374 AQUATIC VERTEBRATES 



it anyway, it is just too bad for the shark. The porcupine fish secretes 

 a red fibrous material around itself, which protects it from the digestive 

 juices of the shark, and then calmly proceeds to eat its way through the 

 shark's stomach and into the water outside, none the worse for its 

 experience. 



Fish Reproduction. The reproductive habits of fish show great vari- 

 ation in the number of eggs and the degree of care or protection given 

 to the eggs and young. It is a general rule of nature that these two fac- 

 tors are in inverse proportion to one another. At one extreme, we find 

 fish, like the sturgeon, that deposit the eggs broadcast in the water and 

 the sperms are released in the same way. The fertilized eggs sink to the 

 bottom and receive no protection of any kind. As a result they are eaten 

 by many animals so that few of them ever hatch and many of the young 

 that do hatch are eaten before they can mature. The sturgeon is able to 

 compensate for this great destruction by laying an enormous number of 

 eggs. A female will lay approximately three million eggs each year for 

 about ten years, or a total of about thirty million eggs from one female. 

 From this huge number only two, on the average, will survive and live 

 to reproduce, one male and one female. Even man is numbered among 

 the animals that eat the sturgeon eggs, for caviar is made from sturgeon 

 eggs. Much of our caviar comes from the paddlefish, or spoonbilled 

 catfish, a relative of the sturgeon, which is common in the southern 

 Mississippi River. 



At the other extreme are quite a number of ovoviviparous fish, such 

 as our common fresh-water minnow, Gambusia. As in the sharks, this 

 little fish retains the eggs in its body until they hatch and the young are 

 born alive so that they have a chance to swim away from their enemies 

 from the first. The anal fin of the male is modified into a long slender 

 tube which serves as a copulatory organ in transferring sperms to the 

 female. A much smaller number of eggs is formed and there may be no 

 more than a dozen young minnows produced at one reproductive cycle. 

 The fresh-water catfish is somewhere in between. These fish scoop 

 a nest out of the mud along the side of the river in which they live and 

 the female lays the eggs in the nest and goes on her way. The male will 

 release sperms over the eggs and stay around waving his fins near them 

 to keep a fresh current of water flowing over them so that they will re- 

 ceive plenty of oxygen. He will protect them with his life and, although 

 the catfish is normally a rather sluggish animal, he is very active and 

 aggressive if any other animal threatens injury to the eggs. When the 

 young hatch he will stay with them for a time and he may even open his 

 mouth so that the young can swim inside for safety when there is danger. 

 This illustrates an interesting principle of fish reproduction. We are ac- 



