274 True Fishes 



The eggs of most fishes are less than an eighth of an 

 inch in diameter; they are usually shaped like a sphere but 

 sometimes may be elliptical in form. There are two types 

 of eggs, pelagic and demersal. Pelagic eggs are buoyant 

 and are found floating at various depths. They contain oil 

 globules which serve as food for the developing embryo. 

 The pelagic egg develops rapidly and the young hatch in 

 a relatively few days. Demersal eggs sink to the bottom. 

 They are frequently heavily yolked and may take several 

 weeks or even months to hatch. However, some demersal 

 eggs resemble the pelagic egg in appearance and hatch 

 quickly. Demersal eggs often have a sticky surface or 

 sticky threads or other structures when extruded. As a 

 result they become attached to each other, plant and 

 animal growths, particles of sand, and other solid objects 

 along the bottom. For the most part newly hatched fish 

 are entirely different in appearance from the adult and can 

 be identified only by the specialist. 



Fishes eat a wide variety of foods including the micro- 

 scopic plants and animals, seaweeds, invertebrates, and 

 aquatic vertebrates. Some fish live almost exclusively on 

 the small plants and animals floating in the water (plank- 

 ton). Others are flesh eaters, feeding mostly on larger in- 

 vertebrates or fish, including members of their own species. 

 The majority of species are covered with scales, but in 

 some forms scales are absent or are replaced, either com- 

 pletely or in part, by bony plates, protuberances, prickles, 

 or spines. A few types have developed special organs such 

 as poisonous spines and electric dischargers for defense 

 or for assistance in capturing food. 



Economic Importance: Fish is an important source of ani- 

 mal protein. In some countries it is the major source of 

 this essential component of the diet. Technological im- 

 provements in preservation and processing of fish devel- 

 oped since World War I have encouraged a greatly ex- 

 panded international trade in salt, smoked, canned, and, 

 more recently, frozen fish. As the world population in- 

 creases the fisheries are being more heavily exploited. 



