A team of scientists from The University of Tokyo is to publish a new study on ponyfishes in the journal Molecular Phylogenetics and Evolution next month.
The family Leiognathidae is a member of the Order Perciformes and its representatives are commonly known as ponyfishes or slip mouths, on account of their specialised mouthparts. To date, no studies have been undertaken on these weird fishes in order to determine what they are related to, what they evolved from, or how the different species are interrelated.
In order to solve the puzzle, the team of scientists looked at two protein coding mitochondrial genes (ND4 and ND5) in 14 species of leiognathid. They found that all of the species in the genere Secutor and Gazza had evolved from a single ancestor, but those of Leiognathus had evolved from more than one species.
They also looked at whether the structure of the mouthparts and their light emitting organs fitted in with the family tree: "The relationships allowed phylogenetic analyses of mouthpart structures and light organ systems. The results suggested that the morphology of the upwardly and forwardly protractile mouth types (latter with canine-like teeth) are phylogenetically informative, and the downwardly protractile mouth type being ancestral in the family."
The results also suggested that internal sexual dimorphism of the light organ system was present in the common ancestor of a sister clade to L. equulus, whereas external sexual dimorphism seems to have evolved subsequently in two monophyletic subgroups."
More information: Ikejima K, Ishiguro NB, Wada M, Kita-Tsukamoto K, Nishida M. (2004) - Molecular phylogeny and possible scenario of ponyfish (Perciformes:Leiognathidae) evolution. Mol Phylogenet Evol. 2004 Jun;31(3):904-9.