174 MARINE PRODUCTS OF COMMERCE 



Cynoscion regalis, spot, Leiostomus xanthurus, and common croaker, Micropogon 

 undulatus, tend to move offshore to spawn, though few seem to go to the deep sea. 

 These are all provisions made by natiire to insure the perpetuation of the species, 

 for it may be pointed out again that each species requires special conditions for 

 the incubation of the eggs and especially for the survival of the young. 



The vast majority of fish have eggs, which are fertiUzed the moment they are 

 laid. However, in certain forms, as in some of the skates and rays, the eggs are 

 fertilized internally and laid afterwards. The "mermaid's purse," known to all who 

 have spent some time on the seashore, is the case of the eggs of a skate. Still 

 other fish give birth to live young. Many, though not all, sharks have live young, 

 as do the so-called viviparous perches of the Pacific Coast and the viviparous top 

 minnows of brackish and fresh waters. The principal difference between the 

 viviparous and the oviparous species is that the eggs in the former are fertilized 

 and incubated internally, instead of externally as in the latter. Some sharks do 

 have a sort of placenta, but to what extent it functions in supplying nourishment 

 to the young remains relatively unknown. Nevertheless, it is among these lower 

 forms of fish that the nearest approach in the reproductive process to the 

 mammals is found. 



In general fish cast their eggs in the water on theii' customary spawning 

 grounds, then provide no further care for them. However, there are exceptions. 

 Pipefish and sea horses, for example, provide excellent care, for the female 

 transfers the eggs to a pouch which the male has for the purpose. There the 

 eggs are incubated, and later the males give "birth" to young. Most, if not all, 

 of the rather numerous marine catfishes, also provide protection for their eggs. 

 In these the female somehow transfers her eggs to the mouth of the male parent, 

 where they are incubated and the young retained until the yolk is absorbed. 



Fish eggs "come" large and small and of different shapes. The marine catfish 

 which practice mouthbreeding, or "oral gestation," have relatively large eggs, 

 those of the common gaff-topsail catfish, Felichthys felis, being approximately an 

 inch in diameter. The cases of skate eggs, that is the "mermaid purses," are flat 

 and may be 2 to 3 inches or so square. At the other extreme are the eggs of the 

 fresh-water eels, which are said to be almost microscopic in size. Pelagic eggs 

 are usually small, frequently only a millimeter or so in diameter. Generally fish 

 eggs are spherical, but those of the anchovies are ovate. Demersal eggs, such as 

 those of the silversides, are often provided with gelatinous strands by which they 

 become attached to submerged objects. 



Some fish have comparatively few eggs, whereas others produce them in tre- 

 mendous numbers. The now well-known mosquito fish, Gambusia, which hatches 

 its eggs internally, generally produces less than 100 at a time, though several 

 batches mature during one summer. The gaff-topsail catfish, which has been 

 referred to as among the mouthbreeders, has, for a fish, enormous eggs, an inch 

 or so in diameter; however, the number produced by a single fish is small, ranging 

 from about 24 to 55. Salmon, too, have relatively large eggs which are com- 

 paratively few in number. The king salmon, for example, is said to produce about 

 5,200 which are around 5 mm in diameter. According to one author as many as 

 40,000 eggs occur in a common Atlantic herring, 635,500 in a sturgeon, 3,500,000 

 in a halibut, and 9,344,000 in a cod. It has also been estimated that as many as 



