CATFISH GENUS NOTURUS RAFINESQUE 209 



Hybridization 



Natural hybrids between species of the family Ictaluridae appear 

 to be rare. The few specimens that are recognized as hybrids are 

 offspring of species exhibiting extremes of morphology. If closely 

 related species interbreed, the resulting hybrids may not be recognized 

 because of the extensive overlap in characters differentiating the 

 parents. 



The hybrids are not accidental, as in many cyprinids, but their 

 scarcity probably results from a rather rigid system of mating and 

 occupation of territory by the breeding pair. The available evidence 

 indicates that pairs spawn alone, in cavities or beneath objects. Nat- 

 ural hybrids would thus appear to result from occasional breakdown of 

 psychological barriers in mating. 



Numerous hybrids of Ictalurus furcatus (LeSueur) and Idalurus 

 pundatus (Rafinesque) have been obtained experimentally after 

 administration of gonadotropin and isolation of pairs in pens or aquaria 

 (Giudice, 1966). Evidence presented suggests that most of the re- 

 sulting eggs were viable, and indicates that offspring are vigorous 

 growers and variously intermediate morphologically between the 

 parent species. Conclusions could not be reached regarding the possible 

 sterilit}^ of the Fi h3'brids. However, the gonads of several natural 

 hybrids in Noturus appear, from gross observation, capable of pro- 

 ducing viable eggs and sperm. 



The effect of hybridization on evolution in the family cannot be 

 postulated. The scarcity of natural h^^brids in collections would sug- 

 gest that hybridization has little effect. Although the fertilized eggs 

 of crosses of any two species, or genera, may be viable, crosses between 

 the most divergent forms such as between members of the genera 

 Idalurus and Pylodidis are doubted on morphological grounds. 

 Offspring from hybrid parents have not been recognized, if indeed 

 they exist. 



The belief of Trautman (1948) that Noturus hybrids are the result 

 of a breakdown of ecological barriers is also doubted for the following 

 reasons. Most of the species of Noturus (few exceptions) have been 

 collected with one or more other species at some part of the range 

 (see discussions of species). Three to five species are commonly taken 

 from small sections of streams in the Ohio and Mississippi drainage 

 as well as elsewhere. These individuals of different species are often 

 found hiding and nesting within a few feet of each other in apparent 

 normal environments. Thus, if the barriers to hybridization are eco- 

 logical, I would expect hybrids to be common rather than rare. Such 

 is not the case. 



It is also doubted that overcrowding is of any significance in icta- 

 lurid hybridization. Rather the opposite may be true — inability of 



