No. 14–Still more on Terrapins

In a previous column I wrote “There are seven subspecies of diamondbacks which inhabit brackish water estuaries from Cape Cod to the Texas coast. Available literature that I have reviewed states that diamondbacks inhabit brackish water exclusively. Our local subspecies, however, is known to frequent fresh water ponds. The two Estuarium residents have a double tank setup with fresh water and water from Battery Creek. They are almost always in the fresh water when I arrive each morning.

“Unlike most estuaries, Port Royal and St Helena sounds are very high salinity systems. This may have something to do with our terrapins spending time in fresh water. I will research the matter further and report…” I made inquiries and have received responses from two authoritative sources. Dr. Whit Gibbons with the UGA Savannah River Ecology Lab replied; “we have seen terrapins enter mildly saline, approaching fresh water on Capers Island, and the ones we keep here at SREL in captivity are in fresh water all the time. I think that their lack of much presence in fresh water systems has more to do with increased competition from other turtle species as well as reduced productivity in their food rather than being a strictly physiological issue. Interesting question and one I will stay more attuned to.”

A slightly different take came from Jack Cover, General Curator at the National Aquarium in Baltimore. ” The diamondback terrapin is generally a brackish water turtle species. However, they can live in fresh water environments. Here at the Aquarium our terrapins have been housed in both fresh and brackish water environments. They can adapt to small changes of salinity. In a full salt water environment they would not do well. The reasoning for your turtles going to fresh water tanks could be a number of changes in your system such as temperature, pH, etc. Most likely it could be due to your high salinity in your system. Since you state that the salinity average is usually higher than 30 ppt for long periods of time, it is most likely that the salinity is too high for them and they will retreat to fresh water to level the salinity in their system.”

The conclusion I draw from the responses is that terrapins do just fine in fresh water in captivity because they are fed. Dr. Gibbons’ observations regarding competition with other turtle species and food availability would explain why terrapins spend the majority of their time in salty water in the wild. Also, Mr. Cover’s response gives credence to the thought that there may indeed be a physiological reason for our local subspecies to actively seek out fresh water in the wild. My thanks to both gentlemen for sharing their knowledge with us.

Out of this has grown the idea for an experiment once the Estuarium has built a permanent facility with dedicated terrapin habitat. The habitat will be of three sections containing fresh, brackish and high salinity water separated by crawl grounds. It will be interesting to observe whether if given the option, our local subspecies will choose a strictly brackish water life. In the mean time, since it is our desire to approximate the natural physical living conditions of our resident creatures as closely as possible, the Estuarium’s terrapins will continue to occupy their present home.