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The Trouble with the Sea Otter's Menu

Otters Need Food Inspectors?

Because of their dietary habits, sea otters living along the central California coast face the menace of disease-causing parasites (www.usgs.gov).
 
Sea otters that dine on small marine snails are at a higher risk of encountering Toxoplasma gondii, a potentially deadly pathogen, than animals that feed exclusively on other prey. And, sea otters along the coast of San Simeon and Cambria are in more peril than sea otters living elsewhere.
 
In a similar vein, sea otters that feed on clams and fat innkeeper worms at the southern end of Monterey Bay have more contact with another perilous little pathogen, Sarcocystis neurona. 
 
So where's an otter to go? Fortunately, the sea otters whose diet includes significant amounts of abalone, a preferred prey species when abundant, have a very low risk of infection with either pathogen.
 
"Recovery of the sea otter in California has been especially sluggish at the center portion of its range, where sea otter densities are highest and where most of the reproduction occurs," said Tim Tinker, a U.S. Geological Survey (USGS) sea otter expert. "Where food resources are limited, individual sea otters tend to become diet specialists, and the specific skills used to secure food are passed on from mother to pup."
 
Limited Dining Options Threaten the Species
 
The result is that individual otters inhabiting the same area can have very different diets from one another, and it now appears that high levels of infection with T. gondii or S. neurona may be the result.  "Our findings indicate that prey choice in sea otters has very real implications for their health," said Christine Johnson, epidemiologist at UC Davis. "Depleted resources and high rates of infectious disease may be acting in concert to limit the recovery of this threatened species."
 
T. gondii and S. neurona both have complex life cycles involving multiple hosts. They complete their life cycle in their respective final hosts, cats (T. gondii) and opossums (S. neurona), which then shed new stages of the parasites into the environment. These infective oocysts can persist for months in the environment outside a host.  Although these parasites are thought to have land origins, many oocysts end up in the marine environment, where they can be consumed by invertebrates in the marine food web. Sea otters that become infected from consuming infected prey may eventually die from encephalitis, an inflammation of the brain.
 
A key challenge for scientists has been to determine exactly how the parasites are getting to sea otters. Identifying routes of infection has been difficult because it is almost impossible to detect the parasites in the environment.
 
Complete findings appear in the February 17, 2009 issue of Proceedings of the National Academy of Sciences (www.pnas.org).
By Adam Herschkowitz
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