Performance limits of low-temperature, continuous locomotion are exceeded when locomotion is intermittent in the ghost grab

摘要

Since a decline in temperature decreases aerobic capacity and slows the kinetics of exercise-to-rest transitions in ectotherms, we manipulated body temperature to better understand the performance limits of intermittent locomotion. Distance capacity (i.e., the total distance traveled before fatigue) of the ghost crab, Ocypode quadrata, was determined during acute exposure to 15 deg C inside a treadmill-respirometer. Instead of exacerbating the near-paralyzing effects of low body temperature resulting from the frequent transitions, intermittent locomotion allowed animals to exceed the performance limits measured during steadystate locomotion. At low temperature, distance capacity for continuous locomotion at 0.04 m s~(-1) (83 percent maximum aerobic speed) was 60 m. When 30 s of exercise at 0.08 m s~(-1) (166 percent 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 speed (83 percent maximum aerobic speed). A 30-s pause following a 30-s exercise period was sufficient for maintaining low lactate concentrations in muscle and for partial resynthesis of arginine phosphate. A greater dependency on nonoxidative metabolism due to slowed oxygen uptake kinetics at low temperature resulted in a decreased duration of the critical exercise period, which increased performance relative to that measured at higher temperatures (30 s at 15 deg C vs. 120 s at 24 deg C). Despite the ghost crab's limited aerobic capacity at 15 deg C, distance capacity during intermittent locomotion at low temperature can be comparable to that of a crab moving continuously at a body temperature 10 deg C warmer. While endurance capacity is generally correlated with maximum aerobic speed, we have demonstrated that both locomotor behavior and body temperature must be considered when characterizing performance limits.