Series Atherinomorpha

pectoral actinosts; caudal skeleton usually with two large triangular hypural plates, never more than four; swim bladder physoclistous. The protrusible upper jaw differs from that of other acanthopterygians in lacking a ball-and-socket joint between the palatine and maxilla (a feature that prevents the pre-maxillaries from being locked in the protruded position) and in lacking crossed rostral ligaments extending between the palatines and the heads of the premaxillaries (however, Odontesthes species have a different form of crossed ligaments, Dyer, 1997). A list of synapomorphic characters is given by Parenti (1993, 2005).

Most species of this group are surface-feeding fishes, and about 75% are confined to fresh or brackish water. This taxon contains the only naturally occurring populations of unisexual (all-female) fishes; these occur in the New World—the atherinid Menidia clarkhubbsi and members of the poeciliid genera Poecilia and Poeciliopsis (shown by A. A. Echelle and co-authors in 1983). The diversity of adaptation for internal fertilization is not found in any other higher taxon of fishes; some freshwater members of several families have independently evolved methods for internal fertilization and may lay fertilized eggs or be viviparous (e.g., Grier and Collette, 1987). The eggs of most oviparous members have one or more long chorionic filaments that adhere to the spawning substrate and, except in most exo-coetoids, have conspicuous oil droplets that coalesce at the vegetal pole. Atherinomorphs have other unusual reproductive features such as the male testis is unique in being of the restricted lobular type where spermatogonia are restricted to the distal termini of the lobules (Parenti and Grier, 2004). Delayed hatching is a common occurrence and this may be a synapomorphy (Parenti, 2005).

The concept of a monophyletic taxon Atherinomorpha containing the present assemblage of Atheriniformes, Beloniformes, (including the Adrianichthyoidei), and Cyprinodontiformes goes back to Rosen (1964), Greenwood et al. (1966), and Rosen and Patterson (1969). Few taxa have such strong evidence for monophyly as the Atherinomorpha, and the unity of this group continues to gain support and its constituent members have remained unchanged over the last 40 years, but our understanding of its relationship to other higher taxa remains uncertain while our understanding of relationships within each of the orders continues to change. A detailed review of our understanding of the phylogeny of the Atherinomorpha with emphasis to the evolution of the unusual reproductive modifications is given by Parenti (2005). The sister group of this taxon is accepted to be the Mugilidae following evidence presented by Stiassny (1990, 1993). Some support for this is given by Johnson and Patterson (1993) and Wiley et al. (2000). This and other proposals are given in Parenti (2005) and Parenti and Grier (2004); as advised by Parenti (2005), broader surveys are required to be confident in identifying its sister group.

Some changes to our understanding of atherinomorph systematics is seen by comparing the classifications used in the last three editions. The research of the above authors on systematic relationships was expressed in Nelson (1976). The higher classification in the 1976 edition was as follows:

Series Atherinomorpha (with all members placed in one order) Order Atheriniformes

Suborder Exocoetoidei (present Beloniformes) Suborder Cyprinodontoidei (present Adrianichthyoidei +

Cyprinodontiformes) Suborder Atherinoidei (present Atheriniformes)

Rosen (1981) and Rosen and Parenti (1981) made a major contribution in recognizing Atheriniformes (in their Division I) as sister to the Cyprinodontiformes and the Beloniformes (in their Division II). They argued that the spined atherinids were primitive, with spines being lost in the cyprinodontiforms. Their conclusions were not fully followed in the 1984 edition, where higher relationships were presented as follows:

Series Atherinomorpha (with all members placed in two orders) Order Cyprinodontiformes

Suborder Exocoetoidei (present Beloniformes) Suborder Adrianichthyoidei (present Adrianichthyoidei) Suborder Cyprinodontoidei (present Cyprinodontiformes) Order Atheriniformes (present Atheriniformes)

The classification presented in 1994 was largely based on a reanalysis of the above and works cited in 1994 and was as follows:

Series Atherinomorpha (with all members placed in three orders) Order Atheriniformes (present Atheriniformes) Order Beloniformes (present Beloniformes) Order Cyprinodontiformes (present Cyprinodontiformes)

The current classification has many changes within the orders based on the extensive works cited but no change in the composition at the higher level from the previous edition (Nelson, 1994).

The Atheriniformes are thought to be sister to the remaining members (Dyer and Chernoff, 1996; Dyer, 1998); therefore, the Atheriniformes are now recognized in the superorder Atherinea, with the Beloniformes and Cyprinodontiformes, being in the superorder Cyprinodontea (the ordinal sequence is as expressed in Nelson, 1994).

Series Atherinomorpha Superorder Atherinea

Order Atheriniformes (two suborders, Atherinopsoidei with one family and Atherinoidei with five families) Superorder Cyprinodontea

Order Beloniformes (two suborders, Adrianichthyoidei with one family and Belonoidei with two superfamilies, each with two families) Order Cyprinodontiformes (10 families)

Three orders, 21 families, 193 genera, and about 1,552 species (about 1,304 are primarily freshwater).

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