In my recent post about the terrapin conservation project at The Wetlands Institute, I wrote that terrapins are are the only species of turtle adapted to live in the brackish water of coastal wetlands. This makes them different from sea turtles, freshwater turtles, and tortoises (which live on land).
Evolutionary biologists have calculated, based on differences in their DNA, that terrapins diverged from freshwater turtles around 7 to 11 million years ago. Their nearest relative is a type of freshwater turtle known as a map turtle.
Presented for your enjoyment, seven ways that terrapins have evolved to live in brackish (semi-salty) water:
- Highly developed lachrymal glands - In humans, these are commonly called tear glands. Human tears are a little salty, but terrapin tears are twice as salty as sea water. This helps flush out excess salt that they have ingested. Sea turtles do the same. People have seen sea turtles "crying" as they lay their eggs on the beach, and assumed it was the pain of childbirth that was causing them to cry. They are just getting rid of their extra salt. Crocodiles have developed a similar mechanism, which is where the term crocodile tears originated.
- Specially shaped lower jaw - Terrapins prefer to drink fresh water, since it saves them the trouble of getting rid of the excess salt (and I'm sure it tastes better). When it rains, terrapins will drink from puddles on the ground, collected on leaves, and even collected in the nooks and crannies of their friend's shells. The lower jaw of the terrapin has evolved into a sloping, scoop shape without a "chin". This allows them to drink from puddles as shallow as 1 mm. The box turtle needs a puddle at least 5 mm deep to get a drink.
- Drinks like a camel - Camels and terrapins have a similar problem - access to fresh drinking water. They have developed a similar adaptation - the ability to quickly drink a large amount of water when it becomes available. A terrapin can drink up to 15% of his body weight in 15 minutes. That would be like me chugging 4 gallons of water! Even better, if the terrapin is attacked by an enemy after a drinking binge, it will regurgitate its water before fleeing. Between the surprise of the barf and the lighter load to carry, this may give the terrapin the edge it needs to escape.
- Skims the top of the water - When it rains, a layer of fresh rain water collects on the surface of the brackish water. Using a behavioral adaptation, the terrapin will drink from this top layer before it mixes with the rest of the water.
- Catches falling rain drops - I had a hard time believing this one, but I did read it in more than one reputable source. I think a terrapin would have to be pretty loaded up with salt, and it would have to be raining pretty hard, for this behavior to be worth the effort.
- Strong jaws - Terrapins can break into some of the tough food found in coastal wetlands, including clams and periwinkle snails (their favorite NJ invasive species).
- Webbed feet - Midway on the evolutionary scale between the stump-like feet of land tortoises and the flippers of sea turtles, terrapins have webbed feet adapted to their middle-of-the-scale environment.