THE ROCKBOUND COAST of Maine has come ashore at the Smithsonian's Museum of Natural History, for the edification of the public and the benefit of the planet.

Waves pound and the tides come and go as lobsters lurk, sculpins skulk, cod cruise and crabs commit cold-blooded murder in this new 3,000-gallon companion exhibit to the living tropical reef, which not only is a favorite of visitors but, according to its creator, has revolutionized ecological science.

"From the first exhibit we discovered that the coral reef isn't just an oasis in the barren tropical waters, it's the most productive system in the world," says Dr. Walter Adey, who heads the Smithsonian's Marine Systems Lab.

"That's the most important ecological discovery since World War II. We don't expect anything quite so earthshaking from this Maine exhibit, because it rproduces a simpler and less productive system, but. . . ."

But, indeed. One spinoff already produced by the project has been an improved system of float-farming that brings blue mussels to eating size in 18 months. Whereas their hardscrabble cousins, freelancing it on the wave- lashed rocks, may take five years to amount to anything, and anyway wind up full of grit.

Adey hopes his system will keep Maine's watermen so busy raising mussels that they won't have time to catch too many lobsters, which they have been catching too many of for generations, and so perhaps ease the pressure on both lobsterman and lobster. He also is helping Caribbean Basin fishermen raise the tropical equivalent of king crabs, which are getting dangerously scarce in Alaskan waters. Everything is connected to everything else.

In the Maine exhibit, for instance, the water is purified by algae that is in turn quite edible and nutritious -- and 30 times more efficient than any farm crop. Adey's working on a similar system he hopes will turn Frank Perdue's chickens' droppings into more Frank Perdue chickens. And Adey's trying to persuade NASA that his algae could convert astronaut offal from an embarrassment into nutriment; some are finding that a little hard to swallow.

All this hard science is being yielded by a project that isn't even classified as research. "We've been working with exhibition funds and grants all along," Adey says. "It's very hard to get research money for basic studies and aquaculture experiments." Adey, who was an engineer before he became a biologist, knows how to get lots of bang per buck: Much of the intricate equipment that maintains the delicate balance of the tanks was invented and constructed on the spot; it runs like clockwork because a lot of it is clockworks.

Adey's passionate involvement with the oceans comes through in the film "Blue Planet," which accompanies the exhibit. Among the points it makes is that we ought to call it Planet Water, since only a fourth of its surface is earth; and that from all the seven seas we draw only two percent of our food.

A lot of fine-tuning remains to be done on the Maine exhibit, "which I'm looking forward to, because in something like this you learn the most from the mistakes you make," Adey said, before going off to investigate what had become of the young sting ray in the tank. It was found in a crevice in the rocks, being eaten by a crab.

"Well," Adey said. "That's life."