SCIENCE LAB SCENICS : MIDLEVEL MOBILITY

It seemed to be just a slug - but scientists caught it swimming. Bruce Robison, a marine biologist, has long used robotic vehicles to explore the Monterey Canyon off California, a gargantuan rift : in the Pacific seabed that descends rapidly from coastal shallows to a depth of more than 2 miles [3.2 kilometers ]. In early 2000, he stumbled on a strange creature he had never seen before.

'' We had no idea what it was,'' Dr. Robison recalled. The gelatinous blob had a giant hood at one end, fingerlike projections at the other and colorful internal organs in between, Dr. Robison and a colleague at the Monterey Bay Aquarium Research Institute set out to discover what it was.

A quarter-century later, having studied 157 of the enigmatic organisms in their dark habitats, as in the laboratory, the two scientists are making their conclusions public.

The newly identified creature, they reported recently, represents a whole new family of living things that reside in the midnight world of the ocean's vast midwaters - the largest and least explored part of the planet's biosphere.

Moreover, it looks and behaves unlike any of its closest relatives on the tree of life.

The discoverers say the creature is a surprising new kind of nudibranch, or sea slug. Nudibranchs [ Latin for ''naked gills'' ] get their name from the fact that they're nude, unlike their cousins on land -snails.

What sets the organism apart from its marine relatives - and what makes the discovery so astonishing -is that it swims. Most nudibranchs known to science previously were described as inching their way over coral reefs, sea grass beds, kelp forests, the deep seafloor and rocky tide pools.

But Dr. Robison and his colleagues, Steven Haddock,found that the newly identified creature is neutrally buoyant - that is, it can float effortlessly underwater, neither sinking nor rising.

Striking video footage shows how, in that weightless state, it moves gracefully through its dark habitat, slowly undulating its entire body up and down.

For dramatic motion, the scientists found, it also expel water rapidly from its hood, producing jet-like bursts that send it moving quickly backward to escape predators. [ William J. Broad ]