Laypersons and professional scientists alike are regularly confounded when trying to distinguish between northwestern/southwestern pond turtle (Actinemys marmorata / A. pallida) and any of a number of common pond sliders (Trachemys spp.) like the red-eared slider (T. scripta elegans). As pond sliders age, their distinct markings can diminish; male sliders in particular often become melanistic (an increase in dark pigmentation) with age that masks any distinctive markings (like the "red-ear" and striping) and/or enhances secondary markings (speckling) that more closely resemble a western pond turtle. 

To help iNat users and the public distinguish between these two species, we created an illustrated guide to help sort out those tricky turtles. Because no single diagnostic trait alone is necessarily the silver bullet, we identified 11 key traits that, between them, should help if you can get a clear look at a turtle from any one angle. The correct identification, and correctly distinguishing between, California's native and non-native turtles is important. With certain populations of the west's native western pond turtle experiencing threats from climate change, habitat loss, disease, etc., it is important for Agency personnel and land managers to have the most accurate representation of where western pond turtles are, and are not; just as important is to know where invasive turtles like Trachemys spp. are encroaching on western pond turtle habitat. Western pond turtles are a California Species of Special Concern, and are now a candidate for listing under the U.S. Fish and Wildlife Service. Establishing the species' range is critical to future management decisions.

Matthew Bettelheim is a herpetologist, Certified Wildlife Biologist, and natural historian in the San Francisco Bay Area. He has been exploring the effects of the historical commercial terrapin fishery on western pond turtles for over 20 years, and organized and presented at the Western Section’s 2005 and 2015 Western Pond Turtle Workshops. He was involved in a 15–y radio–telemetry study of western pond turtle nesting behavior in coordination with the Oakland and San Francisco zoos. His 2005 study, "Marmorata: The Famed Mud Turtle of the San Francisco Market" in California History and 2022 study, "A Review of the Historical Market: Effect of the West Coast Commercial Fishery on Western Pond Turtles (Actinemys marmorata and A. pallida)" provide an exhaustive review of the historical commercial terrapin fishery for western pond turtles.