SCIENTISTS, and science fiction writers, have been telling us for years that the future of our food supply lay under water. But we still tend to think of "seafood" only as including the most familiar fish and shellfish, leaving aside a huge number of plants and finless, un-fishlike creatures that other cultures long since came to terms with. And so when we notice these rather interesting dishes listed onmenus, we have a tendency to wonder about them but shy away toward more reassuring territory.

Many of these have been given names based on their likenesses to more common plants and animals, such as sea anemones and sea cucumbers. Most were named by the French, who always like to make everything sound as if it were entirely under control -- "cultivated" wild species, so to speak -- so they sometimes appear on the English half of a Chinese menu translated from the French version.

And admittedly, to those raised on fast food, the term "sea slug" may impart a certain squeamish sensation. But just think: There was a time when only a handful of people in this country had ever heard of sea urchin, and now it would be hard to imagine living without it. Well, for me, anyway.

Actually, a lot of the more poetic "sea species" aren't really so exotic, but they got semi-earthy nicknames (some more obvious than others) from sailors over the centuries: The sea cow, for example, is a seal, the sea hog a porpoise, the sea hen a fish also called a gurand or piper and the sea owl is actually the lumpfish, best known for its rather sour and off-putting imitation of caviar. The sea needle is a gar or freshwater pike.

That same sea urchin now so common at sushi counters is also known as a sea hedgehog, and in fact it is a marine mammal, not shellfish at all. The part that we eat, which is usually described as the roe but which more recently scientists have decided is really the gonads, is for obvious reasons rumored to be a powerful aphrodisiac. (And if you think that's weird, consider the fermented sea slug ovaries that are considered delicacies in Samoa.) We say "common" because as unfamiliar as it was in this country only a couple of decades ago, sea urchin harvesting is now a $75 million industry in California alone, though both the Pacific Northwest and the Atlantic Coast fisheries are producing really great urchin these days. In fact, if you're really lucky one night, your sushi bar will make you a pair to taste, one with urchin from Maine, say, and the other from Washington.

If you've never tasted sea urchin, I can only describe it by saying that it tastes the way the sea smells: that mix of salt, barnacle, wet wood -- well, you either get it or you don't. It's addictively smooth -- it has great "mouthfeel," in the vernacular -- and plays off saltier, nubbier foods like caviar, as well as other velvets, such as quail egg yolks. It's also important to realize that the flavor of sea urchin very quickly grows much stronger, and so some people who have found it too pungent may conceivably have had urchin that was several days or even a week old.

The Japanese, of course, most often showcase sea urchin in fairly straightforward ways, although an occasional special around town, based on a dish at New York's cutting edge Nobu, is sea urchin rolled in nori and flash-tempura-fried. At the new Kaz Sushi Bistro (1915 I St. NW; 202/530-5500), eponymous chef-owner Kazuhiro Okochi pushed that envelope by making a sort of mousse of urchin, mascarpone cheese and tofu puree, wrapping it in spinach and tempura-frying that. At some of the seafood showcases such as Dupont Circle's Pesce (2016 P St. NW; 202/466-3474), which has used it to top tuna tartare, it may appear in similarly pure form.

But in Europe, sea urchin is not only popular by itself -- along the Riviera, sea urchin dealers dredge them right up out of the water, cut them open and hand them straight to beachgoers who scoop the good stuff right out of the shell -- but as a sauce ingredient. David Craig of the Tabard Inn (1739 N St. NW; 202/833-2668) sometimes uses sea urchin to give sauce a richer marine flavor; so have the kitchens at Galileo (1110 21st St. NW; 202/293-7191) and Ardeo (3311 Connecticut Ave. NW; 202/244-6750). At his brand-new Relish (in the former Fellini spot at 18th and M streets NW; 202/785-1177), former Ardeo chef David Nugent dresses sauteed shad roe with a sea urchin veloute.

At Lespinasse in the St. Regis Hotel (the former Carlton at 16th and K streets NW; 202/879-6900), Sandro Gamba has a luxuriant first course on his tasting menu of cream of fennel, sea urchin and osetra caviar. Fabrizio Aielli of Osteria Goldoni (1120 20th St. NW; 202/293-1511) makes a black pappardelle pasta with sea urchin, fresh tomato and basil. DC Coast's Jeff Tunks has an occasional appetizer special that is just as rich in the mouth, every bite at a time: Malpec oysters, topped with a little osetra, some sea urchin and a touch of butter sauce and run for a heartbeat under the broiler (1401 K St. NW; 202/216-5988).

At Cities in Adams-Morgan, which just unveiled its newest menu, this one with a Barcelona accent (2424 18th St. NW; 202/328-7194), Aret Sahakyan has a sinfully seductive and deceptively light sea urchin gratin, really a sea fish-flavored sabayon (emulsified egg yolks and cream) topped with red trout caviar.

And Michel Richard of Citronelle (in the Latham Hotel at 3000 M St. NW; 202/625-2150), who truly loves food (you just have to hear him talk about it), has created some of the most suitably unctuous presentations in town. Richard, who says that doing anything more than a careful warming of the urchin will spoil its fresh iodine-y flavor, has served it just by itself over a little vinaigrette, turned into an airy mousse with whipped cream and a little gelatin and matched with fresh asparagus, almost purely in its own shell with caviar and a touch of cream and in a ragout blended with fresh mussels and lobster.

Next week: Snips and snails and lobster tails, that's what haute cuisine's made of.