Humble hermit crabs are Kwaj evening entertainment

(or: Which crab is this crab?)

Every Kwaj evening, a surprisingly comical show begins at sunset, when the beach suddenly begins to move. You thought that was just a shell lying there? Nope. Crab. Pebble over there? Nope, crab. Here a crab, there a crab. Underfoot a crab. Climbing in the bush a crab. Crabs everywhere. The ground, mostly covered in a low flowering plant, takes on a polka-dot pattern of tumbling, bumping shells powered by crustacean legs.

After nightfall, we like to visit an oceanside beach across the street from our house. I call it Crabby Beach. We bring flashlights and gingerly step around the crab neighborhood, searching for the biggest or the most interesting crab home. Aside from the typical former snail shells of all shapes and sizes, we’ve seen plastic lids, almond shells, and even a twisted piece of metal adapted into hermit houses. 

A group of crabs is called a cast, and casts can be found everywhere on Crabby Beach after dark.

Last summer while investigating Crabby Beach one evening, I found two crabs living in the same spiral shell. Aside from the space created and formerly occupied by some kind of gastropod, the shell had another pinprick hole at its tip. One tiny hermit had slipped into the larger of the two holes, coiling her vulnerable belly and tail around the internal spiral of the shell, nestling two pairs of her 10 legs inside the protected space, leaving just enough room to reach two front pincers — modified legs, really — and two sets of legs into the salty sea air and down into the sand to scurry around. In the other hole, the width of a grain of sand, a second hermit crab sought refuge — and a free ride across the beach.

Thanks to a hole in the shell this crab chose for its home, you can see its tail and the tiny last pair of legs.

Which crab is this crab?

Now that I’ve read the 1983 Pacific Science study about the scavenging heroics of hermit crabs, when I go to Crabby Beach I attempt to identify the three species of hermit crab that the scientists counted in their observations of crab feeding on Kwajalein: Coenobita perlatus, C. rugosus, and C. brevimanus, also called a strawberry hermit crab, a ruggie crab and an Indos crab respectively.

C. perlatus, at home close to the beach but not quite in the water, looks like an apple rolling onto white marble when it ventures onto the sand. With dark reddish or red-orange claws and legs polka-dotted with white spots, the common name strawberry crab is a near-perfect visual description, if strawberries were the size of fists. These guys are easy to identify.

A large C. perlatus scrabbles up a rock on Crabby Beach.

C. rugosus, a ruggie crab, can be brown, tan, or greenish, depending on what it eats. According to most descriptions of the species that I can find, ruggies can be distinguished from other hermits by a series of ridges or stitch-like lines on its large claw. 

Algae glistens on the shell this C. rugosus (I think? Correct me if I’m wrong?) totes across the sand.

Inexperienced crab identifier that I am, however, I’m never certain if I’m looking at a C. rugosus or a C. brevimanus. Apparently, C. brevimanus is more purple in color than rugosus, and brevimanus has a long brown stripe down the center of its back with two brown bands along its sides. The real key in the descriptions for a nonexpert like me seems to be the purple color in the adult brevimanus crabs. 

C. brevimanus clambers over coral chunks.

Here a crab, there a crab

Aside from Crabby Beach, hermits for identification practicing can be found almost anywhere on Kwaj. This week, a friend told me about a crab crawling across her living room, probably brought into the house with some kitesurfing gear.

My biggest crab surprise was on a boat dive a couple weeks ago, when I stuck my foot in my dive boot. My toes found a large hermit crab – I’m pretty sure based on the purple claw it was a brevimanus, and thankfully my toes weren’t pinched – who had abandoned his previous shell home inside the toe of my boot and was making my footwear his new home.

This hermit crab’s soft underbelly was completely exposed when it tried to exchange its shell for my dive boot – an unlucky choice for me and the crab!

Having relinquished his snail shell home, I could see all 10 of his crabby legs and examine his soft, vulnerable underbelly. Since I didn’t want to throw him in the water where he would drown, I kept him in a small container until we returned to the marina where I gently released him into a bush. 

After all, I’d hate to lose any important member of the evening Crabby Beach show.