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 CO, in pore-water sampdes from three cold seep sites in Monterey Bay indicate that fluid chemistry is a strong determinant of the distribution of chemolithoautotrophic vesicomyid clams. The distribution of Culyptogena pacijca and Culyptogenu kilmeri were aligned closely with sulfide concentrations at all cold seeps and reflected species-specific capabilities for sulfide binding. Live clams occurred only in sediment where sulfide was detectable. Sulfide was not detected in the absence of vesicomyid clams. The relative abundances of five vesicomyid species varied greatly among seeps. C. kilmeri accounted for 85-99% of all vesicomyids at seeps with high sulfide content, and C. puciJicu dominated (73%) seeps with low sulfide levels. These species were also partially segregated along sulfide gradients from the center to the margin of seeps, analogous to zonation of rocky intertidal communities. We hypothesize that the absence of thiotrophic or methanotrophic mytilid mussels from Monterey Bay cold seeps is related to the lack of physiological specializations for concentrating reduced sulfur compounds or methane and the absence of hypersaline brines that could extend the persistence of methane or sulfide-rich fluids very near the sea floor. Chemosynthetically based communities associated with sulfide and methane seeps are known from various geologic settings along continental margins but remain poorly understood in terms of their geochemistry, biology, and ecology. Seeps in the Pacific Ocean are associated with tectonically induced over-pressure of hemipelagic sediments along subducting (e.g. Oregon, Peru, Japan) and transpressional plate margins (California; Barry et al. 1997), artesian flow at marine exposures of freshwater aquifers (Hovland and Judd 1988), and buried debris flows in axial valleys of submarine canyons (Embley et al. 1990). Most chemosynthetic metazoan symbioses at sea floor seeps depend on sulfur-oxidizing bacterial endosymbionts, especially in the Pacific Ocean where vesicomyid clams and vestimentiferan worms are the dominant metazoan groups. Although seeps in the Atlantic Ocean have a similar fauna, including both vesicomyid clams and vestimentiferan worms, these sites differ from PaAcknowledgments We are grateful to Patrick Whaling for assistance in collecting and processing samples. Discussions with K. Buck, C. Harrold, L. Ferioli, H. Jannasch, S. Lisin, D. Orange, and B. Robison improved the content and quality of the manuscript, as did comments from two anonymous referees. This study would not have been possible without the expert assistance of the crew of the Pt. Lobos and pilots of the ROV Ventunu. Funding for this project was provided by the Monterey Bay Aquarium Research Institute. cific seeps owing to the presence of mytilid mussels harboring methanotrophic symbionts that are abundant at several sites in the Gulf of Mexico (MacDonald et al. 1990a, b) and along the base of the Florida Escarpment (Paul1 et al. 1984). As in all natural communities, the species composition and organjzation of seep communities depends upon factors influencing the supply of larvae for potential seep species and also factors affecting larval settlement and postsettlement survj.val or growth. Larval supply is affected by reproductive rates, larval survival, and planktonic transport, of which very little information is available concerning seep species. Settlement and postsettlement processes are also little known but are likely to include the effects of physical characteristics of seep sites (e.g. sulfide concentration) and biological interactions among species. The habitability of seeps for species in the larval pool probably depends largely on the concentration of sulfide or methane at cold seeps, which may also play an important role in der:ermining the local distribution of species within seeps. The survival and growth of seep species must also be linked to variation in their requirements or tolerances of sulfide or other compounds. Although sulfide and methane have been detected at nearly all Pacific cold seep sites, little information is available concerning spatial gradients or patchiness of sulfide and methane within seep habitats. The molluscan bivalve family Vesicomyidae is represented by several species in both the Pacific and Atlantic oceans.