Evolution and systematics

The order Atheriniformes originally was conceived by Donn Rosen in 1964 to include the exocoetoids (flying fishes and allies), scomberesocoids (sauries), adrianichthyoids (rice-fishes), cyprinodontoids (killifishes and allies), atherinoids (silversides and allies), and phallostethoids (priapium fishes). Today this disparate assemblage of fishes still is thought to be a natural group, but it has been parceled into three orders. Now called the Atherinomorpha, the group contains the orders Cyprinodontiformes and Beloniformes (put together as Division II atherinomorphs by Rosen and Parenti in 1981) and the Atheriniformes (Division I atherinomorphs according to Rosen and Parenti). Characters of the testis, egg, embryo, rostral cartilage and skull, and dorsal gill arch skeleton, among others, unite the Atherinomorpha. This chapter concerns the Atheriniformes, which at present includes the silversides, phallostethids, rainbowfishes, and related taxa.

At more than 50 million years old, fossils of the genus Atherinidarum from the early Eocene of France are the oldest known atheriniforms. Researchers disagree about the identity of the most basal atheriniform lineages and have variously proposed both freshwater (bedotiids and melanotaeniids) and partly or wholly marine (atherinopsids and notocheirids) groups. Through the course of their evolution different groups of atheriniforms have repeatedly breached the marine-freshwater barrier. Such an ability to colonize patchily distributed freshwater habitats has fostered speciation within the order, especially in the Americas and Australia. At present there are about 315 species and 49 genera of atheriniforms known to science.

Ichthyologists disagree on whether the Atheriniformes are a monophyletic group, descended from a single branch on the atherinomorph tree, or a hodgepodge of lineages on multiple branches that are associated closely with the Cyprinodonti-formes and Beloniformes (which are considered sister groups). Even the number and constitution of families is in dispute: recent classifications have cited from six to ten families. In light of the disagreement in the literature, this treatment follows Joseph Nelson's book Fishes of the World, in which eight families are recognized within the order: Bedotiidae (Malagasy rainbowfishes), Melanotaeniidae (rainbowfishes), Pseudo-mugilidae (blue-eyes), Telmatherinidae (Sulawesi rainbows), Atherinidae (silversides), Notocheiridae (surf silversides), Dentatherinidae (pygmy or tusked silverside), and Phal-lostethidae (priapium fishes).

In 1996, the most recent treatment of the issue, Dyer and Chernoff marshaled 10 morphological characters supporting the Atheriniformes as a monophyletic, or natural, group. In addition, they proposed a new classification for the order, in which the number of families was reduced to six. Familial allocations within this new classification are in stark contrast to Nelson's scheme. Differences include an expansion of the family Melanotaeniidae to subsume the families Bedotiidae, Pseudomugilidae, and Telmatherinidae; an expansion of the family Phallostethidae to include Dentatherinidae; and an elevation of the Old World silverside genus Atherion to family status (Atherionidae). Furthermore, in agreement with a 1994 article by Saeed and collaborators, the New World silverside subfamilies Menidiinae and Atherinopsinae are shunted from the family Atherinidae into their own family, Atherinopsidae. Dyer and Chernoff's morphology-based hypothesis awaits further testing, especially using DNA sequence data. It is too soon to say whether their proposal will gain wide acceptance. Certainly, though, theirs will not be the last word on atherini-form relationships.

A school of silversides (Atherinidae), near Thailand. (Photo by Animals Animals ©Joyce & Frank Burek. Reproduced by permission.)
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