CHARACTERISTICS OF MARINE FISHES 155 



are cycloid and arranged in regular series. The croakers (Micropogan) , weakfish 

 (Cynoscion), and many others have moderately small ctenoid scales. Scales so 

 small that they are generally overlooked partly cover the mackerels, as, for ex- 

 ample, the Spanish mackerel {Scomheromorus maculstus) . The question, "Do 

 mackerel have scales?", has been answered in the affirmative many times by the 

 Fish and Wildlife Service and the former Bureau of Fisheries. It is true that the 

 scales in some species of mackerel are rudimentary, and in others only the anterior 

 part of the body is covered. However, it is correct to say that the mackerels belong 

 to the "scaly fishes". 



The scales of fish are of further interest because their age can be determined, 

 especially in temperate regions where there are definite summer and winter 

 seasons, resulting in periods of rapid and retarded growth. The differences in the 

 rate of growth of the fish is "registered", in its scales, which bear growth rings, 

 called "circulae". When growth progresses slowly, as during the winter, the rings 

 are formed close together, while they are formed farther apart during the summer 

 when rapid growth occurs. Thus, with some practice the student is able to de- 

 termine the approximate age of many kinds of fish from temperate regions. 



The Fins. It was stated near the beginning of this chapter that fish, with few 

 exceptions, have limbs, or at least rudiments of limbs, which are represented by 

 fins. In addition to the paired fins representing arms or legs most fish have median 

 or vertical fins. The pair of fins corresponding to the front legs of higher verte- 

 brates are known as pectoral fins; the pair corresponding to the hind legs are 

 called ventral or pelvic fins; those on the median line of the back, of which there 

 may be 1 to 3, are dorsal fins; the one forming the tail is the caudal fin; and 

 those on the median ventral line behind the vent, of which there may be 1 or 2, 

 are called anal fins. All these fins differ widely in size, shape, and position and the 

 dorsal and anal fins vary in number. 



In reference to the fins we may refer again to the common Atlantic herring, 

 Clupea harengus, chosen earlier in this chapter as a usual type of fish. The herring 

 has the ventral fins attached to the abdomen, and consequently belongs to a large 

 group of fish known as Abdomenales, all of which have these fins similarly at- 

 tached. Furthermore, the herring has one dorsal and one anal fin. This is quite 

 the usual arrangement among the Abdovienales. However, salmon, trout, smelts, 

 and several others have, in addition to the fins possessed by the herring, a small 

 ray less flesh fin behind the dorsal fin. This small fin is known in ichthyology as 

 an "adipose fin." 



The greatest number of well-developed fins with several or many rays occurs 

 in some of the members of the codfish family, as well as in the codfish itself, and 

 in pollock and haddock, each of which has 3 dorsal and 2 anal fins (Fig. 10-7). 

 However, the hakes (Urophycis) , which are classed with the codfish (Gadidae), 

 have only 2 dorsal fins, a short and a very long one, and a single long anal fin. 

 Although the codfish rays are all soft and lack true spines, they are not Abdom- 

 enales as their ventral fins are attached to and in advance of the thorax. 



The fins in the vast majority of fishes are composed partly of spines and partly 

 of soft rays. These are the "spiny-rayed fishes". Many of them have the same 

 number of fins as the herring, but some of the fins begin with spines which are 

 followed by soft rays. This is the arrangement in the sea bass (Centropristes), 



