PHYSIOLOGICAL ECOLOGY OF A MUSSEL WITH METHANOTROPHIC ENDOSYMBIONTS AT 3 HYDROCARBON SEEP SITES IN THE GULF-OF-MEXICO

In situ growth rates were determined, using two, 1-yr mark/recapture e xperiments, conducted between September 1991 and July 1993, for an und escribed mytilid, Seep Mytilid Ia, at three hydrocarbon seep sites in the Gulf of Mexico. The sites are located at depths of 540 to 730 m, a pproximately 27 degrees 45'N; 91 degrees 30'W, and are separated by di stances of 6 to 18 miles. These seep mytilids harbor methanotrophic en dosymbionts and use methane as both a carbon and energy source. The mu ssel habitats were chemically characterized by analysis of water sampl es taken from precisely located microenvironments over, among and belo w the mussels, using small-volume, interstitial water samplers and the ''Johnson Sea Link'' submersible. Substantial differences were found in habitat conditions, growth rates, and population structure for the mussels at the three sites examined. The growth rate of these seep myt ilids reflects the methane concentration in their immediate habitat. M ussels at sites with abundant methane had growth rates that were compa rable to shallow water mytilids at similar temperatures (5 to 8 degree s C) with increases in shell length up to 17 mm yr(-1) documented for smaller mussels (< 40 mm shell length). In conjunction with measuremen ts of growth rates, three condition indices (glycogen content, tissue water content, and the ratio of ash-free dry weight to shell volume) w ere used to determine the relationship between the condition of the mu ssels, their growth rates, and their habitat chemistry. The three cond ition indices were correlated with growth rate and were often signific antly different between mussels in different samples.