There's no barrier to love for a tiny crab that tickles its way into mollusks to find a mate, a new study has found.

How these so-called pea crabs, which live alone inside shellfish, find love has long been a mystery to scientists. (See "Wild Romance: Weird Animal Courtship and Mating Rituals.")

Now their secret is finally out, according to researchers from the University of Auckland in New Zealand—and they have intimate video footage to prove it.

View Images

Video courtesy: Oliver Trottier

Infrared cameras set up in the lab caught male New Zealand pea crabs (Nepinnotheres novaezelandiae) leaving the safely of their green-lipped mussel homes to search for females.

Having pinpointed a mussel occupied by a potential mate—likely via chemical cues—the males spent up to four hours tickling away at the opening to the bivalve's fleshy edge until it let them in, according to the study, published recently in the journal Parasite.

It's the first time such behavior has been recorded in a crustacean, but why tickling works isn't yet clear, Oliver Trottier, who co-authored the study, says in an email.

Why Tickle?

One possibility is that the male crab tickles to relax or desensitize the shellfish so it doesn't snap shut and crush him when he attempts to access the female, Trottier speculated.

If the males "keep rubbing [the mussel] in the same place until it goes numb," maybe they're able to enter without being felt, he says.

This would also help explain why the males are active at night—the team found that the plankton-feeding mussels aren't nearly as sensitive then, though why is unknown.

Crabs "can be crushed [by mussels] both night and day, but it's much, much more likely during the day as the mussels are hypersensitive," Trottier said.

Not only that, the mini-crustaceans are easily picked off by predators if they leave their armored bachelor pads during daylight, the marine scientist added.

Tough Love

Martin Thiel, a marine biologist at the Catholic University of the North in Coquimbo, Chile, said how the female pea crabs are fertilized has long been a puzzle.

Scientists had suspected that males sought out females, partly because of their thinner shape and smaller size. "But this is the first study to show experimentally that this is happening," says Thiel, who wasn't involved in the new research.

He adds that "what these guys have found for this pea crab from New Zealand is most likely happening in many other pea crabs all over the world."

They won't all be shellfish-ticklers, though—pea crabs also live in sea squirts, sea urchins, and a range of other animals—and they all face the same challenge of how the sexes come together, he said.

While male New Zealand pea crabs are estimated to make up less than one in five of the adult population—an unsurprising stat given the mating risks they run—the study team found they're very successful at locating and fertilizing females.

They may do this by detecting pheromones, according to experiments in which female-occupied mussels were placed upcurrent of males. (See "Women Can Sniff Out Men Without Knowing—And Vice Versa.")

While the use of pheromone attractants by pea crabs has yet to be proven—the males could be responding to other chemical cues—it is known in other marine crustaceans, such as crayfish and hermit crabs, study co-author Trottier said.

Blocking an Invader

If this is the case, Trottier has a cunning plan To synthesize the female pea crab's scent and use it to lure males into traps on commercial mussel farms.

The crabs are considered a significant pest of green-lipped mussels, an important aquaculture species in New Zealand.

The parasitic crabs, which steal food gathered by the bivalve and therefore stunt its growth, infect up to 60 percent of mussels on some farms, Trottier noted. (Also see "Meet 5 'Zombie' Parasites That Mind-Control Their Hosts.")

But not everyone is unhappy to find pea crabs lurking in their seafood meal.