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Patterns of Abundance and Distribution among Members of a Unisexual-Bisexual Complex of Fishes (Atherinidae: Menidia)
Anthony A. Echelle and Alice F. Echelle
Vol. 1997, No. 2 (May 13, 1997), pp. 249-259
Stable URL: http://www.jstor.org/stable/1447744
Page Count: 11
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Patterns of abundance among members of a unisexual-bisexual complex of silversides fishes (Atherinidae: Menidia) are described from a series of 54 seine collections made from a diversity of habitat situations in the Copano Bay area of Texas. The complex includes the two bisexual species M. beryllina and M. peninsulae, their hybrids (primarily F1 s), and a variety of unisexual clones representing the M. clarkhubbsi species complex. The unisexuals arose as gynogenetic hybrids between M. beryllina and an as yet undetected (possibly extinct) form similar to M. peninsulae. Protein electrophoresis detected 108 unisexuals in this survey, 88.9% of which were from the previously defined clone A (97 specimens in 27 collections); clone C was represented by seven specimens in five collections, and the remaining unisexuals consisted of one specimen from each of four clones (B, I, H, and J). In an ordination gradient based on composition of the fish community exclusive of Menidia, scores for hybrids and clone A were intermediate between those for the two bisexual species. Measures of association indicated that the two bisexual species were negatively associated in abundance and distribution, whereas hybrids and the various unisexual clones were positively associated. The bisexual species were the predominant forms of Menidia in all habitat situations, M. peninsulae in estuarine bays and connected tidal pools and M. beryllina in freshwaters and brackish and saline pools isolated from bay waters. In this survey and elsewhere, the unisexual complex of Menidia is consistently less abundant than the bisexuals, a trait that is unusual among vertebrate unisexual-bisexual complexes. The consistently low abundance of unisexuals excludes competitive interaction models and favors frozen niche-variation as an explanation for the persistence of the unisexual-bisexual complex of Menidia. Various aspects of the biology of Menidia suggest that competitive interaction is, however, involved in producing the consistently low abundance of the unisexual complex. A corollary of the frozen niche-variation hypothesis is that the ultimate factor limiting both present abundance and long-term persistence of the unisexual complex is the apparent absence (possibly extinction) of one of the bisexual progenitors of the unisexual complex, thereby precluding new unisexual origins.