ANDRZEJEWSKI, JE*; BRIGGS, LE; JOYNER-MATOS, JL; JULIAN, D; Univ. of Florida, Gainesville: Functional responses to high temperature, hypoxia, and hyposalinity in the stress-tolerant clam Mercenaria mercenaria

Mercenaria mercenaria, an estuarine clam, tolerates high temperature, hypoxia and hyposalinity, but it is unknown how these stressors affect functional responses such as condition index (tissue mass:shell volume), ability to bury, water clearance rate, whole-animal glycogen content and survival. To test this, we first conducted two sets of experiments across three seasons, focusing on hypoxic and thermal stress: 1) clams were exposed for 14 d at 24C to normoxia, mild hypoxia or moderate hypoxia (21.1 0.4, 13.6 0.1 or 6.5 0.1 kPa O2, respectively); 2) clams were exposed to normoxic conditions at 23.9 0.6 C, 28.6 0.9 C or 33.5 0.7 C. There were no significant changes in condition index or glycogen content. In the winter, clams at high temperature showed decreased burial, and died by day 6. Since these clams likely experience simultaneous stressors in their estuarine habitat, we next examined the effects of high temperature combined with hyposalinity. Clams were acclimated to 19 ppt and 23 C for 1 week, and then exposed for 14 d to control salinity (23 1.7 ppt) or two levels of hyposalinity (15.5 1.1 and 4.6 0.7 ppt), all at control temperature (20 C) or 34 C. Compared to clams exposed to dual stressors, those exposed to single stressors were more likely to bury, had higher clearance rates and higher survival. Dual stressors had a particularly marked effect on survival: all clams in the 5 ppt/34C treatment died by day 5, and all clams in the 20 ppt/34C and 5 ppt/20C treatments died by day 14. There were no deaths in the 20 ppt or 15 ppt treatments at 20C. While the clams are remarkably capable of withstanding moderate levels of thermal and hypoxic stress with relatively small changes in the measured functional responses, exposure to the high temperature in combination with hyposalinity reduces the health of these animals.