Diet of Northern Map Turtles (Graptemys geographica): Relationship to Sexual Dimorphism and Potential Impacts of an Altered River System in Maryland

Abstract

Last recorded in Maryland in the early 1990’s, the Northern Map Turtle (Graptemys geographica) is listed as state endangered and is currently only known in the lower Susquehanna River drainage. This river is divided by a hydroelectric dam, used for recreation, and has increasing shoreline development, all of which may negatively impact this population. Knowing this population’s diet and if alterations to the river affect their prey base is important to understanding the species’ current status. Northern Map Turtles are mostly molluscivorous but will also consume other invertebrates. Studies have shown that map turtle species will subsidize their diet with invasive mussels, potentially decreasing dietary diversity. The Susquehanna River contains both native and nonnative mollusks species. Recently, Zebra Mussels (Dreissena polymorpha), an invasive species known to have large effects on aquatic food webs and nutrient cycling, have been found in the lower Susquehanna. Given the absence of basic data on Northern Map Turtle diet in Maryland, fecal samples of 32 turtles were examined. Three operational taxanomic units predominated across samples: gastropods, trichopteran larvae, and Corbicula fluminea. Pronounced sexual differences occurred for these taxa, as adult males (n=14) fed primarily on trichoptera, a small unidentified gastropod species (~2 mm), and Corbicula sp. while the females (n=18) fed almost exclusively on Pleurocerid snails. This research has provided us baseline data on Northern Map Turtle diet and monitoring of the diet will allow us to not only understand impacts of dam operations on composition of their prey base, but the potential shift to zebra mussels as well.