THE INFLUENCE OF PORE-WATER CHEMISTRY AND PHYSIOLOGY ON THE DISTRIBUTION OF VESICOMYID CLAMS AT COLD SEEPS IN MONTEREY BAY - IMPLICATIONS FOR PATTERNS OF CHEMOSYNTHETIC COMMUNITY ORGANIZATION

Analyses of sulfide, methane, oxygen, and CO2 in pore-water samples fr om three cold seep sites in Monterey Bay indicate that fluid chemistry is a strong determinant of the distribution of chemolithonutotrophic vesicomyid clams. The distribution of Calyptogena pacifica and Calypto gena kilmeri were aligned closely with sulfide concentrations at all c old seeps and reflected species-specific capabilities for sulfide bind ing. Live clams occurred only in sediment where sulfide was detectable . Sulfide was not detected in the absence of vesicomyid clams. The rel ative abundances of five vesicomyid species varied greatly among seeps . C. kilmeri accounted for 85-99% of all vesicomyids at seeps with hig h sulfide content, and C. pacifca dominated (73%) seeps with low sulfi de levels. These species were also partially segregated along sulfide gradients from the center to the margin of seeps, analogous to zonatio n of rocky intertidal communities. We hypothesize that the absence of thiotrophic or methanotrophic mytilid mussels from Monterey Bay cold s eeps is related to the lack of physiological specializations for conce ntrating reduced sulfur compounds or methane and the absence of hypers aline brines that could extend the persistence of methane or sulfide-r ich fluids very near the sea floor.