PERFORMANCE LIMITS OF LOW-TEMPERATURE, CONTINUOUS LOCOMOTION ARE EXCEEDED WHEN LOCOMOTION IS INTERMITTENT IN THE GHOST CRAB

Since a decline in temperature decreases aerobic capacity and slows th e kinetics of exercise-to-rest transitions in ectotherms, we manipulat ed body temperature to better understand the performance limits of int ermittent locomotion. Distance capacity (i.e., the total distance trav eled before fatigue) of the ghost crab, Ocypode quadrata, was determin ed during acute exposure to 15 degrees C inside a treadmill-respiromet er. Instead of exacerbating the near-paralyzing effects of low body te mperature resulting from the frequent transitions, intermittent locomo tion allowed animals to exceed the performance limits measured during steady-state locomotion. At low temperature, distance capacity for con tinuous locomotion at 0.04 m s(-1) (83% maximum aerobic speed) was 60 m. When 30 s of exercise at 0.08 m s(-1) (166% maximum aerobic speed) was alternated with 30 s of rest, distance capacity increased to 271 m , 4.5-fold greater than continuous locomotion at the same average spee d (83% maximum aerobic speed). A 30-s pause following a 30-s exercise period was sufficient for maintaining low lactate concentrations in mu scle and for partial resynthesis of arginine phosphate. A greater depe ndency on nonoxidative metabolism due to slowed oxygen uptake kinetics at low temperature resulted in a decreased duration of the critical e xercise period, which increased performance relative to that measured at higher temperatures (30 s at 15 degrees C vs. 120 s at 24 degrees C ). Despite the ghost crab's limited aerobic capacity at 15 degrees C, distance capacity during intermittent locomotion at low temperature ca n be comparable to that of a crab moving continuously at a body temper ature 10 degrees C warmer. While endurance capacity is generally corre lated with maximum aerobic speed, we have demonstrated that both locom otor behavior and body temperature must be considered when characteriz ing performance limits.