HETEROCHRONY IN JAW MORPHOLOGY OF NEEDLEFISHES (TELEOSTEI, BELONIDAE)

The jaw morphology of needlefishes (Beloniformes: Belonidae) passes through distinctive ontogenetic stages. When hatched, larvae have short jaws of equal length. The lower jaw first elongates to produce a "halfbeak" form, then the upper jaw elongates to produce the adult "needlenose" morphology. Onset and duration of the halfbeak stage vary among belonid species. We tested the hypothesis that heterochronic evolution produced this diversity in jaw morphology. First, a cladogram was developed for 27 belonid species. Ontogenetic trajectories for upper and lower jaw elongation were determined for 18 of these species. For upper and lower jaw elongation, trajectories of these 18 species were statistically compared to determine character transitions, which were then mapped onto the cladogram. The results showed that the growth trajectories of upper and lower jaws changed independently of each other, and that neoteny and acceleration occurred repeatedly during belonid evolution. Evidence suggests postdisplacement and predisplacement were also important mechanisms. The resulting morphological diversity may be interpreted as adaptations to food availability, because jaw morphology (halfbeak vs. needlenose) seems to be correlated with feeding modes (planktivory vs. piscivory).