As with snakes, lizards, lion cubs and the like, it's hard for a human to achieve a meaningful relationship with a crab. You can love crabs, but they may not love you back.

"They're not exactly the kind of pet you can cuddle," says Ann Cohen, a specialist in the Smithsonian's Department of Invertebrate Zoology who happens to own four pet hermit crabs. "They don't like to be handled and can bite through a fingernail if you rile them. But they're not really dangerous."

Cohen, mother of several children, does not depend on her pet crabs for role reinforcement. But she has a friend who considers her crab as something more than a pet and allows it the run of the apartment, along with the dog. When the crab gets the munchies, say Cohen, who has witnessed the drama, it scoots up to the dog dish and chows down. Fido backs off.

Such domestic freedom is highly unusual for crabs, as they are inclined to get lost in deep-pile carpet, even darting out at odd moments to ambush toes.

Hermit crabs are the little critters you see skittering on all 10 legs down on the beaches and swamplands of Florida - the same "hermit" or "soldier" crabs whose Latin moniker, coenobita clypeatus , or "shell-bearing monk," describes an apparent yearning for solitude but ignores a hidden desire to dress up in different shells and scamper about.

They're the crusty creatures that, in pet stores across the country, have tried for the past year to fill the gap left in children's hearts by baby green turtles - which the FDA banned for commercial sale in 1975 for carrying nasty organisms like salmonella.

Amy Carter got one as a Christmas present from her former Plains lemonade stand partner. The crab rode to Washington in Air Force One with the President and Mrs. Carter, moved into the Amy's bedroom terrarium (shocked with a wardrobe of extra shells) and, according to the First Lady's press secretary, may keep Amy company on future trips.

Amusement with pesky crustaceans shows no sign of letting up. Tysons Corner Pet Mart, one of the local pet stores that report a healthy interest in the crabs, imports them from the Florida Keys and, says assistant manager Jim Grant, sells about a hundred hermit crabs a month.

One of the major wholesalers, The Great American Crab Co. of Orlando, Fla., has reportedly quadrupled sales each month since it started marketing hermit crabs last winter. After borrowing $50,000 to go into business in last year, David Oslin, 33, and his partner, Dan Scheffer, 29, have experienced the joy of deluge.

The firm sells crabs to 500 department stores in 45 states, advising the stores to use them as novelty items - and holiday specials - in stationary, toys and gifts. In December, they shipped 150,000 crabs to retailers. And for Valentine's Day, they anticipate a rush by spurned romantics who are expected to snap up the creatures as hints for those too crabby to love.

Last summer Gimbel's, the New York department store, praised the crabs' personality in a two-column, page-length ad in The New York Times. A girl of about seven was pictured holding a live crab in her open palm. "The crabs bring new life to a tired terrarium or lackluster coffee table," read the copy. "Not just a novelty, these live hermit crabs can live for many years and make wonderful pets. They display unique personalities and will respond to your loving care and attention."

Crabophiles continue to extoll their virtues: curious ballets, a scavenger's lust for all kinds of food (from pizza to dog food), low cost (from $1.49 for a twerp to $4 for a fist-size crab) and ease to please.

For hermit crabs, happiness is a small cage, preferably glass or wire mesh, a water dish (not too deep), a bit of foliage or twig to climb on, a shelf of rocks to hide under, a diet you would feed your garbage disposal, and temperatures from 72 to 93 degrees Fahrenheit (22-34 degrees centigrade). Below that, crabs become sluggish and may even die, as many did last winter in shipment from their tropical homelands to pet stores in the North.

Oh, and don't forget to toss in a few extra shells. Crabs are thrift-shop fashionplates and like nothing better than to cat about in natty castoffs. As nature's orphans, hermit crabs are unable to manufacture their own shells, so they must adopt the homes of others. They outgrow shells frequently, though sometimes they change out of vanity. When the urge strikes, they dash out their front door, poke a claw inside a nearby shell to check for tenants, rush inside, and often dart back to the old home turf.

Little definitive research exists on the biology and behavior of hermit crabs, but that doesn't mean they are without fans. Senior Smithsonian zoologist Dr. Fenner Chace has found himself fielding so many recent inquiries that he has assembled a pamphlet on crabs' lives and loves from the findings of a Dutch zoologist, one P. de Wilde.

De Wilde, who observed the creatures in Curacao, claims they do best in hot, dry climates, preferring to migrate to a hard-soil habitat a good distance inland from their birthplace in the sea's salt-water tide pools. Such complicated roots mean it's almost impossible to breed the crabs at home. "You'd need a very large setup," warns Cohen, who says that raising crustaceans even under lab conditions can be tricky business.

Yet anyone can delight in observing what appears to be an intelligent, fun-loving species with a pecking order Cohen sums up as: "The biggest crab is the top crab."

In their travels, crabs make chirping, croaking sounds - a mating racket that is said to have scared off early invaders on Curacao. In captivity, Cohen says, she has never heard such cacaphony among her crabs. "But I've seen them carry on a lot of social intercourse."

In some Caribbean bars, says Geerat J. Vermeij, a University of Maryland associate professor of zoology, crab racing takes the place of darts. "It's quite a sport," he day.

Like snakes, crabs shed their skin. So don't worry if yours burrows underground every 18 months or so. That's what it's up to; and some believe that they consume their old skins for the calcium content.

Even under a magnifying glass, the sexes are hard to tell apart. Males boast tufts of hair that conceal the openings on the first segment of the last pair of legs and lack the egg-carrying appendages on female abdomens. Females have bare opening on the first segment of their third set of legs.

You won't need a leash for your crab, although some pet stores may try to sell you one - along with special crab food you can skip. You should also watch out for some South American species of hermit crabs that live for only six months, compared to five to 10 years for the true coenobita clypeatus . The real thing has a large, purple claw, that tends toward blue if it hails from the Carribean. But hermit crabs with tan claws are okay, too, and usually come from the Pacific coast of Central America (Panama or Costa Rica).

There's a continuing debate over the susceptability of crabs to disease, like green turtles. THe FDA reports no such complaints, though some zoologists reserve judgment. Some also fear that if the craze goes on, nature may be denuded of its tropical scavengers. But others say there are e nough crabs to make everyone happy.