Aquatic organisms inhabiting estuarine areas experience multiple natural (e.g. temperature and salinity) and anthropogenic physiological stressors. Stressed animals undergo many physiological adjustments, including the production of glucocorticoid stress hormones. Many stress responses are energetically expensive resulting in increased metabolic rate. If energy intake is finite, animals with higher metabolic rates may alter energy allocation. Such allocation adjustments occur at both the tissue level (e.g. decreased organ size) and the whole-organism level (e.g. decreased body size).
In this study, I examined the levels and effects of pollutant exposure in the obligate estuarine turtle, the Diamondback terrapin, Malaclemys terrapin . First, I determined PAH concentrations in turtle eggs and showed eggs from the eastern shore of the Patuxent River, Maryland had higher total hydrocarbons than eggs from the western shore. Thus analysis of terrapin eggs may provide insights into how contaminant impact geographically varies.
The remainder of this study examines the lethality and growth responses of three PCB 126 doses and the sublethal metabolic and growth responses of PCB 126 and salinity in combination. I exposed laboratory-reared, male terrapins to intraperitoneal injections of PCB 126. The terrapin ten-month LD50 was 29.8μg/g ±13.7, substantially higher than the LD50’s reported for other vertebrates. PCB 126 exposure reduced growth at all doses (4, 20, 40 and 80μg/g). Animals at 20μg/g PCB 126 exhibited impaired respiratory patterns and reductions in metabolic rate, hematocrit, and heart and gastrointestinal mass. There were no interactions between the two stressors, but salinity alone decreased growth and liver mass and altered respiratory patterns. There was no effect of PCB or salinity on corticosterone, however, plasma levels may have already returned to baseline at the time of the assay. These data suggest PCBs represent a more severe stressor, perhaps because terrapins are able to cope with salinity stress (e.g. salt glands).
This is the first study to provide experimental evidence for PCB 126 effects alone and in combination with another stressor in turtles. If we are to understand how stressors affect populations, we must provide causative, not merely correlative, evidence of the physiological responses to stress of all ecologically relevant stressors.