GULF OF MEXICO 



485 



domestic sewage ^ is of great importance, not as 

 it affects the sm-vival of oysters, but as it affects 

 their food value to man. On the west coast of 

 Florida alone, more than a dozen shellfish-pro- 

 ducing areas have been closed to harvesting as a 

 health measure (Vathis 1950). To some extent 

 all of the Gulf States are faced with this problem. 

 Actually, the closing of polluted areas to harvest- 

 ing constitutes a conservation measure as far as 

 the continuity of the oyster population is con- 

 cerned, but it results in a deplorable waste of a 

 valuable food resource since satisfactory control 

 measures are known. 



Disease. — ^The oyster is undoubtedly afflicted 

 with a large number of diseases, but their effects 

 upon the animal are, for the most part, imperfectly 

 known. The recent demonstration of the infec- 

 tion by the fungus Dermocystidium marinum is a 

 significant contribution to om- knowledge of the 

 pathology of the oyster in the Gulf. The harmful 

 effects of this parasite are greatest at high temper- 

 atures and high salinity levels (Mackin 1951). 



Some specific parasite may be the causative 

 agent of the disastrous mortalities which occur 

 among transplanted oysters in some areas of 

 Louisiana during the hot summer months. These 

 mortalities, ranging from 35 to 95 percent of the 

 population in different areas, were unreported 20 

 years ago. Their frequent occurrence since that 

 time has necessitated entirely different cultm-al 

 techniques. Now, oysters for rebedding purposes 

 in these areas must be large enough so that a 

 few months' growth will make them suitable for 

 marketing. In the past it was the practice to 

 bed much smaller seed oysters and let them grow 

 for 18 months or longer before harvesting (McCon- 

 nell 1950). 



Oysters are widely subject to infestation with 

 the sporozoan parasite Nematopsis. There is no 

 evidence, however, that this micro-organism 

 debilitates or is the cause of mortalities in the 

 oyster (Landau and Galtsoff 1951). Of less 

 conmion occurrence is the digenetic trematode, 

 Bucephalus, whose larvae develop in and cause 

 the deterioration of the oyster gonad. Several 

 other unspecified diseases, as well as infections 

 with bacteria, have been reported recently for 

 oysters in Barataria Bay (Mackin 1951). It is 



' A detailed account of pollution in Quit water is given in the last chapter 

 of this book, pp. 556-675. 



probable that none of these diseases greatly 

 affects the reproductive potential of the oyster 

 community. 



Predators. — Man has been the most serious 

 threat to the continuity of oyster populations 

 because of his wanton methods of overharvesting. 

 Overharvesting in the past has taken the form not 

 only of removing all the oysters from the reef but 

 also of removing the underlying cultch which 

 made the reef possible. Reefs so destroyed do not 

 rehabilitate themselves naturally, and the cost of 

 replacing the foundations is not feasible econom- 

 ically. Fortunately, our increased awareness of 

 this problem during the past 50 years has reduced 

 the threat of overharvesting except in isolated 

 areas. 



The most serious natural predator of oyster 

 populations in the Gulf area is the conch or 

 oyster drill, Thais. The two forms of this species 

 have slightly different appearances; Thais j. 

 floridana occurs mostly east of the Mississippi 

 River and Thais f. haysae, primarily west of the 

 river (St. Amant 1938). Depredations due to 

 this conch are incalculable. In the Pensacola area 

 and in the southern reaches of Barataria Bay, for 

 example, it makes oyster culture impossible. The 

 snail is distributed wherever oysters are found at 

 salinity levels averaging above 15 parts per thou- 

 sand. Its populations are periodically decimated 

 in times of flood and show enormous increases in 

 times of drought. The snail reproduces during 

 the summer months simultaneously with the 

 oyster. Large individuals may deposit a half 

 mdlion eggs which develop into free-swimming 

 larvae. After a plankton stage of unknown dura- 

 tion the young snails settle to the bottom and 

 commence feeding on oysters and other sedentary 

 forms. Snails a millimeter in length and a week 

 old can be found actively drilling oyster spat of 

 the same size and age. Large snails have been 

 observed by the author eating spat at the rate of 

 four per hoiu* under experimental conditions. 

 The snails are known to live for at least 3 years 

 and possibly many more under normal circum- 

 stances. Its voracious feeding habits, high re- . 

 productive capacity, and the fact that its larvae 

 are distributed by water currents combine to 

 make this snail the most destructive oyster pred- 

 ator in the Gulf environment. A few other gas- 

 tropods present, such as the moou snail, Polynices, 



