Hybridization between Fish Species in Nature

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Abstract
Fish spp. cross often in nature. In sunfish certain supposed spp. are hybrids, as for example Lepomis euryorus and L. ischyrus. Hybrid males, although large and vigorous, are sterile. Experiments with the top minnow, Mollienisia formosa. are described. It is viviparous and apparently parthenogenetic and the hybrid young are always females exactly like the mother. "Hybrids" were even produced when this spp. was mated with Gambusia affinis, belonging to a different subfamily. In other instances hybrids are nearly always intermediate in their characters between the parental spp. Hybrids, although common among fresh-water spp., are rare among marine spp., but in one group, the flatfishes, hybrids are fairly common. Examples are mentioned of hybrids among the chars, trouts, salmons, herrings, whitefish and suckers. Charts illustrate hybridization in various families with indications of known experimental and natural hybrids between subgenera and genera. Hybridization is largely due to environmental factors. Spp. that hybridize rarely in nature may cross commonly in the laboratory. Environmental changes are more common in fresh waters than in the sea, hence the more common occurrence of hybrids in the former than in the latter. Frequency of hybridization is inversely related to the number of spp. in a region. This fact, as well as the sterility of the hybrids, indicates that crossing of spp. is not an important factor in speciation of fishes, as a general rule.

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