been introduced into eggs in cryptic forms. AmongThese bacteria resembled gill endosymbionts in the following 3 polnts: dimensions and general outllne, occurrence of elecdeep-sea tZlftia pachyptila was tron-transparent vacuoles in the periplasm, and occurrence of establish syn~biosis by talung up bacteria from freeelectron-dense materials in the periplasm. Eggs which harbored bacteria were all in the vitellogenic stage. This finding suggests some restricted time schedule of the entry of bacteria into oocytes. It is hypothesized that these bacteria represent transmission stage of gill symbionts, and continuum of the symbiosis is maintained by maternal inheritance through egg infection.

Abstract:Additional, intracytoplasmic membrane-stacked bacterial symbionts were found to colonize the same bacteriocytes of a hydrothermal vent snail, Alviniconcha hessleri, along with previously found slender rod-shaped symbionts.These membrane-stacked bacteria (MSB) were observed only in a part of the bacteriocytes in gill sections examined. Electron microscopy revealed that the bacteriocytes of A. hessleri possessed phagocytic activity.The phagocytic incorporation of MSB by bacteriocytes, in addition to uneven distribution of these bacteria among gill filaments, strongly suggest that MSB were acquired by the bacteriocyte as guests from the external environment. Electron micrographs revealed an intermediate phase of intracellular and extracellular existence of both types of bacteria. This mode of occurrence can be explained by the compromise between the avoidance of self defense mechanisms of host cell and keeping intimate contact with their host. Phage-like particles (PLPs) were found in the slender rod-shaped symbionts of A. hessleri. This is the first observation of PLPs inside symbiotic chemoautotrophic bacteria.

Since 1984, dense biological communities dominated by giant vesicomyid clam, Calyptogena soyoae, have been located on 1000 to 1200m deep seabed along the foot of the western escarpment of Sagami Bay Japan, by the manned submersible, "Shinkai 2000" and deep-tow TV and photography of Japan Marine Science and Technology Center. Chemical and isotopic study of the clams, sediments and bottom seawater, sampled during the field study of the area using "Shinkai 2000" in 1986, revealed that the clam com munities are in close association with methane rich seep systems and that chemoautotrophic sulfur bacteria, living symbiotically in the clam's gill tissues, play an important nutritional role for the seep communities.