Hybridization and the Detection of Hybrids in the Fairy Shrimp Genus Streptocephalus

Abstract

Viable adult hybrids were obtained in laboratory mating tests involving five species pairs of Streptocephalus fairy shrimps. Three hybrid types were distinguished from parental types using prominent diagnostic features of parental species: two hybrid types resembled only one parental species in diagnostic features. Laboratory hybrids provided a search image for detecting natural hybrids in collections of mixed-species populations. These studies suggest, but do not prove, that S. mackini hybridizes to.a limited extent with both S. dorothae and S. texanus where they co-occur. Introduction Hybrid animals are not often found in nature (Mayr, 1963), and it is generally assumed that matings between species are correspondingly rare. For many animal groups this is undoubtedly true. However, our ability to recognize hybrids is directly related to the extent of our knowledge about the morphological variation within and between species, and all too often we know species only by their diagnostic characters. Since hybrids sometimes reflect the diagnostic features of only one parental species, natural hybrids may go undetected. Laboratory mating studies provide information which aids in the detection of hybrids in natural mixed-species populations. These laboratory tests indicate which species pairs are likely to hybridize and provide a search image for natural hybrids based on the morphology of laboratory hybrids. This approach is particularly useful for evaluating material, such as museum collections, which cannot be altered for more detailed morphometric or biochemical studies. Laboratory mating studies have demonstrated a high degree of reproductive compatibility between species of Streptocephalus fairy shrimps (Wiman, in press). Crosses were attempted between eight species pairs, and five of these mating combinations resulted in adult hybrid offspring. All five successful crosses involved S. mackini as one parental species. The present study documents some of the morphological features of laboratory hybrids which may be of use in distinguishing hybrids from parental species in natural populations. Natural hybrids are most likely to be found where closely related allopatric species come into secondary contact (Remington, I960). Belk (1977) recently presented evidence suggesting that the ranges of Streptocephalus fairy shrimp species may be expanding due to the creation of many new man-made temporary pond habitats. Even though species ranges overlap to various degrees and two species sometimes occur in the same pond, species are largely allopatric (Wiman, 1978). Thus some of the Streptocephalus species may be characterized as allopatric species which have recently come into secondary contact. Given the geographic and reproductive relationships of these fairy shrimp species, one would expect hybridization to occur in natural populations. This study presents evidence for natural hybridization in the few available collections of mixed-species populations of Streptocephalus. Materials and Methods Laboratory hybrids were obtained from no-choice mating tests involving six species of Streptocephalus: S. mackini, S. dorothae, S. texanus, S. seali, S. linden and S. bou vie ri. When fertilized eggs were obtained from interspecific crosses, they were allowed to dry in 25-ml vials for at least 2 weeks at 19-22 C. The vials were then