564 



MARINE PRODUCTS OF COMMERCE 



with a rasplike structiore, called a radula; by rapid movements of the radula small 

 pieces of oyster meat are scraped and devoured. 



The common oyster drill or screw-borer of the Atlantic Coast ( Urosalpinx cinerea 

 Say) and the conch {Thais haemastoma fioridana Conrad) of the Culf States cause 

 very heavy damages, especially to young oysters. There are several places along 

 the coast of the United States where oyster spat has no chance to reach maturity, 

 for it is usually destroyed by drills or conchs within a year. For example, oyster 

 grounds in lower Chesapeake Bay are so heavily infested with screw-borer that 



Fig. 26-10. Tangle used 

 for catching starfish in Long 

 Island Sound. 



the oyster growers have to be satisfied with a return of M bushel of market oysters 

 to 1 bushel of seed planted. A yield of 1 to 2.5 would have been easily realized 

 if these grounds were free of drills. 



Oystermen themselves help to spread the drills and conchs to noninfested areas 

 by transplanting their stock without taking any steps to clear it of snails or 

 their eggs, encapsulated in the gelatinous egg cases attached to the shells. In this 

 way the drills, which themselves can crawl only over short distances, have spread 

 over the entire coastline and have even been introduced to Europe. Through the 

 importation of Japanese seed oysters to the Pacific Coast, a very destructive Japa- 

 nese conch (Tritonalia japonica Dunker) was introduced to Puget Sound waters, 

 where it established itself and became a menace to the oyster fishery. 



Control of marine snails is expensive and difficult. The present methods of 



