MARINE BIOLOGYSAN FRANCISCO, CALIFORNIA-- Populations of some jellyfish appear to be exploding in several parts of the world, scientists reported at a marine science meeting here last week, raising fears that they are taking over ecosystems that nurture key commercial fish stocks. The Gulf of Mexico and the Bering and Black seas have been particularly hard hit. In some cases, however, researchers don't know whether the blooms are unusual or just natural population fluctuations.