Few aquatic organisms in freshwater environments elicit such strong negative reactions as a leech. They are gross, slimy, bloodsuckers and they creep people out. For those willing to dig a little deeper into their biology, however, leeches are very fascinating animals. For starters, leeches are hermaphroditic and some provide parental care to their young. Then consider the fact that many species of leech have a narrow range of hosts and wouldn’t know what to do with a big mammal like a human even if there was no other source of food for miles and suddenly the topic of leech biology starts to pique people’s interest. So, this month’s topic: the Smooth Turtle Leech (Placobdella parasitica).
Finding leeches on aquatic turtles is pretty common, and I would venture to say that most adult freshwater turtles have at least one leech on them right now, perhaps even hundreds if you count their young. Quite often the leeches are just attached to the turtle’s shell where they are unable to feed, so until recently I just assumed the leeches were hitching a free ride on the turtles, much like the leeches I occasionally pluck off of my kayak. It turns out, however, that the leeches usually found on freshwater turtles, including the Wood Turtles I spend most of my time thinking about, are not attached to the turtles for pure convenience, but actually spend most of their lives on turtles, and even rear their young on them. Smooth Turtle Leeches, which are widespread through much of the Eastern United States, have been documented on most species of freshwater turtle within their range and, as their name suggests, feed almost exclusively on the blood of turtles. When the leeches feed they attach themselves to the flesh, but are careful to select locations where turtles would have great difficulty biting or scratching them off, such as the base of the tail or on the flesh behind the legs. Like other leeches, the Smooth Turtle Leech can also tolerate some exposure to the air, and on many occasions while measuring, weighing, and collecting other data on turtles, I have witnessed leeches on the turtle’s plastron or carapace move with deliberation to the edge of the shell and then behind the legs where they will remain moist long after the shell dries out. This trick would seem particularly useful on species such as Wood Turtles, which spend much more time out of water than most other turtles, even when they are staying close to streams.
All leeches are hermaphroditic, meaning they possess the anatomy necessary to both lay eggs and fertilize other leeches. During copulation, the Smooth Turtle Leech will deposit a capsule of sperm, called a spermatophore, on the dorsal surface of another leech. Shortly thereafter, the skin immediately around the spermatophore begins to soften through a process called histolysis, allowing the spermatozoa to pass directly through the skin. In fact, pressure from the spermatophore may even push the sperm through the softened skin, after which the spermatozoa distribute to the surrounding tissue, with the greatest concentration eventually ending up in the uterus. Four or five days later, eggs are laid. The parent will lay eggs into multiple cocoons containing up to over 200 eggs all together, which it will then brood under its ventral surface, often while still attached to the turtle host. After hatching the young attach themselves to the underside of the adult and will be carried around in such a manner, in some cases never leaving the turtle host until maturity. Eventually, when the young are developed enough to take their first blood meal, the adult will transport them to an appropriate location on the turtle to release them and it is not uncommon to see patches of hundreds of tiny leeches around the base of a turtle’s tail, especially in the fall. After at least one blood meal the young disperse, but will need at least one more meal in order to survive the winter.
So what does this all mean for the turtles? Probably not much. While leeches are certainly an annoyance to the turtles, it is unlikely they are doing any real harm. One study looking at leeches on Musk Turtles found that adults without leeches were less likely to have leeches during a subsequent capture than other turtles that biologists had previously removed leeches from, suggesting that the leech-free turtles either avoid being latched onto by the parasites or are better able to remove them. Leeches are known to transmit certain diseases (one of which is associated with newt die-offs), but which pathogens the Smooth Turtle Leech can transmit to turtles is an area that so far has not received much attention in academic circles (so far as I know). Parasites are a very big part of the natural world, though, and modern humans are an unusual exception in that regard. Leeches, ticks, botflies, and countless other external parasites are common place in the wild, and then there is the even larger world of internal gut parasites with mounting evidence that some such species even provide benefits to their hosts. Of course it is possible that a non-native species of leech or pathogen could be introduced that could cause problems for turtles here, or that changes in the environment may disrupt the natural balance between parasite and host (read up on how a warming climate results in fatal loads of winter ticks on moose for a good example). But when it comes to the natural scenario of native leeches on turtles, this is something that, in all likelihood, turtles have been dealing with for as long as turtles have existed, possibly 300 million years. And yes, sea turtles have their own saltwater leech specialist, the Marine Turtle Leech (Ozobranchus margoi). #TheMoreYouKnow