of the valves, and registered the deflection of a 

 small plate placed in front of the cloacal current. 

 As indicated in chapter IX this method is not 

 reliable for a quantitative determination of the 

 volume of water transported through the gills but 

 is adequate for determining the relative strength 

 of the cloacal current. The results show that the 

 adaptation to new conditions depends upon the 

 degree of change. Recovery was more rapid when 

 the salinity was increased than when the same 

 degree of change was made in the opposite direc- 

 tion. At a sahnity of about 13°/oo very little 

 water was transported even after several days 

 were allowed for adaptation, but recovery to 

 normal activity followed rapidly after the return 

 of the oyster to a normal environment in water of 

 267ooto297oo. Increased salinity, from 257oo to 

 39°/oo, produced no significant changes in the 

 water transport by C. gigas. An unfavorable 

 effect was recorded at 567oo, which is considerably 

 above the normal range of the oyster's habitat. 



In experiments at the Bureau of Commercial 

 Fisheries Biological Laboratory, Milford, Conn. 

 (Loosanoff, 1952), C. virginica from Long Island 

 Sound accustomed to water of a stable salinity of 

 about 27 %o were placed directly in water of 20, 

 15, 10, and 5 °/oo made by the addition of a cor- 

 responding amount of fresh water. The loss of 

 food caused by the addition of plankton-free water 

 was compensated by providing measured amounts 

 of phytoplankton culture. The decrease in the 

 rate of water transport was proportionate to the 

 degree of change and varied from 24 to 99.6 per- 

 cent of the normal rate. Six hours of exposure to 

 the lowest salinities tested resulted in no perma- 

 nent injuries, and within a few hours after transfer 

 to the salinity of their natural habitat the oysters 

 fed, reformed the crystalline styles, and dis- 

 charged true feces and pseudo feces. Other ex- 

 periments at Milford at the same time demon- 

 strated that oysters conditioned to live in low 

 salinities can tolerate lower concentrations of salts 

 than oysters living in more saline waters. Al- 

 though the oysters were observed to feed in water 

 of 5 %o salinity their shell movement and water 

 transport were abnormal and growth was inhibited. 



The reproductive capability of oysters is in- 

 liibited by low salinity. Butler (1949a) showed 

 that this is due primarily to the failure of gonad 

 development in oysters of the marginal area of 

 upper Chesapeake Bay; his findings were con- 

 firmed by experiments with Long Island Sound 



oysters (Loosanoff, 1952). These experiments 

 have not demonstrated whether the failure of 

 gonad development is the direct result of lowered 

 salinity or is due to inadequate feeding. 



Long-continued exposure to salinities above the 

 32 %o level also has an unfavorable effect on oyster 

 populations. This can be seen from the conditions 

 of Texas oyster beds. During the 6-year drought 

 from 1948 to 1953, the salinity in the bays of the 

 central Texas coast generally rose well over 36%o 

 and at times reached the 40%o level without an 

 appreciable decrease in tlie winter (Parker, 1955). 

 Previous records, from 1922 until 1948, show that 

 during most of the year salinity in this area ranged 

 from 5%o to 25%o with somewhat higher sal- 

 inities in the summer. With the increase in salin- 

 ity there was a gradual replacement of C. virginica 

 populations by 0. equestris. In 1952 over half of 

 the young oysters (spat) were 0. equestris, whereas 

 in years of low salinity the reefs were comprised 

 primarily of C. virginica. It is not known whether 

 the observed change was due to the inhibition of 

 gonad formation or to the failure of oyster larvae 

 to reach setting stage. From an ecological point 

 of view it is, however, significant that the replace- 

 ment of one species by another took place at the 

 time of the increase in salinity of water. The 

 surviving C. virginica were observed to develop 

 different shell characteristics: the valves became 

 crenulated, thin, sharp, and higlily pigmented. 



Under certsiin circumstances the influx of fresh 

 water into estuaries may be beneficial. Some of 

 the carnivorous gastropods, flatworms, and star- 

 fishes, which are highly destructive to oysters, are 

 killed by brackish water that constitutes a barrier 

 through which they cannot penetrate. Decrease 

 in the salinity of water protects the populations of 

 oysters at the heads of the bays. Periodical flush- 

 ing wipes out the predators and restores the pro- 

 ductivity of beds. The population of oysters in 

 areas highly infested by their enemies, as in the 

 Apalachicola Bay, tlie upper half of the Delaware 

 Bay, and many others, cannot exist if the access 

 of fresh water to oyster bottoms is diminished and 

 the salinity increases above the 15%o level. 



The evaluation of the salinity factor can be esti- 

 mated by determining the total percentage of 

 time the brackish water of less than 10%o 

 or water of salinity exceeding 34 %o remains on 

 an oyster bottom. The zero value is assigned to 

 conditions unsuitable for the oyster's existence; 

 marginal conditions are indicated by 1, and opti- 



406 



FISH AND WILDLIFE SERVICE 



