Feeling Crabby?

By Judy Drew Fairchild

It’s difficult to feel crabby in a place as beautiful as the Lowcountry, and yet there are a lot of crabs! When walking the beach at sunset there are crabs scurrying along the sand. Tiny hermit crabs in tidal pools means spring is definitely here. And everyone is fascinated with horseshoe crabs. But have you ever wondered more about the crabs that catch your eye?

Ghost Crabs

Island-Magazine-Pawleys-Crab-1

Ghost Crab

Beachgoers in the early morning or late evening are most likely to see this fast-footed crab. Perhaps called ghost crabs because they are nearly transparent and appear to glide across the beach, they live out of the water most of the time, although they make a quick dash to the ocean at least twice a day to wet their gills. If you’re lucky enough to spot one, they are fascinating creatures to watch.

Like other crabs, ghost crab eyes are on stalks that extend way up over their carapace, giving them 360-degree vision and the ability to spot avian predators. They can retract the eye stalks in order to maintain a lower profile in the sand to escape predators. However, when ghost crabs are in the ocean, the most visible part of them is the eye stalks, which can often be seen extended above the water’s surface as the rest of the crab hunches down in the waves.

Ghost crabs live above the high-tide zone in burrows they excavate themselves. Burrows can be more than 4 feet deep, and they end in a chamber. The chamber stays open because the sand at that depth remains wet enough to keep its shape. Ghost crabs are so light on their feet that they can scamper quickly across the harder sand without leaving tracks except for the places they dig.

When they are out, ghost crabs stop regularly to scrape around for coquina clams and moon snails just beneath the surface of the sand and often investigate garbage or other disturbances on the beach. In fact they have been known to haul garbage back to the entrances of their burrows. One of their favorite meals is sea turtle eggs or hatchlings. And if you see the remains of a bunch of coquinas in one spot, it’s a pretty good bet a ghost crab pinched the small clams to break their shells and have a hearty dinner.

Horseshoe Crabs

These fascinating creatures don’t look much like other crabs and are in fact more closely related to spiders and scorpions. Their name originates from the fact that their carapace, or shell, is shaped like a horseshoe. Horseshoe crabs are incredibly successful, prehistoric animals that existed before dinosaurs and have been around for 445 million years.

The full moons in April, May and June are spawning times for horseshoe crabs. Females come ashore above the high tide line to lay several nests. The eggs are buried in the sand and within them, the baby crabs will grow and molt four to five times. Two weeks later, when the new moon creates an above-average tide again, the tiny crabs hatch and return to the sea.

Horseshoe crab eggs are a vitally important source of food for many migratory birds, such as the red knot, a tiny shorebird that usually winters in Chile and nests in the arctic. These birds are known to time their rest stops in places where horseshoe crabs are spawning.

As strange as it may sound, horseshoe crabs are valuable to the medical research community. Their blue blood is bound by copper and contains a clotting agent that can be used to detect toxins and impurities in pharmaceuticals. Medical testing labs harvest blood from horseshoe crabs and return them to coastal areas with similar salinity.

Their Latin name, Limulus polyphemus, refers to the fact that they appear to have a single eye. In fact, they have nine eyes, some simple and some compound. There are also light sensing organs in their telson, or tail. And while the tail looks a bit menacing, it does not have any sort of barb or stinger and is mostly used to help flip the animal upright if it gets turned over accidentally.

Many other marine organisms make their homes on the carapaces of horseshoe crabs. Barnacles, slipper shells, leeches, sponges and other invertebrates use horseshoe crabs as a kind of slow-moving piece of real estate that provides them with stability and secondary locomotion. Often when you find a horseshoe crab carapace, there is evidence of these hangers-on!

Fiddler Crabs

Island-Magazine-crab-4

Fiddler CrabsPhotograph by Judy Drew Fairchild

Fiddler crabs are most often seen on the mud flats of outgoing tides in Lowcountry salt marshes. There are three different species of fiddlers: sand fiddlers, mud fiddlers and red-jointed fiddlers, depending on whether they are on sheltered beaches, mud flats or low-salinity brackish marshes, respectively.

