[29] FISHEKIES OF THE UNITED STATES. 213 



themselves were to survive there can be no doubt that they would 

 exhaust the available supply of lime before they failed to obtain enough 

 organic food. 



" It is well known that shell-fish of all kinds thrive best when the sup- 

 ply of lime is greatest. The fresh -water mussels which live in streams 

 and ponds where the supply of lime is scanty, grow slowly, and their 

 shells are so thin that they are very subject to accidents, and their num- 

 bers are limited ; but in limestone regions the shells are large and 

 heavy, and the bottoms of the streams are almost paved with mussels, 

 and it is well known to conchologists that coral reefs and islands are 

 the most favorable regions for the abundant growth of all kinds of 

 shelled molluscs." 



. The investigations of the United States Coast and Geodetic Survey, in 

 the neighborhood of Crisfield, Maryland, which were coincident with 

 those of Dr. Brooks, led to the conclusion that there is little or no 

 regularity in the recurrence of successful " spatting" seasons, and that 

 all irregularities are due, probably, to variations from the normal tem- 

 perature and density of the water; the higher the temperature during 

 the spring months the earlier will be the advent of the spawning sea- 

 son, and an increased temperature will also hasten the development of 

 the spat, and of the young oysters after they have become attached. 

 Sudden and extensive changes of density will likewise affect the ad- 

 vent, duration, and success of the spawning, though to a less extent. 



Subsequent to the attachment of the animal, changes of the con- 

 ditions surrounding it are not of so much importance, though naturally 

 such changes will more severely affect the delicate organism of the 

 young oyster than that of the older and more hardened adult. 



It is during the first six months of its existence that the oyster is ex- 

 posed to the greatest danger from its numerous enemies. The thin, 

 delicate shells, from one-sixteenth of an inch to one inch in diameter, 

 are readily bored by the drills, whelks, and other gastropods, or torn 

 off by the crabs, and the immense number of all leaves no room to 

 doubt their destructive effects. As an instance, the inspection of the 

 spat collectors placed in the Big Annemessex River by the United 

 States Coast Survey, shows that during the early months of their ex- 

 istence about 50 per cent, of the young oysters were destroyed.* Nat- 

 urally, as the animal progresses, it becomes more hardy and better able 

 to resist the attacks of enemies and changes of environment, and on 

 unworked beds, where the oysters are practically in a natural state, 

 the decrease in passing from young growth to mature oysters is about 

 30 per cent., or about one-third of a given number perish in passing 

 from the first to the fourth year of their existence. 



The only essential for securing the attachment of the spat is that the 

 object exposed should be clean. The specimens illustrating attach- 

 ment show how many and various, these objects are; and the formation 



* Photographs of this collector are exhibited. 



