Ocean keepers: California's sea otters are mysteriously dying in record numbers. How could their decline affect other ocean life?

Seated in a small boat, biologist James Estes scans the water in Monterey Bay, off the coast of California. He spots his target: a sea otter frolicking in the middle of a large patch of kelp, a type of seaweed. The otter disappears underwater, then resurfaces and bobs--belly up--in the rolling waves.

The otter reaches its paw into the loose pocketlike skin under its arm, and pulls out a clam and a small rock it had retrieved on its dive. It lays the rock on its chest, then--CRACK!--it skillfully slams the clamshell onto the rock. Estes watches intently as the otter whacks open its meal and starts snacking on the flesh inside the shell.

Why is Estes keeping such a close eye on this California sea otter, or Enhydra lutris nereis (en-HY-dra LOO-tris NEAR-ee-iss)? The reason: Record numbers of the species have been washing up dead or dying onto California shores. Estes, a sea otter researcher from the U.S. Geological Survey and the University of California-Santa Cruz is trying to find the causes. "Over the last couple of decades, the otter's mortality rate (number of animals that die each year) has gradually gone up."

If sea otters disappear, it could forever change the coastal ocean ecosystem (interactions between a region's living and nonliving things). That's because sea otters feast on sea urchins, a spiny, hard-shelled animal that eats away at the thick patches of kelp. If hungry otters don't keep urchin populations in check, the urchins overgraze the kelp--creating a barren wasteland on the seafloor. To keep the coastal oceans healthy, Estes and other scientists are trying to solve the mystery of what's killing the otters before it's too late.

OTTERS TAKE A DIVE

Sea otters--a member of the weasel family--once flourished in the coastal marine habitat around the rim of the Pacific Ocean (see map, p. 9). Scientists believe that 300 years ago, more than 500,000 otters lived in the Pacific, including 20,000 of the California sea otter subspecies, a group defined by its limited geographic range along the coast of California.

Then, in the mid-1700s, commercial hunters discovered the value of the otter's luxurious fur--the thickest in the animal kingdom. Over the following years, humans hunted sea otters to near extinction. "By the early 1900s, [the otter population in California] was essentially wiped out, with the exception of a few dozen animals," says Andrew Johnson, the sea otter program manager at California's Monterey Bay Aquarium.

In 1911, the International Fur Seal Treaty banned the hunting of sea otters, and the animals slowly started to bounce back. Since then, the California sea otter population has rebounded to about 2,800 animals. Despite the population increase, the otters now face a new threat. Increasing numbers of otters have been discovered dead on beaches each year--scientists counted 135 in just the first five months of 2004.

What's to blame? To find out, scientists have performed necropsies, or examinations of animal carcasses, on the dead sea otters. The studies have revealed a startling trend: A top killer of sea-loving otters are microorganisms that originate on land.

LAND BUGS

Studies have suggested that up to 40 percent of the recent California sea otter deaths may have been caused by infections from one of two land-based parasites. These microorganisms live inside their host, the otter, and cause it harm. Toxoplasma gondii (TOX-oh-PLAZ-ma GONE-die) is a parasite that is released in the feces of cats. Opossums spread another parasite in their droppings, Sarcocystis neurona (SAR-coh-SIH-stus new-RONE-ah).

How does a parasite that is released on land end up inside otters, which rarely come ashore? It turns out that cats and opossums release a very hardy form of the parasites. "[The tough form of the parasite] can survive in water for long periods, which allows it to be washed from the land into the sea," says Patricia Conrad, a veterinarian and biologist at the University of California-Davis. In areas near the coast, rain can carry the tiny parasites out to the ocean.

Once there, it can become concentrated in the tissue of a sea otter's favorite snacks, like clams and mussels. How? These bivalves eat by filtering seawater. They capture microscopic bits of floating food by sucking water through their shells. If parasites are in the seawater, they get captured along with the food particles and build up in the bivalves' tissues. Then, when an unsuspecting sea otter nibbles on the bivalve, it gets a dose of the parasite. Once inside the sea otter's gut, the parasite enters the otter's bloodstream. There, it can cause

infections that result in brain inflammation called encephalitis, which often kills the otter.

FOREST DEFENSE

The recent rise in sea-otter deaths may have serious consequences to the ocean ecosystem. That's because the otters are a keystone species (see Nuts & Bolts, above). "When [sea otters] are present, the ecosystem looks one way, and when they are absent the ecosystem looks dramatically different," explains Johnson.