Growth and condition of seep mussels (Bathymodiolus childressi) at a Gulf of Mexico Brine Pool

The banks of Brine Pool NR1, a brine-filled pockmark in the Gulf of Mexico, are host to a population of Bathymodiolus childressi, a hydrocarbon seep m ussel with methanotrophic endosymbionts. The mussel community surrounds the pool, covering an area of similar to 540 m(2) and can be divided into two distinct zones separated by a transitional area. The inner zone extends inw ard from the edge of the pool similar to 1 m. The water among the mussels i n this zone has high methane concentrations (>200 mu mol/L) and moderate ox ygen concentrations (nondetectable to 161 mu mol/L). Hydrogen sulfide was r arely detected in this zone. The outer zone extends similar to 1 m inward f rom the outer edge of the mussel bed. This zone also has high methane conce ntrations in the water among the mussels (>200 mu mol/L), but lower average oxygen levels and areas with very high levels of hydrogen sulfide. The two zones are linked with a transitional area, the middle zone, ranging in wid th from 1 to 4 m, where intermediate environmental conditions are found. Th e inner zone was relatively homogenous, while spatial heterogeneity was hig h in the middle and outer zones, both in the characteristics of the mussel population and in their environment. Size-frequency distributions indicate that the inner zone is an area of active recruitment, with very little recr uitment occurring in either the middle or outer zones, Physiological condit ion indices of the mussels were significantly higher in the inner zone than in the outer zone in some years. Comparisons of growth parameters also ind icate better growth conditions in the inner zone. Physiological condition a nd growth did not change between the first two years of the study, However, a significant change in both was detected between 1994 and 1995, suggestin g that this deep-sea community is exposed to an unexpected temporal variabi lity in environmental conditions.