Popular literature frequently claims Diamondback Terrapins (Malaclemys terrapin) are the only US turtles to inhabit brackish waters (e.g. Brennessel 2006) however it would be more accurate to say that terrapins are the species most adapted for brackish conditions. Several other US turtles can occur in brackish waters and some populations of Common Snapping Turtles (Chelydra serpentina) and Eastern Mud Turtles (Kinosternon subrubrum) are apparently adapted for brackish conditions (Ernst and Lovich 2009). I found the almost-hatchling-sized (SCL 3.5 cm) snapper pictured above (literally) beached and lying motionless in a pile of Bladderwrack, which made me wonder if it was in the best condition. Hatchlings can grow well in brackish water, but not beyond 14.35 ppt (Dunson 1986); the snapper I found was near water with a salinity of 22-25 ppt and was hundreds of meters from the mouth of a tidal stream. I reasoned that the turtle would probably not have survived much longer and decided to give it a head-start, getting it as large as possible over the summer and then releasing it into the tidal stream where it would have a better chance of successfully osmoregulating and avoiding predation.
Brennessel, B. (2006) Diamonds in the Marsh: A Natural History of the Diamondback Terrapin. University Press of New England
Dunson, W. A. (1986) Estuarine populations of the snapping turtle (Chelydra) as a model for the evolution of marine adaptations in reptiles. Copeia 1986, 741-756.
Ernst, C. & Lovich, J. (2009) Turtles of the United States and Canada. John Hopkins University Press.