Summary

We examined physiological and biochemical responses to temperature and aerial exposure in two species of intertidal porcelain crabs (genus Petrolisthes) that inhabit discrete vertical zones. On the shores of the Northeastern Pacific, P. cinctipes (Randall) occurs under rocks and in mussel beds in the mid to high intertidal zone and P. eriomerus (Stimpson) occurs under rocks in the low intertidal zone and subtidally to 80 m. Because of their different vertical distributions, these two species experience very different levels of abiotic stress. Individuals of P. cinctipes can be emersed during every low tide, but P. eriomerus is only emersed during the lowest spring tides and on most days is not emersed at all. Temperatures measured underneath rocks in the mid intertidal zone were as high as 31 °C, 15 °C higher than maximal temperatures measured under rocks in the low intertidal zone. In air, at 25 °C, large specimens of P. cinctipes were able to maintain a higher respiration rate than similarly sized P. eriomerus. No interspecific differences in the respiratory response to emersion were seen in small specimens. Examination of the response of heart rate to temperature revealed that P. cinctipes has a 5 °C higher Arrhenius break temperature (ABT, the temperature at which there is a discontinuity in the slope of an Arrhenius plot) than its congener (31.5 °C versus 26.6 °C). The heart rate of P. cinctipes recovered fully after exposure to cold (1.5 °C), but the heart rate of P. eriomerus did not recover after exposure to 2 °C or cooler. The ABT of heart rate in P. cinctipes was very close to maximal microhabitat temperatures; thus, individuals of this species may be living at or near their thermal tolerance limits. P. cinctipes were able to maintain aerobic metabolism during emersion, whereas P. eriomerus shifted to anaerobic metabolism. A pronounced accumulation of whole-body lactate was found in specimens of P. eriomerus incubated in air at 25 °C over a 5 h period, but not in P. cinctipes similarly treated. P. cinctipes possesses a membranous structure on the ventral merus of each walking leg, but this structure is not found in P. eriomerus. To test the function of the leg membrane, we measured the aerial respiration rates and the lactate accumulation of P. cinctipes with their leg membranes obscured. These individuals had significantly lower aerial respiration rates at 30 °C than control crabs. Crabs with leg membranes obscured also accumulated a considerable amount of lactate during a 5 h period of emersion at 28 °C, but control crabs showed no accumulation under the same conditions. These data suggest that the leg membrane functions as a respiratory structure. The results of this study illustrate that a suite of morphological, physiological and biochemical features allows P. cinctipes to live higher in the intertidal region than P. eriomerus.