Leiognathus elongatus (Perciformes: Leiognathidae): Two Distinct Species Based on Morphological and Light Organ Characters

Abstract

The morphological description of the bacterially bioluminescent elongate leiognathid, Leiognathus elongatus, unites those ponyfishes having cheek scales and a standard length of 0.33 or less in body depth. This report reassesses the taxonomic status of fish identified as L. elongatus by these criteria using specimens obtained in the Philippine Islands and Japan in 1982 and 1983. Based on differences in external morphology and components of the internal light-organ system, the specimens fall into two distinct groups (I and II). Group I fish are 20% deeper bodied, have 30% more lateral line scales and a more finely detailed dorso-lateral mottling pattern than group II specimens. Differences in the supraorbital spine, posterior margin of the adipose eyelid and snout pigmentation further distinguish these fishes. Additionally, group I fish exhibit a less pronounced sexual dimorphism; males have a sex-specific blue lateral stripe and a light organ 2 to 3 times larger in volume than that of females. In contrast, group II fish are distinctly dimorphic, with males differing from females in three morphological features of the light-organ system: a sex-specific clear flank patch, transparent lateral gas bladder walls and a greatly enlarged light organ (20 to 100 times larger than that of females). Based on these differences and on comparisons with type specimens, we recognize two species: L. stercorarius Evermann and Seale (group I), a previously buried synonym of L. elongatus; and L. elongatus (Glinther) (group II), which includes fishes previously named Equula elongata Giinther, L. elongatus Smith and Pope and L. popei (Whitley). We propose the use of the light-organ system in clarifying questions of leiognathid taxonomy.