In September, we told you about xenotransplants, a medical technique involving transplanting tissue from animals into human bodies. This week, at a meeting in Washington D.C., public health officials met to discuss whether or not xenotransplantation's risks outweigh its possible benefits. Some are calling for a moratorium on the clinical use of the xenotransplantation technique.

It's a difficult issue on which to make a decision. Organs for transplant are in extremely short supply. In 1996, an estimated 45,000 Americans under the age of 65 could have benefitted from a heart transplant, yet only 2000 hearts were donated that year. Many patients die waiting for organs to become available. Cloning has been suggested as a potential source of transplant organs, but at present is just a dream, and artificial organs still have not been perfected.It is easy to see why using hearts from baboons or livers from pigs to help save human lives is tempting to some researchers.

On the other hand, xenotransplantation is a new and uncertain field. Many scientists are concerned at the potential for animal diseases to cross over into the human population. Recent findings that certain viruses that normally live in pigs may be able to live on in human hosts have increased fears of possible outbreaks of new diseases. Other people are concerned about xenotransplants on ethical grounds.

Join us for look at whether or not it's wise to push ahead with xenotransplants, on this segment of Science Friday.

An Asian clam, Potamocorbula amurensis, was first sighted in the bay in 1986. It is now the most common organism in the bay, clustering in groups of hundreds, even thousands per square meter. Scientists think that the clam may have hitched a ride from overseas in the millions of gallons of ballast water discharged by ships that visit the bay. Other newcomers to the bay within the last 150 years include the European green shorecrab, the soft shell and littleneck clams, and the striped bass. A type of smooth cordgrass, Spartina alterniflora, has taken over from native grasses in much of the marshland and wetland areas around the bay.

Most species are thought to have arrived in ballast water, like the Asian clams. Other species have arrived stuck to the outside of ship hulls or attached to the shells of imported oysters. Some were even deliberately introduced, either for food, sport, or to control other species - but all have taken to the bay with a vengeance.