Years ago when I first encountered a Wood Turtle with a viscous brown goo oozing out of its mouth I thought something was wrong. Generally speaking, whenever a turtle has a runny nose, eyes, or slime coming from its mouth it is bad news, and a disease pathologist I sent some pictures to suggested the condition looked somewhat similar to a type of herpesvirus that can infect turtles. A Wood Turtle expert working for the Vermont Department of Fish and Wildlife offered a much simpler explanation, and just said, “that a classic case of slug face”. I felt like an idiot as my brain cycled through memory after memory of all the times I used slugs for fishing bait as a kid and recalled the orange brown slime they would secrete while being handled or placed on a hook. Of course it was slug face, and since then I’ve seen a lot of Wood Turtles similarly afflicted.
Wood Turtles eat a lot of slugs. Snails too. But, as an opportunistic omnivore, they eat lots of things. Indeed, of all the studies that have evaluated Wood Turtle diets, one big thing that ties together all of the different findings is that Wood Turtles vary widely based on what is available to them in their home range. Hatchlings and juveniles eat a larger proportion of protein rich foods (invertebrates), and adults eat a larger proportion of vegetation, but enthusiastically gobble up invertebrates and carrion when they can (one study even documented a female Wood Turtle eating one of her own eggs!). While slugs and snails are certainly a big part of the Wood Turtle’s diet, the propensity of slug slime and snail shells to get stuck to the turtle’s beaks for prolonged periods of time may mean that the frequency at which Wood Turtles eat such gastropods is sometimes overestimated. That being said, almost every Wood Turtle I have ever personally seen eating something at the moment I found it was eating a snail.
In Vermont, the most abundant snails I encounter during Wood Turtle surveys (based on my limited ability to key out gastropods) are called ambersnails (family Succineidae), likely the blunt ambersnail (Oxyloma retusum). Typically less than a centimeter long, this species, like other ambersnails, commonly inhabits wet meadows where they feed on the decaying leaves of dead or dormant plants. Given that Wood Turtles spend considerable time foraging in floodplains, especially in the spring when conditions are wet and the floodplains are full of decaying plant matter, it would make sense that their foraging habitat is chock full of ambersnails (as, it would seem, are the turtle’s stomachs). Furthermore, anecdotal evidence would suggest that adult Wood Turtles desperately crave protein when they first emerge from their winter dormancy, so an abundant supply of ambersnails would be readily taken advantage of as soon as the turtle emerge in the spring, much to the turtle’s benefit.
Unlike the Wood Turtles that prey on them, ambersnails have incredibly short lifespans. Those that survive the winter breed at about the same time Wood Turtles begin to move away from their streams in early to mid-spring. Asexual, most snail couplings result in both snails becoming fertilized and laying eggs before dying sometime in late spring or early summer. Around the end of August most of the snails from the new generation reach maturity, mate, and then die, while the rest (along with a new fall generation) overwinter to repeat the cycle the following year. During that entire process, they are food for absolutely anything large enough to eat them, even many herbivores such as deer that aren’t even aware of their existence.
Not only are the ambersnails prey, they are also hosts. Though not the preferred host species, ambersnails can be parasitized by the trematode Cotylurus flabelliformis, more commonly known as duck flukes. Ambersnails, along with other snails and slugs, play important roles in the life cycles of many parasites, including the duck fluke, which needs to be passed from one host species to another in order to grow and breed. In the case of duck flukes, there is actually a third host in the process (mollusks) that must also be infected in order for the fluke’s life cycle to be completed. At the end of that cycle, ducks pass the fluke eggs in their scat, at which point all a snail needs to do is haplessly wander near the scat to become infected and start the cycle over again. Though not known to occur in ambersnails, moose brainworms, which actually prefer to live in deer because such an infection is fatal to moose, have similar life cycles involving different gastropods.
To the best of my knowledge, these snails do not play any known role in the transmission of parasites to Wood Turtles, but it would not surprise me to one day learn that they do. The world of snail ecology is relatively understudied, and it would seem much more is known about the parasites they transmit than the snails themselves, which is saying a lot because there is much we don’t know about the parasites either. Even if we eventually do learn that the snails to play a part in Wood Turtle parasite transmission, it is clear that these snails play an important part in the Wood Turtle’s diet, and their presence on the landscape is surely a benefit to them.