Field Guide: The Incredible Crabs of Middle Island

by Morgan Greene, Education Part-Timer

During the summer, the Conservancy’s tour of Middle Island and its famous Ibis Sanctuary treats tourists and locals alike to stunning wildlife sightings. One might spot a Bald eagle soaring high over the marsh, or maybe catch a glimpse of a beautiful Painted bunting singing at the top of a pine tree. The variety of birds that call these islands home may tempt you to keep your eyes on the sky, but don’t forget to look down every now and then during the tour—they may be small, and they may be muddy, but Middle Island’s crabs can be just as amazing as the birds above.

Fiddler crab

One of my favorite crustaceans found on Middle Island is the Fiddler crab! These small brown crabs sometimes have brightly colored claws, one of which is much larger than the other on male crabs. If you tour Middle Island during low tide, you’re in for a treat: when the mudflats are exposed, large colonies of these little crabs can be seen dancing beside their burrows, waving their massive claws up in down as if to say “hello” (a habit that earned them the nickname “calling crabs”). This waving motion is actually a mating dance the male crabs perform to attract females. Male Fiddler crabs may also fight one another with their giant claws, but these brawls rarely result in injury—the two males will size each other up and shove each other with their claws, the fight ending when one crab intimidates the other into retreating.

Squareback marsh crab

Another crustacean you may see during the Middle Island tour is the Squareback marsh crab! Somewhat of a Fiddler crab lookalike, these crabs are also on the smaller side, but they’re a darker shade of grayish brown and lack the giant claw Fiddler crabs are known for. That heavy claw would only slow the marsh crabs down—these little guys love to climb! You may see them walking up dock pilings, crawling through exposed oyster beds, or climbing up the sides of trees, and even buildings, near the marsh. Unlike most crabs on our tour, these crabs are terrestrial, which means they can live out of water as long as they wet their gills periodically. As their name suggests, you can recognize the Squareback marsh crab by its square-shaped shell, or carapace.

Blue crab

While Fiddler crabs and marsh crabs are at least semi-terrestrial, our Blue crabs prefer to spend all their time underwater. If you look over the edge of a dock on the marsh, you may see these crabs scuttling amongst the rocks and oysters or eating a seafood snack (probably a small fish, or even a smaller crab). These much larger crabs can be up to 9 inches long when measured across their carapaces. The females have bright red claw tips, but you can also tell male and female Blue crabs apart by flipping them upside down to see the flaps on their abdomens—the male’s flap, or apron, is thin and long, somewhat resembling the Washington monument, while the female’s apron is wider and more triangular. This is where the female will carry her eggs after mating. 

Spider crab

Another aquatic crab you might see during our Middle Island Tour is the Spider crab! Like Blue crabs, Spider crabs will eat almost anything they can get their pincers on, from plant matter and algae to decaying organisms. However, these crabs are a bit more shy than our other crustaceans, so you’ll have to pay close attention to spot them—they’re masters of camouflage, concealing their carapaces in algae and other debris to hide from predators. Spider crabs also like to hitch a ride inside the bells of Cannonball jellyfish, so if you find a jellyfish floating through the marsh or washed up on the beach, beware—there might be a crabby surprise inside! Another fun fact: while most crabs are known for walking side to side, Spider crabs are able to walk forward.

Thinstripe hermit crab

The Thinstripe hermit crab is also quite shy and will duck into its shell if approached too quickly. Named for the brown and white stripes on its legs, Thinstripe hermit crabs are actually quite social with other hermit crabs and even other invertebrates, sometimes allowing hitchhiking snails and other animals to ride on their shells. Thinstripe hermit crabs do not make their own shell and instead live in the discarded shells of marine snails, such as whelks, which means they need to find bigger shells as they grow up. These friendly crabs are great at communicating with one another and will tap on each other’s shells if they want to make a trade. Sometimes, Thinstripe hermit crabs will even line up in a big hermit crab conga line to pass on their shells more efficiently: when a crab finds an empty shell that’s too big for it, it will wait with the shell while other crabs arrive to try it out for themselves. Once a big enough crab arrives, it will move into the new shell and pass its old shell down to the next largest crab, until every crab has a new shell of its own!

Have you found any of these crabs at other North Carolina beaches? Whether you’ve seen them all before or you’re hoping for a first look, the Conservancy’s tour of Middle Island is a great opportunity to meet some cool crustaceans up close. We hope you’ll join us during your time at Bald Head Island! To learn more and register, click here.

Sources: 

• NOAA Fisheries
• ScienceDaily
• Chesapeake Bay Program
• Texas A&M University
• National Park Service