I. 







Centimeters 



Figure 392. — T. haemnstoma haysae Clench from Bastian Bay in the lower part of the Mississippi River delta. 



1 — apertural view; 2 — abapertural view. Natural size. 



The larvae that escape from the egg capsules are 

 veligers, which pass through a free-swimming 

 period of unknown duration, and are widely dis- 

 persed by tidal currents before they settle on the 

 bottom and begin attacking small oysters and 

 other bivalves. 



The distribution of Thais is checked by fresh 

 water. The conch is immobilized by a salinity of 

 10%o, and a 1- or 2-week exposure to a salinity 

 of 77oo kills them (Scliechter, 1943). 



The effect of sudden changes in water salinity 

 on the rate of crawling of Thais was corroborated 

 by my observations at the Bureau of Commercial 

 Fisheries Biological Laboratory at Gulf Breeze in 

 northwestern Florida. The crawling of these 

 snails in the tanks was automatically recorded on 

 a kymograph. The movements stopped immedi- 

 ately when the salinity of the water was artificially 

 reduced from 157oo or 17%o to 87oo or 97oo. 



The snails became active again when the salinity 

 returned to the former level. 



Two species of conchs found on oyster grounds 

 of tiie Pacific coast are Thais lamellosa Gmelin 

 (fig. 393), a native snail, and Ocenebra (Tritonalia) 

 Japonica Dunker (fig. 394), introduced from Japan. 

 T. lamellosa has been considered by some fishery 

 biologists as a predator on 0. lurida, but Kincaid 

 (1957) discards this view as not substantiated by 

 his 50 years of familiarity with the marine fauna 

 of the region. He states that since T. lamellosa 

 feeds mainly upon barnacles and mussels, the snail 

 sliould be classified as "tlie only invertebrate 

 friend" of the oyster, presumably because it 

 destroys its competitors. Chapman and Banner 

 (1949) found that under experimental conditions 

 T. lamellosa drilled some 0. lurida, but that in a 

 natural environment it showed a preference for 

 mussels (Mijtilus edulis). 



434 



FISH AND WILDLIFE SERVICE 



