In 1768, only 27 years after its discovery in the Bering Sea, the last Steller's sea cow was killed -- a fate shared by the great auk in 1840s, the Caribbean monk seal in the 1950s, and unknown numbers of other marine species. Far less publicized than loss of biological diversity on land, the loss of marine genetic, species, and ecosystem diversity is a global crisis in its own right.

Although fewer marine than land species have been described, in some respects the marine realm is more diverse. It hosts 31 of the world's 32 extant animal phyla, 14 of them exclusively marine. Coral reefs, like tropical forests, are renowned for their dazzling species diversity, though recent evidence suggests that the deep sea might also have a remarkably high species diversity. Because many marine species defend themselves chmically, marine biochemical diversity is an exciting source of new medicines. The diversity of live in marine systems also afords recreational and aesthetic pleasures.

The oceans' biotic wealth extends beyond numbers of species; the highest measured productivity on the Earth is in North Pacific kelp beds. Seafoods provide much of humankind's protein supply. Marine photosynthesizing and shell-forming organisms tie up carbon dioxide that would otherwise intensify global warming. The diversity of marine ecosystems, from structurally complex mangrove forests to seemingly featurelss oceanic midwaters, is at least comparable to the land's.

Marine scientsits are continually reminded of how little ie known about the seas. Not until 1938 was it learned that coelacanth fish, until then known only as fossils, still survive in the Indian Ocean. And it was as recently as 1977 that hydrothermal vents, with diverse and unique associated ecosystems, were discovered in the East Pacific.

As technology and international trade have intensified, their impact has extended even to the remote oceans, which bear the "fingerprints" of humanity. Even in Antarctica, penguins far from any agricultrue contain DDT, shorlines have been fouled by oil spills, and blue whales are critically endangered. The species and ecosystems suffering most, howeve,r are in the coastal waters closest to humankind.

Several distinctive aspects of the sea complicate the task of conservation. First, marine ecosystems are at the receiving end of drainage from the land, and most wastes eventually wind up there. Second, reproduction of marine organidms can be very uneven in speace and time. In widespread species, such as teh tropical West atlantic Nassau grouper, spawning may occur in just a few places. Long-lvied species, such as the geoduck clam of Northeast Pacific coastal bays, may recruit successfully only once in many years.

Many marine organisms release their eggs into the surface waters of the sea. Planktonic larvae can disperse hundreds, even thousands, of kilometers, and because of this widespread dispersal marine fishes, invertebrates, and plants might seem to be a low risk of extinction. Not so. The endangered totoaba fish of the Sea of Cortez, the extinct (1930s) West Atlantic eelgrass limpet, and an extinct (1980s) hydrocoral of the East Pacific's Gulf of Chiriqui all had wide-dispersing planktonic larvae.

Several categories of marine species are particularly vulnerable to threats caused by human activities. Surface-dwellers (including larvae of many commercial fishes) are vulnerable to oil and other floating pollutants and increased ultraviolet radiation. Species requiring more than one habitat during development (such as Pacific salmon populations) are threatened by activities in any one of them. Species that mature slowly and produce few young (such as sea turtles, seabirds, and sharks) are vulnerable to over-exploitation. So are the exceptionally large species favored by people for food and other products, who have decimated once-sizable populations of giant clams, king crabs, bluefin tunas, and the great whales.

Preventing extinctions is essential but not sufficient. Maintaining the integrity of the sea and, hence, its sustained production of resources and services requires attention to whole ecosystems as well as to their component species. Estuaries and salt marshes, mangrove forests, and seagrass beds near cities and towns are severely degraded worldwide. And, because ships carry millions of larvae in their ballast tanks, alien species are common in the busiest harbors, where more than half of the species can be interlopers. Many estuaries draining rural watersheds are contaminated with agricultural chemicals and choked with silt eroded from farming and forestry. The increasingly observed worldwide bleaching of corals could portend massive ecological changes for coral reefs and other marine ecosystems. Global atmospheric change will touch even the remotest areas.

Marine conservation has only become an issue of global concern within recent decades. There are three main reasons for this delay:

• First, because the sea is not their element, people seldom notice damages that would be readily observed on land. Wastes, for example, simply seem to disappear. So widespread is the notion that the seas are infinite and inexhaustible that few become alarmed even when fisheries crash and ecosystems become sewers.

• Second, there is no tradition of managing marine areas for conservation, whereas protected areas have existed on land for over a century. Strategies and plans for marine protected areas are a product of the last 15 years and have still to gain general acceptance. Integrated resource management in the coastal zone, though the key to conservation and sustainable use, is hardly being applied anywhere.

• Third, most of the seas and oceans lie outside the jurisdictions of states, and even territorial waters and those within Exclusive Economic Zones (EEZs) are communal property. Because the ocean has been an "open access resource," competitive exploitation has been the norm. Even though fisheries conventions and international agreements on the management of whales and seals have existed for some time, only within recent decades have regional seas conventions, conventions to prevent marine pollution, and the United Nations Convention on the Law of the Sea begun to impose a framework of international law on the 70 percent of the Earth's surface that is ocean.