OYSTERS AND METHODS OF OYSTER-CULTURE. 289 



meuliaden, the alewife, and otlier fish equipped with delicate sifting 

 devices at times find the oyster fry of some importance in their dietary. 



After the attachment of the spat other enemies, active and passive, 

 wage war upon it. The passive enemies affect its welfare by consum- 

 ing its food or by smothering it beneath their own more active growth. 

 Of tlie former class, mussels, lingulas, etc., are examples, but as the 

 food upon an oyster-bed is usually sufficient for all, this is not a very 

 important consideration, ijarticuiarly as in the end an equilibrium is 

 established through the intimate reciprocity which exists between the 

 various forms of life. 



The conditions of life upon an oyster-bed are favorable to the rapid 

 growth of dense sponges, mussels, barnacles, hydroids, and tube-build- 

 ing worms, which establish themselves upon the young growth, often 

 increase more rapidly than their hosts, and, in many cases, overgrow 

 them to such an extent as to cut off the supply of food and oxygen. 

 (Plate XYii). Aquatic vegetation sometimes has the same effect when 

 its growth becomes extensive. Cert lin worms, such as Serpiila, and 

 especially Sabellaria (plate xv, fig. 3), often build their tubes of lime or 

 sand so rapidly as to i)roduce dense accumulations upon the surface 

 of the shells, thus forming a nidus for the collection of sand and mud. 

 Considerable loss has at times resulted from the suffocation of oysters 

 by sponges, worm tubes, and vegetable growths, but most of these 

 passive forms have a compensatory use in the food which their spores, 

 eggs, and young furnish to the oysters. 



The active enemies of the adult oyster are those which injure it by 

 direct attacks, such enemies being found in most of the classes of 

 zoological life having aquatic representatives. 



Fishes of several kinds are found habitually on the oyster-beds. 

 Most of these offer no direct injury and they may even benefit the 

 03'ster by keeping down the crowding masses of hydroids and vegetable 

 life, but a few species, of which the drumfish is aj^parently the most 

 destructive upon the Atlantic coast, consume considerable quantities 

 of oj'sters as food. At times much damage has thus been wrought to 

 the beds in the vicinity of New York and along the New Jersey coast. 

 In iSan Francisco Bay the stingray is the most feared enemy of the 

 oyster, and schools of them frequent^ly "clean out" the beds to which 

 they gain access, their teeth being such that the shells are crushed into 

 fragments iu their grasp. Some of the ska»tes and rays on the eastern 

 coast no doubt have similar habits, but they do not appear in suflficient 

 numbers to cause much harm. 



The drills are the most destructive enemies of the oysters in the 

 Chesapeake and adjoining regions, as well as upon most of the more 

 important inshore beds northward. There are, perhaps, several species, 

 but the most destructive is the form known to naturalists as UrosaJpinx 

 cinercn (plate xv, fig. 1). It is a snail-like niollusk, which, by means of its 

 raspiug tongue, drills a tiny hole in the shell of the oyster, through which 

 it extracts the soft parts. It is only the younger oysters which are thus 



F. C. R. 1897 19 



