boscides to the edge of the oyster's mantle and feed 

 on the mucous and tissues. These ectoparasites 

 are probably a great nuisance to the oyster, but 

 there is no evidence that they can be regarded as 

 important enemies. Two species have been found 

 associated with C. virginica: 0. {Menestho) bisu- 

 turalis Say which has a range from New England 

 to Delaware Bay, and 0. (Menestho) impressa 

 Say which is found from Massachusetts to the 

 Gulf of Me.xico. 



Starfish 



The starfish of the Atlantic Coast is also a 

 highly destructive predator on oysters. The 

 common species, Asterias jorhesi (Desor), is the 

 most familiar animal in tidal pools, on rocks, and 

 beaches of the Eastern Coast of the United States, 

 often found exposed by the receding tide. Accu- 

 rate statistics of the destruction caused by this 

 species are not available, but a few selected 

 examples emphasize its deadly efficiency. In 

 1887 the State of Connecticut estimated the loss 

 caused by starfish at $463,000; the sum repre- 

 sented the destruction of over 634,246 bushels 

 of oysters or nearly half of the total harvest for 

 the year (1,376,000 bushels). The numerical 

 strength of a starfish population over a relatively 

 small area can be visualized from the record of 

 only one company which in 1929 removed over 

 10 million adult starfish from 11,000 acres of 

 oyster grounds in Narragansett Bay. 



As a rule the starfish populations on various 

 parts of the coast fluctuate within wide limits 

 with years of great abundance usually followed by 

 relative scarcity. These fluctuations cause many 

 oystermen to believe that starfishes invade their 

 grounds periodically. Studies of the problem con- 

 ducted simultaneously in Buzzards Bay, Nar- 

 ragansett Bay, and Long Island Sound (Galtsoft" 

 and Loosanoff, 1939) demonstrated that sudden 

 increases in the abundance of A. jorhesi are due 

 primarily to the high percentage of survival of 

 its free-swimming larvae and their successful 

 setting (fig. 397). 



The reproductive season of A. forbesi in New 

 England waters slightly precedes that of C. 

 virginica. When oyster larvae reach setting stage, 

 the space available for their attachment is already 

 occupied by young starfishes only several mm. in 

 diameter, hungry, and ready to attack the spat. 

 The new set of oysters may be completely wiped 

 out by young starfish. 



Millimeters 



Figure 397. — Photomicrograph of live larva, brachiolaria, 

 of A. forbesi from a plankton tow in Buzzards Bay. 



The movements of ^-1. forbesi in concrete tanks 

 are slow, random, and apparently not directed by 

 tactic reactions. Initially it was difficult to 

 reconcile this fact with the experience of oyster 

 growers in Long Island Sound who reported that 

 oyster bottoms thorouglily cleaned by mopping or 

 dredging were invaded within the next 24 hours 

 by swarms of starfish. Underwater observations were 

 made in Long Island Sound by members of the 

 Bureau of Commercial Fisheries Biological Lab- 

 oratory in Woods Hole who used an underwater 

 television camera. The underwater photographs 

 showed clearly that starfishes are passively 

 transported by the tidal currents which in Long 

 Island Sound are fairly rapid. The animal curls 

 up the tips of the rays, releases its hold on the 

 substratum, and floats just above the bottom. 



FACTORS AFFECTING OYSTER POPULATIONS 



437 



