Oxygen limitation of thermal tolerance defined by cardiac and ventilatory performance in spider crab, Maja squinado

Geographic distribution limits of ectothermal animals appear to be correlat ed with thermal tolerance thresholds previously identified from the onset o f anaerobic metabolism. Transition to these critical temperatures was inves tigated in the spider crab (Maja squinado) with the goal of identifying the physiological processes limiting thermal tolerance. Heart and ventilation rates as well as PO2 in the hemolymph were recorded on-line during progress ive temperature change between 12 and 0 degreesC (1 degreesC/h) and between 12 and 40 degreesC (2 degreesC/h). Lactate and succinate were measured in tissues and hemolymph after intermediate or final temperatures were reached . High levels of hemolymph oxygenation suggest that an optimum range of aer obic performance exists between 8 and 17 degreesC. Thermal limitation may a lready set in at the transition from optimum to pejus (pejus = turning wors e, progressively deleterious) range, characterized by the onset of a decrea se in arterial PO2 due to reduced ventilatory and cardiac performance. Hemo lymph PO2 values fell progressively toward both low and high temperature ex tremes until critical temperatures were reached at similar to1 and 30 degre esC, as indicated by low PO2 and the onset of anaerobic energy production b y mitochondria. In conclusion, the limited capacity of ventilation and circ ulation at extreme temperatures causes insufficient O-2 supply, thereby lim iting aerobic scope and, finally, thermal tolerance.