SUMMARY OF OUR KNOWLEDGE OF THE OYSTER IN THE GULF OF 



MEXICO 



By Philip A. Butler, Fish and Wildlife Service, United States Department of the Interior 



The American oyster, Crassostrea virginica, in 

 the Gulf of Mexico is characterized perhaps most 

 of all by its versatility in adapting itself to differ- 

 ent habitat niches. There are few places where 

 at least a scattered growth of oysters is not found 

 along the 3,600 miles of tidal shore line from Cape 

 Sable in Florida to the Rio Grande in Texas. 

 The distribution of this oyster population is more 

 impressive if we consider its area. There are 

 probably more than 1,400 square miles of water 

 bottoms along the Gulf coast suitable for and 

 more or less populated with oysters. In some 

 areas the oyster communities consist of small 

 isolated patches only a few yards in area which 

 are elevated like islands in a "sea" of too soft 

 mud. Where the bottom is more firm the com- 

 munities may cover many acres, and reefs up to 

 25 miles in length are not unknown. 



Oysters establish successful colonies in the 

 range from a foot above mean low water to a 

 depth of 30 to 40 feet in some of the deeper chan- 

 nels. Their occurrence is everywhere dependent 

 on a suitable or rather a physically possible sub- 

 stratum. They compete with the tree oyster for 

 space on the mangrove roots; they cover bridge 

 footings and piles of old ballast rock, and have 

 even been found 5 miles out in the open Gulf on 

 an oil well rigging (Gunter 1951a). 



In well-protected areas small clusters and 

 single oysters become established on pure sand 

 bottoms. On mud bottoms, too soft to support a 

 reef, single oysters may grow until their own 

 weight carries them below the surface. New spat 

 settle on the edges of such oysters and grow until 

 they, in turn, sink into the mud. Formations 

 such as these with a living oyster attached to 

 buried older generations may extend like poles 

 3 feet or more into the soft substratum. Oysters 

 persist in isolated marsh ponds whose waters con- 

 nect with the open sea only for short intervals in 

 time of flood or storm. Scattered individuals 



attach to the roots of marsh grass fringing the 

 bayous and bays or settle secondarily on mussels 

 growing in the marsh. With natural cultch fre- 

 quently at a premium oysters are found attached 

 to crabs, turtles, and even on the shell of their 

 worst enemy, the drill. 



More typically, reefs build up on the sticky mud 

 bottom found in most of the coastal bays and 

 estuaries along the Gulf shore.' The foundations 

 of some of these reefs have persisted since pre- 

 historic times. Successive generations of oysters, 

 attaching to the older oysters and dead shells, 

 have created deposits many feet in thickness. 

 The living population forms an infinitesimal per- 

 centage of the total mass on such reefs. In other 

 areas, although the reef may have existed for in- 

 numerable years, the substratum of shell is rela- 

 tively thin, comprising little more bulk than that 

 of the living oysters. In these cases, the high pop- 

 ulation density of commensals, which excavate the 

 valves, and perhaps chemical factors in the environ- 

 ment lead to the rapid disintegration and disap- 

 pearance of the old shells. 



A fundamental requirement in the ecology of 

 the oyster is the mixture of salt water from the 

 Gulf or ocean with the fresh water from land 

 drainage. The characteristics of the growing oys- 

 ter community, dependent in large measure on 

 the salinity level resulting from the admixture of 

 these waters, are governed not only by the average 

 salinity levels but also by the extremes of seasonal 

 fluctuation in the total salt content. The size of 

 the population and of the individual oyster, ap- 

 pearance of the shell and quality of the meat, 

 reproductive potential, commensals, predators, 

 parasites, all of these and probably a host of other 

 factors have an obligatory relation to the salinity 

 level of the environment. 



1 The formation of oyster reefs in the Oulf of Mexico is discussed in detail 

 by \V, A. Price, p. -Wl. 



479 



