entire oyster bottom but apparently exerts no ill 

 effect on oyster populations. This is not the case, 

 however, with another aquatic plant, the Eurasian 

 watermilfoil, Myriophyllum spicatum, which by 

 1933 was established on the Virginia and Maryland 

 sides of the Potomac River; since 1959 it has 

 increased rapidly in the Chesapeake Bay area, 

 including the Potomac River, and is found also 

 in the fresh and nontidal waters above Washington 

 D.C. In recent years the growth of the plant 

 has become spectacular and is a threat to brackish 

 water oyster grounds, which may become covered 

 with a heavy layer of decomposing leaves and 

 stems of the milfoil (Beaven, 1960; Springer, 

 Beaven, and Stotts, 1961). 



The effect of commensals and competitors on the 

 productivity of an oyster bottom can be evaluated 

 for each species if the intimate relationship to the 

 host is clearly understood and the relative abun- 

 dance of a species is determined. A single species 

 which appears to be innocuous imder normal 

 conditions may become destructive and dangerous 

 because of its mass development. All these 

 conditions should be evaluated in order to express 

 their effect in numerical terms. Commensals 

 such as bryozoans, barnacles, and tunicates so 

 completely cover the shell sm'face that the 

 settlement of young oysters upon it is prevented. 

 Thus the negative effect of fouling may be con- 

 sidered in relation to the productivity of setting 

 groimds. On the other hand, in southern waters 

 where setting continues for the greater part of the 

 year and oysters become overcrowded with suc- 

 cessive generations of young, the reduction and 

 prevention of settlement of spat may be beneficial 

 because it reduces overcrowding and permits 

 better growth and fattening of oyster stock. The 

 struggle for space is an essential factor in the life 

 of an oyster community. 



PREDATORS 



The list of many enemies that prey on oysters 

 includes flatworms, mollusks, echinoderms, crus- 

 taceans, fishes, birds, and mammals. Not all of 

 them are equally destructive to oyster popula- 

 tions. The most dangerous are those which prefer 

 oyster meat to other types of food, and in search 

 of it invade the oyster grounds. 

 Carnivorous gastropods 



The deadliest enemies of oysters are various 

 gastropods inhabiting coastal waters. The most 

 widely distributed species is the common oyster 

 drill, Urosaipinx cinerea (Say), which is found 



along the entire Atlantic Coast from Canada to 

 Florida. With a shipment of C. virginica the 

 common oyster drill was introduced to the Pacific 

 Coast of the United States (1870 and the following 

 decades), and to Great Britain (1920 and probably 

 earlier) where the American oysters were planted 

 at Brighthngsea and West Mersea. In a short 

 time the drill became very abundant along the 

 coast of Essex and across the Thames estuary. 

 At present Urosaipinx is the most dangerous and 

 the most widely distributed of all the predators 

 of oysters in Em-ope. 



Oyster planting by shellfish growers is the major 

 factor in the wide dispersal of Urosaipinx in this 

 country and its introduction into areas which 

 formerly were free of the pest. The migration 

 of drills is rather limited. Wlien hungry, they 

 may move at an average rate of 15 to 24 feet per 

 day in the direction of food. To a certain extent 

 the drills are dispersed by floating objects to which 

 they may cling and by hermit and horseshoe crabs 

 wliich have been seen bearing as many as 140 

 drills per animal (Carriker, 1955). 



The drill is particularly destructive to young 

 oysters. In Cape Cod coastal waters, which are 

 infested by these snails, the oyster spat has very 

 little chance of surviving the first year; often small 

 seed oysters are wiped out before the end of the 

 second year. Adult oysters with thick shells suffer 

 less, and the losses sustained during 1 year by the 

 4 and 5 year classes are insignificant. There are 

 many localities in Long Island Sound, on the 

 eastern shore of Virginia between Chincoteague 

 and Cape Charles, and in other regions where 

 drills commonly kill 60 to 70 percent of the seed 

 oysters and sometimes annihilate the entire crop. 



Fortunately, brackish water effectively bars the 

 drills from the upper parts of estuaries and tidal 

 rivers. Survival of drills in water of low salinity 

 depends on temperature and on the concentration 

 of salts to which they were adjusted. It may be 

 accepted as a general approximation that mini- 

 mum survival salinities at summer temperatures 

 vary from 12°/oo to 17%o in different regions. 



Extensive literature on the biology and control 

 of oyster drills has been critically reviewed by 

 Carriker (1955), who has also made a detailed 

 study of the structure and function of the proboscis 

 and drilling apparatus of the drills (Carriker, 

 1943). 



The maximum height of adult drills varies in 

 different localities between 25 and 29 mm.; a 



430 



FISH AND WILDLIFE SERVICE 



