BRACKISH VS. FRESH

There has historically been much debate over whether diamondback terrapins can thrive in freshwater as they do in their natural brackish environments. Most hobbyists would rather not have to contend with the hassles of a brackish water setup, while others would prefer to house their terrapins with other freshwater species in a community environment.  Numerous herpetocultural writers and suppliers alike have touted the ability of diamondbacks to be housed in freshwater, while others cite research suggesting that diamondbacks are susceptible to skin and shell ailments when kept in freshwater.
   The Facebook Diamondback Terrapins group is of the opinion that captive bred terrapins can be kept exclusively in freshwater without detriment.  The fact that there are keepers who maintain their diamondbacks in both fresh and brackish environments seem to suggest that the species is tolerant of extreme fluctuations in salinity levels.  There are numerous documented cases of diamondbacks that have adapted well to living in freshwater.  This seems to be the case particularly in captive bred specimens.  An unpublished description of diamondbacks compiled by Kathy Nemec of U.S. Fish & Wildlife has the following interesting observations:
   "the appetite of terrapins held in seawater, without access to freshwater, gradually becomes depressed, reducing their normal intake of food by 46-78% (Davenport and Ward, 1993), and possibly reducing their incidental intake of sodium."
   "adult terrapins are capable of spending several weeks in seawater without access to freshwater, but they cannot survive indefinitely.  Hatchlings are also severely affected by long exposure to seawater.  In the laboratory, hatchlings were not capable of growth in concentrations of 100% seawater.  Limited growth was achieved in 50% and 35% seawater with periodic access to freshwater, and in pure freshwater.  However, a very distinct growth optimum was seen in hatchlings raised in 25% seawater concentration (Dunson, 1985)."
   "early trappers holding terrapins for sale found that providing freshwater ensured greater survival (Dunson, 1970), and reported seeing them drinking rainwater in the wild."
   Consequently, the decision to keep terrapins in freshwater or brackish water is left solely to the discretion of the keeper.  The exception to this would be in the case of wild caught individuals or captive bred specimens in poor health - these should best be kept in brackish water.  Salt acts as a very effective disinfectant and very likely protects terrapins from various shell and skin infections that could occur in freshwater.  There are various bacteria and parasites that are found in both freshwater crustaceans like crayfish and shrimp, which if fed to diamondbacks can cause severe shell and skin infections.  However, sufficient salt in the system (1/2 cup of rock salt per ten gallons of water) would quickly neutralize such pathogens.  In fact, commercial grade marine salt mixed to a specific gravity of 1.006-1.007 is probably pretty close to the 25% seawater concentration cited earlier.