They live and forage in large groups, digging burrows into the sediment. Burrows can be as deep as 2 feet, and the crabs will retreat into the burrows for resting or shelter, plugging the entrance at high tide to keep the water out.

Females have two small claws used for feeding, while males have one large claw and one small feeding claw. On warm days in the spring, you can see lots of males lined up near burrows, waving their large claws. This is a form of communication to attract females or engage in ritual combat with other males. The waving action looks as if the crab is playing the fiddle, which is how they got their name.

When fiddlers dig their burrows, they leave large pellets outside on the ground, which you are likely to see if you stop to examine one. Along with the larger pellets from excavation, you may also see smaller pellets or balls. Fiddlers feed primarily on detritus (microscopic particles of decaying organic material) that they extract from mouthfuls of sand or mud. Once they have gotten their meal, they very neatly ball up the remaining sand or mud and throw it away!

Hermit Crabs

Like fiddler crabs, hermit crabs have one claw larger than the other, although the difference is not as pronounced. They also eat detritus and carrion. What makes these crabs unique is the way they occupy empty sea snail shells as a portable home. The hermit crab abdomen extends into a tapered spiral that provides a hooked appendage capable of holding the crab in place in the shell. Like other crabs, hermits molt and increase in size as they grow, but as they do that, they seek larger and larger shells to provide their homes.

Finding and keeping the right “house” is very important to a hermit crab! If a crab is too big for its shell and can’t retract into it enough to be safe from predators, it might quickly become another animal’s meal. And those in shells that are too small can’t grow as fast. As soon as a larger hermit crab leaves its shell for another one, there is often a smaller crab is waiting to move in. Two pairs of walking feet enable hermit crabs to carry a shell up to five times their body size!

There are several types of hermit crabs in Lowcountry waters. Flat-clawed hermits use their asymmetrical claws to completely close off the opening to their shell, just like the original snail would have used their operculum to close off the opening. They also play host to a variety of other organisms – slipper snails will attach to the outside of the shell, and flat slipper snails attach to the inside. Some sea anemones have also been known to hitch a ride. All of these animals take advantage of the extra particles the hermit crab doesn’t eat and use the crab as a means of locomotion through the water.

The other two kinds of Lowcountry hermit crabs are the long wristed, which can gather sea foam to feed on the fats and protein it contains, and the thin striped, which have more symmetrical front claws and is more common on muddy beaches and mud flats.

Blue Crabs

The primary crab for human consumption, the blue crab is the largest crab found in the Lowcountry. Each year, millions of pounds are caught by commercial and recreational fishermen. The Latin name is Callinectes sapidus, or beautiful swimmer. Their flat back legs serve as paddles or flippers to move them smoothly through the water, and strong front claws can catch a crabber unawares if he’s not careful!

They are beautiful: males have bright blue claws, while mature females have red-tipped claws. And like most crabs, they are opportunistic eaters that will feed on vegetation, clams, worms, fish, snails, carrion and even other crabs. Blue crabs often dig into the mud with just their eyes peeking out, waiting to ambush their next meal.

Mating usually occurs from March to July and October to November. Females carry the eggs under their abdomen, which starts out as a bright orange spot and turns darker red as the embryos develop. Females with eggs are called sponge crabs, which are protected in South Carolina (if you happen to catch one of these, be sure to put it back in the water). In about two weeks, sponge crabs make their way to water with higher salinity where eggs hatch into tiny zooplankton.

Male blue crabs are larger than females and usually reach about 7 or 8 inches. The best way to differentiate male from female, however, is to look at the abdomen. Males have a sharply pointed appendage (Washington Monument), while females have a more rounded appendage (Capitol Building). If caught, all female crabs should be thrown back in order to allow them to grow to maturity. It is illegal to keep any crab that does not measure 5 inches from point to point.

The next time you see one of these fascinating crabs on the beach or in the marsh, take a moment to stop, watch and appreciate the role they play in the Lowcountry’s complex ecosystem.

Judy Drew Fairchild is a South Carolina master naturalist and photographer who lives on Dewees Island and spends entirely too much time lying on the dock watching fiddler crabs battle each other.

2018-01-19T11:44:59+00:00