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 mussel with methanotrophic endosymbionts. The mussel community surrounds the pool, covering an area of ~540 m² and can be divided into two distinct zones separated by a transitional area. The inner zone extends inward from the edge of the pool ~1 m. The water among the mussels in this zone has high methane concentrations (>200 μmol/L) and moderate oxygen concentrations (nondetectable to 161 μmol/L). Hydrogen sulfide was rarely detected in this zone. The outer zone extends ~1 m inward from the outer edge of the mussel bed. This zone also has high methane concentrations in the water among the mussels (>200 μmol/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 width from 1 to 4 m, where intermediate environmental conditions are found. The inner zone was relatively homogenous, while spatial heterogeneity was high 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 recruitment occurring in either the middle or outer zones. Physiological condition indices of the mussels were significantly higher in the inner zone than in the outer zone in some years. Comparisons of growth parameters also indicate better growth conditions in the inner zone. Physiological condition and growth did not change between the first two years of the study. However, a significant change in both was detected between 1994 and 1995, suggesting that this deep-sea community is exposed to an unexpected temporal variability in environmental conditions.