Salinity Tolerance of Estuarine and Insular Emydid Turtles (Pseudemys nelsoni and Trachemys decussata)

Abstract

ABSTRAcr. - Pseudemys nelsoni and Trachemys decussata inhabit brackish water in mainland areas of extreme southern Florida and on Grand Cayman Island. They appear to be intermediate in their salinity tolerance between truly freshwater forms and the highly specialized estuarine terrapin (Malaclemys). Unfed P. nelsoni (730-1240 g) had especially low rates of mass loss (primarily net water loss) in 100% sea water (about 0.4% initial mass/day). Smaller T. decussata (200-240 g) had higher values (about 0.8% /day), yet these rates were still lower than four values obtained on typical freshwater species (1.8-7.6% /day) held in 100% sea water. Mean whole body water effluxes in 100% sea water of adult P. nelsoni and T. decussata larger than 60 g were low (0.24-0.47 ml/100 g-h). Hatchling T. decussata had much higher rates of water efflux (1.0 ml/100 g-h). Sodium effluxes in 100% sea water were low (less than 100 umol/100 g-h) in all size classes. There was no stimulation in sodium efflux after salt loading in P. nelsoni, nor in T. decussata after dehydration in saline solutions. This implies the lack of salt glands in these species. Sodium influx in 100% sea water was very low in adult P. nelsoni and in T. decussata larger than 200 g. There was a progressive increase in sodium influx with declining size in T. decussata, so that hatchlings underwent a considerable net uptake of sodium in sea water. Hatchling T. decussata fed fish ad libitum were unable to maintain mass or grow when the salinity exceeded 41% sea water. Juveniles of about 80 g grew at salinities up to 59% sea water, representing a significant increase in tolerance above that of the hatchlings. Additionally, 80 g turtles grew significantly faster in 25% sea water than in fresh water. Large individuals (>200 g) of both species tolerated immersion in 100% sea water for prolonged periods (at least 10-24 days). Their natural habitats vary seasonally in salinity, but remain on average quite dilute due to rainfall.