chemosynthetic, bacterial endosymbionts (Kennicutt et al . , 

 1985; Brooks et al . , 1985, 1987a, b). These discoveries 

 significantly expand the geographic area of the deep ocean 

 where one might expect to encounter dense populations of 

 vent-type taxa. Subsequent studies on the upper Gulf of 

 Mexico continental slope using submersibles and surface 

 ships have: 



( 1 ) Identified chemosynthetic organisms or their 

 remains (either tube worms, mussels and/or clams) at 

 17 northwestern Gulf of Mexico continental slope 

 sites (Fig. 1 ) ; 



(2) Confirmed, based on enzyme activities, elemental 

 sulfur content and electron microscopy, that 

 tubeworms and clams from these sites do contain 

 chemoautotrophic, bacterial endosymbionts (Brooks et 

 al., 1987b); 



(3) Found a mussel that is potentially capable of 

 utilizing methane as its sole carbon and energy 

 source, the first demonstrated symbiosis between a 

 methanotrophic bacteria and an animal (Childress et 

 al., 1986); 



(4) Identified shallow seismic "wipe-out" zones as high 

 probability sites for chemosynthetic ecosystems 

 (Fig. 2); 



(5) Shown that active oil seepage is associated with 

 all of the chemosynthetic ecosystems described on 

 the Gulf of Mexico slope (Fig. 3); 



(6) Demonstrated that carbon, nitrogen and sulfur 

 isotopes can be useful in differentiating 

 heterotrophic, sulfur-based and methane-based 

 ecosystems (Brooks et al . , 1987b); 



(7) Confirmed the transfer of carbon from the 

 chemosynthetic organisms to background heterotrophic 

 organisms; 



( 8 ) Discovered ten gas hydrate and several active oil 

 seepage locations in the Gulf of Mexico; and, 



(9) Determined that shell beds are being produced in 

 and around areas of petroleum seepage. 



METHODS 



Three series of dives have been conducted at the 

 hydrocarbon seep communities. The first six dives aboard 



120 



