Studies of cyprinodont Fishes. XX. A new subfamily from Guatemala, with ctenoid scales and a unilateral pectoral clasper

" Xenodexia ctenolepis, a new genus and species from an area of endemism in the upper part of the Rio Usumacinta system in Guatemala, is so highly specialized as to call for its segregation in a distinct subfamily, Xenodexiinae. Its most distinctive features are an elaborate unilateral pectoral-fin clasper of the adult male-a structure unique among fishes; ctenoid scales, hitherto unknown and unpredicted in the order Cyprinodontes ; and an extreme extension of the body cavity into the urosome. A large variety of characters, including genital and genitalia1 structures of high taxonomic value and 2 recently discovered features that are independent of sex, indicate that this bizarre fish should be classed in the family Poeciliidae, but alternative possibilities are not wholly eliminated. Xenodexia may have been derived from the Poeciliopsinae, but the very complex tubular form of the gonopodium may have been evolved independently. In Xenodexia this structure (the intromittent organ) is extremely long and very complex.

It seems probable that the ctenii on the scales of Xenodexia represent reversible evolution, but probably only in the sense that the ctenii were long lost in phenotypic expression, not in genetic potential. The discovery of ctenoid scales in a member of the order adds weight to the view that the Cyprinodontes were derived from an ancestor, possibly m acanthopterygian, well advanced along the teleost line of evolution.

Insemination in Xenodexia is presumably effected when the male holds the female in the recess behind the highly modified base of his right pectoral fin, the upper part of which is replete with hard and fleshy hooks and pads. The forward-turned gonopodium is probably held securely in place by the main hook of the clasper. In this position its fleshy tubular tip would fit into the clasped female's genital papilla, which is grooved on the right side, to complete what seems to be a lock-and-key mechanism. Perhaps in correlation with the perfection of the inseminating mechanism, the males seem to be greatly outnumbered by the females. The sexes appear to attain about the same size, but sexual dimorphism in form and structure is rather extreme. Xenodexia ctenolepis is one of the poeciliids, the development of which involves superfetation. As many as 5 broods i11 different stages of development are present at one time in the ovary "