs you will see that the back legs of the former two are flattened to help them swim. Green crabs are not swimmers so don’t need this adaptation.

Most crabs walk sideways, and all have gills. They must have moisture in order to breathe. They can be out of water for a brief period but must stay wet if they are to survive. Crabs can’t bite you due to their tiny mouths, but some can give you a wicked pinch. Be careful when walking the sand flats at low tide, because if you step too close to a buried crab, it will let you know. This is also true if you’re walking in shallow water.

Crabs won’t run after you to attack but they will respond to what they may feel is an intrusion into their personal space, i.e. your toes in their face. My dad always said if I wanted to find a crab, just wiggle my toes. 

Many crabs are edible, such as the blue and Jonah crabs. Both can be a challenge to catch and can give a very strong pinch. In fact, if one gets hold of you, it may not be in a hurry to let go. In his classic book “The House on Nauset Marsh,” Wyman Richardson tells a tale of fishing for blue crabs in Nauset Marsh that is the best and most hysterical description of trying to catch blue crabs without losing any body parts that I’ve ever read. The whole book is a treasure, by the way.

All Cape Cod kids are familiar with hermit crabs, the little guys without their own shells. They are often found in periwinkle shells and can be found at almost any beach. 

Mole crabs used to be quite common here. They are little, about two inches long, and as their name suggests, they live underground. My sister and I used to dig them up while making sand castles. You can still find them on some south-facing sandy beaches.

Spider crabs are sort of creepy looking and can get quite large. They don’t have big pincers, in spite of their long legs and claws. Their claws are for picking up things like seaweed and little goodies from the sand, not attacking or defending.

Fiddler crabs are unique in our area in that they live in colonies, often very large colonies. If you come across a lot of holes in the sand with little balls of compressed sand by the entrances, you’ve found fiddler crab burrows. Male fiddler crabs carry a big claw, but it isn’t a pincer. Rather, it is a means of attracting a mate and at the same time warning off competitors or predators.

While you’re out and about at the beach this summer you may find the shed shells of crabs that have outgrown them. A dead crab has weight. The shell is light and very fragile in many cases.

As for horseshoe crabs? Alas, they aren’t real crabs, but of course they are still very cool.

If you are interested in collecting crabs for a meal, please check with your town for regulations, as they may vary. In the meantime, see how many species you can find.