CHARACTERISTICS OF FISHES 9 



Reproduction 



Fishes may give birth to living young which develop as embryos in 

 the uterus. They may incubate the eggs internally, or they may spawn 

 the eggs into the water where they are subsequently fertilized. In some 

 sharks the embryo is completely developed internally and the female 

 gives birth to fully developed young. The gestation period in sharks is 

 relatively long, and in the dogfish shark (Squalus) it may be as much as 

 20 months. In some of the sharks and rays the embryos are partially 

 developed internally, and subsequently, the female deposits the develop- 

 ing embryo in a capsulated ''egg case" from which further development 

 and hatching takes place. In the bony fishes the viviparous perch (Embio- 

 tocidae) are an example of a group which gives birth to living young. 

 In the majority of fish, however, the eggs are spawned and fertilization 

 takes place in the water. The number of eggs spawned varies considerably 

 between species and may exceed a million eggs (cods and halibuts). The 

 eggs in some species are deposited directly onto the sea floor (demersal) ; 

 in others, they may be laid in large sticky masses where they adhere to 

 rocks or plant growth. In many of the pelagic (offshore) fishes the eggs 

 drift freely with currents. 



Some fishes such as the Pacific salmon (Oncorhynchus) spawn only once 

 and then die; however, the majority of fishes spawn several times, and 

 fishes such as halibut which have a long life span may spawn many times. 

 Generally a female will spawn only once each year, but the peak spawn- 

 ing period of a species may extend over several months. 



Air Bladder 



Any fisherman or sportsman who has cleaned marine fishes has proba- 

 bly noticed an elongated, double-lobed sac lying close to the back bone 

 in the abdominal cavity. This thin-walled, semitransparent sac is referred 

 to as an air or swim bladder, and it presumably functions as a hydro- 

 static organ. Fishes living at mid-depths or in surface waters are required 

 to expend energy to maintain their position whenever their body density 

 differs from that of the surrounding water. When a fish moves vertically, 

 it passes through pressure gradients, and thus the gasses in the air bladder 

 are expanded or contracted and the fish's density altered, depending on 

 the direction of movement (up or down). Because of the change in the 

 volume of gasses in the air bladder, the fish then must bring itself back 

 into equiUbrium with the ambient water. This is accomplished through 

 exchange of gases across the body wall of the bladder or by means of a 

 duct which connects directly into the gastrointestinal tract. Gases are 



