Guide to Distinguishing Western Pond Turtles (Actinemys spp. ) from Common Pond Sliders (Trachemys spp.)

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). 

You are not alone!

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. While the marbling and speckling of certain western pond turtles is unmistakable, these characteristic markings can become muted as well through age and wear/tear; some western pond turtles are drab brown and any marbling only becomes apparent when submerged in the water, if at all. 

Why is this 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. The western pond turtle is a California Species of Special Concern, and is now a candidate for listing under the U.S. Fish and Wildlife Service. Establishing the species’ range is critical to future management decisions.

To help iNat users and the public distinguish between these two species, I’ve partnered with @cnddb_brian, Rachel Freund, and Annie Chang to create an illustrated guide to help us sort out those tricky turtles. Remember, not one diagnostic trait alone is necessarily the silver bullet; we’ve identified 11 key traits that, between them, should help if you can get a clear look at a turtle from any one angle. Some traits might contradict others; turtles are variable, not only between species but also within species. You might not be able to state whether the turtle you are looking at has short-to-medium or medium-to-long claws, or whether it is more-than, or less-than 12 inches long from 200 feet away – that’s understandable. Hopefully, there are enough clues among these 11 to help you make an educated decision. 

That being said, the two key diagnostic traits we would hang our hats on are (#7) the arrangement of the 1st vertebral and 1st marginal scutes (see Footnote #1) on the carapace immediately behind the turtle’s head/neck, and (#9) whether the rear marginal (outermost) scutes are serrated/toothed within each scute (= slider), or only show interruptions (if at all) at the seam between each scute (= western pond turtle). If you can get a good look, or a sharp photo, that shows either or both of these traits, you’ll make your job and ours a lot easier :). 

The other 9 traits are also important, but there is so much variability that some of these can be open to interpretation depending on time-of-day/lighting, distance, wear and tear on the shell, the turtle’s angle of repose, and/or the quality of the photograph. Personal biases should be considered too: 

• Don’t assume it is a slider because you are in an urban setting;
• Don’t assume it is a native turtle because you are in a natural wilderness area;
• Don’t assume it’s either species because the other has never been reported there before.

Those can be context clues, but not the sole basis of an identification. That being said, consider the natural range of the species and use common sense. Many pond sliders reported in southeastern states get improbably flagged by the image recognition software as western pond turtles despite a user’s location data. If you are in Texas or Maine, you can probably (usually?) make a safe assumption the turtle in front of you is not a western pond turtle. Unless you have solid evidence to the contrary – stranger things have happened, and unfortunately, people move turtles around.

Many of the photos uploaded to iNaturalist are nowhere near as sharp or detailed as the representative photos we’ve curated here. We’ve selected sharp, diagnostic photos to help train you and your eye to make heads or tails of those heads and tails. These photos are meant to condition you for when conditions in the field are less than optimal. And trust us – in the field, the turtle is going to going to haul itself onto a log with a carapace coated in algae, or the marginal scutes will be gnawed to tatters by a predator, or there will be a tule or blade of grass across the photo blocking that one key diagnostic trait we suggested you look for. 

And remember, this guide is only meant to help users distinguish between native western pond turtles and non-native pond sliders, rather than identify either to a specific species. The north and southwestern pond turtles are more or less indistinguishable except by their geographic location and, if the turtle is in hand (please don’t capture/handle them), the sometimes-presence or absence of an “inguinal scute” hidden along the groin area of the shell. Meanwhile, there are any number of pond sliders to choose from. This guide, in essence, is a binary guide: native turtle, or nonnative turtle. If that is all you are able to determine, or if this guide helps you look for that one key diagnostic trait so that you can make that decision, we’ve done our job.

Good luck!

You can also download a pdf of the guide using this link.

Footnote #1: scute = the individual plates on the carapace [top shell] and plastron [bottom shell]
Photo Credits (left): © Sarah Bettelheim and © Matthew Bettelheim
Photo Credits (right): © James Muehlner (a), Merav Vonshak (b), sqrlfrndz (c), grdrennan (d), Aaron Runkle (e), genobirder (f), and Sabin1 (g) under CC BY-NC 4.0 DEED; © Matthew Bettelheim and © Sarah Bettelheim
Created by: 
Matthew Bettelheim, Brian Acord, Rachel Freund, and Annie Chang