I enjoy having hermit crabs in the yard. They are cute and fun, and come by to snack on leftover cat food.

Cats may not be so happy to see these intruders. But even if they catch the snackers red-handed, they can’t do much damage to a hermit crab drawn up inside its shell.

Though hermit crabs can hide inside their shells, their legs and claws usually stick out some. As they grow larger, their legs and claws hang out further, and they have to start searching for a bigger one.
Recently I started to wonder why, after over 150 million years, hermit crabs are still scrounging around for discarded mollusk shells to cover their butts.
Caribbean hermit crabs, or soldier crabs, (Coenobita clypeatus) commonly use cast-offs from West Indian Top Shells (Cittarium pica), otherwise known as whelks or wilks.

West Indian Top Shells, like other marine snails, build an exterior exoskeleton through a process called biomineralization. They have an organ called a mantle that secretes a thin layer of protein as a base, followed by layers of calcium carbonate, which they create using carbon and calcium ions drawn from the seawater. The calcium carbonate then solidifies to make the hard shell. Later on, the mantle can add to the edge of the shell, so the same shell can grow as the snail gets bigger.
Crabs also use calcium carbonate to make hard shells for themselves that cover their whole bodies. But their shells are fitted to their legs and bodies, and not expandable. In a process called molting, as the crab grows bigger, it forms a new, thin exoskeleton cover underneath the existing shell, and breaks out of the old one. Once out, the crab then quickly makes the new, soft exoskeleton harder using calcium carbonate.
Hermit crabs do the same kind of growing and molting as other crabs, but only their claws and legs get hardened up. The abdomen stays soft and the hermit crab needs to find a new larger shell to cover it. In late summer there are often mass molting events as the hermit crabs all migrate down to the shoreline and take off their shells, hoping to exchange too-small shells for bigger ones.
They also might do some quick mating when their shells are off. It’s a matter of life or death for the hermit crabs to get into new shells, not just for protection from predators but also to prevent them from getting dried out and cooking in the tropical sun.

Female hermit crabs deposit their eggs in the water, where the eggs go through several free-floating phases of development before the baby hermit crabs start their transition to land. While in the water, they need to locate a discarded shell from a (dead) micro-mollusk, which are species of snails that remain tiny even as adults. Over time they will then work their way onto land and into ever larger shells.

Depending on finding abandoned shells from other dead animals seems pretty iffy as a life strategy, although it does allow successful hermit crabs to avoid spending a lot of energy building and maintaining their own mobile homes. They are renters, not owners, which in some situations can be an advantage.
Still, the hermit crabs are heavily dependent on continued housing availability. There needs to be a large population of whelks – and intact shells left behind after they die or get eaten. In the Caribbean, an octopus or lobster will eat a whelk, as well as bonefish, porcupinefish, and rock hinds. The whelks are also attractive to shorebirds like oystercatchers, which are locally known as whelk-crackers. And then there a number of people who collect whelks for stew. Supplies of whole, empty shells can quickly become limited.
Hermit crabs are essentially recyclers, taking cast-off resources and reusing them, and in the process cleaning up the beaches. If they can’t find a shell that fits, they will sometimes end up wearing beach trash instead, which is a bit sad to see, but adaptive and enterprising of them.
A hermit crab’s survival plan is also a bit poetic, as each temporary shelter it finds will carry a history of prior lives, which it will carry on its back, and then pass along.
Gail Karlsson is the author of a photo book Looking for Birds on St. John, as well as two other books about nature in the Virgin Islands – The Wild Life in an Island House, and Learning About Trees and Plants – A Project of the Unitarian Universalist Fellowship of St. John. Follow her on Instagram @gailkarlsson and gvkarlsson.blogspot.com. More info at gailkarlsson.com.