Ecomorphology of Longnose Gar (Lepisosteus osseus): on the influence of size, sex, and river location

Ecomorphology is essential to understanding the evolution and biogeography of species. However, morphological studies that focus on nongame or ‘rough’ fish groups such as Gar (Lepisosteidae), outside of a phylogenetic context, are comparatively rare. Herein, we investigate the effects of sex, size, and location in a river to assess the potential for sexual dimorphism, allometry, habitat, or some combination to drive observed morphologic variation. Our dataset includes 230 Longnose Gars (Lepisosteus osseus) made up of both males and females of a wide range of sizes collected from three different sites corresponding to the upper, middle, and lower Great Pee Dee River in South Carolina. Dorsal and lateral photographs were taken from each specimen and landmarked for 2D geometric morphometric analyses. Discriminant function analyses (DFA) were also conducted on a few linear trusses to compare the accuracy of sex-related shape aspects that emerged from morphometrics as a possible field-based tool to help fisheries managers understand their stock populations. Overall, location was a statistically significant predictor of shape, yet the differences were not biologically meaningful. Additionally, sex and size were strong predictors of shape, with the greatest differences seen in the largest individuals. DFA of field measurements found that the Snout Length-to-Total Length ratio accurately predicted sex 71% of the time, with females having longer snouts than males. Differences in ratio between sexes can allow tentative sex assignments without lethal gonadal inspection. This study presents the first standardized whole-body 2D geometric morphometrics protocol for Longnose Gar.