All these years they’ve been hiding out on the Monterey Bay shoreline, passing for something they’re not. Now, with the help of genetic sequencing, Hopkins Marine Station scientists have cracked their secret and given them a name: Pagurus hopkinsiensis.

The newly discovered hermit crabs are 1 to 2 centimeters long, with greenish antennae and curled hind ends. They’re native to the salty stew of Monterey Bay’s high intertidal zone, where – beneath the notice of surfers and beach bums – the males make gripping displays of territorial lust.

“It is the most common tide pool species, and is very similar to one found elsewhere, but it has different claws and different genetics and we have named it as a new thing,” explains Hopkins Director Steve Palumbi.

When Stanford post-doc researcher Ryan Kelly took a bag of hermits from the nearby intertidal zone, he figured they were all the same species. “But when I was looking at the genetic data, it was two groups that were quite different,” he says. “To find it in our own backyard was really exciting.”

By sight, the newly discovered hermit crabs are virtually indistinguishable from their hairy cousins, P. hirsuitiusculus. Five other species that scuttle the West Coast from the Bering Strait to Baja murk up the taxonomic crab stew. The fact that the Hopkins crab looks similar to its relatives qualifies it as “cryptic”: visually indistinguishable, but genetically distinct, from another species.

“It would be nice if it had some sexy horns,” Kelly says. “But it doesn’t look much different from the other related hermits.”

After collecting specimens from Southern California to Oregon, Kelly determined that, unlike its far-flung cousins, P. hopkinsiensis sticks to a narrow range from northern Big Sur to just north of Santa Cruz.

“This is biodiversity at play,” says Kelly, now a law student at UC Berkeley. “It’s similar to the Victorian age of discovery, when people went out to find different species. We’re doing that now, but with genetics.”

While Kelly did the genetic analysis, then-lab tech Emily Jacobs-Palmer used digital calipers to measure the crab’s tiny parts. “The morphological differences are really, really subtle,” she says. “If they weren’t, someone would have discovered it long ago.”

Outside of their shells, the males measure up to twice the length of the females, at an intimidating 2 centimeters. That’s one difference compared to the related hairy hermits, whose females are roughly the same size as the males, explains Jacobs-Palmer, who’s now pursuing a doctorate in evolutionary biology at Harvard.

She recalls finding a cluster of the crabs at the tidepools by Garrapata State Park in 2007-08. “We must’ve hit the season just right,” she says, “because we found a bunch of the crabs engaged in mate-guarding: The male will grab onto the female’s shell and hold onto her to make sure that no other male can get to her until he’s done with her. They just hold on for dear life. Basically until death do they part.”

Kelly, Jacobs-Palmer and Palumbi wrote a paper about the newly discovered Monterey Bay native and submitted it to the riveting Journal of Crustacean Biology. It was rejected, Kelly says, so they’re now pitching it to broader scientific journals in hopes that someone will see the mini crab as big news.

“It’s just an example that we know so little, even in really well-studied habitats,” Jacobs-Palmer says. “The Marine Station has been around 121 years, and still we’re finding new things. It’s important to keep our noses in the tidepools.”

Stanford marine science professor George Somero sees the Hopkins crab as a harbinger of global change. DNA tests are revealing other cryptic species, including limpets and mussels, that are quietly replacing their doppelgangers in the intertidal zones.

“You have one species replacing another, but they don’t look any different,” he says. “This is probably a trend with climate change. The ecosystem is warming, and the animals that are showing up will be able to carry on at higher temperatures. It’s kind of maintaining the status quo by substituting the northern species with the southern species.”

That’s a good sign in uncertain times – until you get to the planet’s hottest waters.

“If you’re at the equator, there ain’t no more hotter places for those animals to evolve,” Somero says. “No one’s waiting in the wings to replace their role in the ecosystem.”

Not even a hermit.