BACTERIA large enough to be visible to the human eye have been discovered within the gut of a surgeonfish (Acanthurus nigrofuscus) caught off the Great Barrier Reef in Australia (Science, Vol. 256, No. 5064). These symbiotic bacteria, about one million times larger than ordinary bacteria, probably digest the algae the fish graze on.

Initially, researchers assumed the bacterium, because of its size, was a protozoan -- a single-celled organism that ordinarily is larger than bacteria. The giant bacteria have some other features that make them different, including the absence of a membrane around the cell nucleus. A molecular study based on gene sequences has conclusively proved the organisms are bacteria and the data suggest the bacteria are related to the bacterial genus Clostridium.