Embryos and larvae survive only in salinities 

 between 2%o and 1 5%o. After reaching the setting 

 stage (6 to 7 days after fertilization), the juvenile 

 clams become more tolerant of salinity fluctuations. 

 Rangia is incapable of reproducing or of maintaining 

 permanent populations at salinities higher than about 

 15%o. The stabilization of salinity at any level wiU re- 

 sult in the dying out of the population in 15 to 20 

 years, when old clams reach the limit of their life 

 span. 



Optimum temperatures for larvae occur at 24° C 

 (75° F), but fastest growth occurs at higher tempera- 

 tures (32° C or 90° F). Temperatures of 30° to 35° C 

 (86° to 95° F) are critical; damage occurs above 35° C 

 (95° F) for rangia. Temperature affects respiration 

 most drastically at the extremes of the salinity range 

 (2%o to 32%o). Lower temperatures usually have no 

 lethal effects on adult rangia, although rates of respi- 

 ration and growth are reduced. 



Predators may also limit the abundance of rangia. 

 Rangia is a major food of lesser scaup, blue crab, and 

 bottom-feeding /ishes (croaker, drum, etc.). 



5.6.2 AMERICAN OYSTER (Crassostrea virginica) 



The planktonic eggs and larvae of the American 

 oyster are at the mercy of currents. However the larvae 

 can swim vertically and take advantage of the horizon- 

 tal movement of salt wedges that allow populations to 

 be transported shoreward or inland. At the end of their 

 larval stage, young oysters (now called spat) attach to a 

 firm substrate where they remain and grow to adults. 



Natural oyster reef areas are located where bottoms 

 characterized by firm mud, rock, or shell. Typically, 

 the bays bottoms of south Louisiana are firm around 

 their periphery, increasing in softness toward the center 

 (Van Sickle et al. 1976). Therefore, bay perimeters are 

 usually the best habitat for oysters. Along the Gulf 

 coast, especially in Louisiana, oyster reefs are often 

 associated with raised features of the water bottom. 



The formation of a natural oyster reef begins with 

 the attachment of larvae to a piece of shell or to other 

 hard objects. Other larvae will attach to those already 

 set, forming a small cluster of juvenile oysters. There is 

 a high rate of mortality among oysters. Dead shells 

 provide additional surfaces for attaclmient. Successive 

 sets begin the cycle again, and the reef grows horizon- 

 tally and vertically (Galtsoff 1964). The annual accre- 

 tion of oyster shells provide additional stability. 

 Gunter (1976a) found shells at the base of some 

 Galveston reefs to be more than 6,000 years old. 



In attempts to reestablish natural oyster reefs or to 

 provide additional material for spat attachment in the 

 vicinity of producing reefs, cultch materials are often 

 deposited. The most common cultch materials are 

 oyster and clam (rangia) sheUs. Clam shell is more 

 abundant and is preferred by many oystermen because 

 it generally promotes the development of more larger 

 unclustered oysters (Van Sickle 1977). 



Salinity levels are crucial to oysters. The produc- 

 tivity of an oyster community is governed not only by 

 average levels of salinity, but also by extreme seasonal 

 fluctuations (Butler 1949). According to Galtsoff 

 (1964), oysters can tolerate a salinity range from 5%o 

 to 40%o, but optimum salinity for Louisiana oysters 

 is 15%o. In Louisiana and Texas waters, the optimum 

 salinity range for natural oyster growth and survival 

 lies between 5%o and 20%o (Hofstetter 1977). 



Because free exchange of water is essential for 

 growth and survival of oysters, stagnant water is detri- 

 mental to oyster reefs. The spat must set on firm sub- 

 strate located where bottom currents are strong 

 enough to bring in sufficient food and oxygen and to 

 carry away metabolic waters (Galtsoff 1964). 



The velocity of water currents helps determine the 

 amount of sediment deposited on an oyster reef. The 

 more productive oyster reefs are usually located in 

 areas free from siltation. Reefs are often located with 

 the long axis perpendicular to the direction of prevail- 

 ing water currents. Such reefs are common along the 

 Texas coast (Hedgepeth 1953). 



Oyster larvae feed on phytoplankton and detrital 

 particles. Spat are suspension feeders (i.e., they ob- 

 tain food by pumping large quantities of water across 

 their gills and filtering out suspended particulate mat- 

 ter, even oyster larvae). A single oyster can pump up to 

 341/hr (9 gal/hr) of water across its gills (Galtsoff 

 1964). Although the American oyster is adapated to 

 do weU in moderately turbid water, a large increase in 

 turbidity can cause a decline in feeding by impairing 

 the feeding mechanism (Loosanoff and Tommers 

 1948). 



Oysters in Louisiana spawn from spring to late 

 fall. Enormous numbers of eggs and sperm are released 

 into the water column, yet only a small proportion of 

 the eggs are fertilized. About 2 weeks elapse from the 

 time of fertilization until the larvae are fully develop- 

 ed. 



Oyster production is a function of available habi- 

 tats, hydrological processes, and natural and man- 

 caused stresses within each basin. Water salinity is the 

 most important parameter. 



In addition to an optimum salinity level, oysters 

 must have suitable substrate for attachment and suf- 

 ficient water movement both for transporting the 

 planktonic phase and for exchanging food and wastes 

 during the attached phase. 



Dredging may severely damage oyster reefs by de- 

 stroying the reef or by causing increased turbidity in 

 the vicinity of the reef. Sediments impair the oysters' 

 feeding mechanism. Dredging may alter sahnity re- 

 gimes by creating passages for salt water to move 

 closer to or farther from reef areas. In 1940-41, a nav- 

 igation channel 30 feet (9 m) deep with a bottom 

 width of 250 ft (76 m) was dredged through Calcasieu 

 Lake and Pass to the Gulf of Mexico, resulting in sig- 

 nificant salinity increases in the Calcasieu River and 

 Calcasieu River-Mermentau River section of the Gulf 



261 



