Testing hypotheses concerning the phenotypic plasticity of escape performance in fish of the family Cottidae.
A knowledge of the natural history of two species of marine Cottidae was used to construct a set of complex a priori hypotheses to test the idea that seasonal temperature acclimation conferred a fitness advantage and to examine whether acclimation responses were constant through development. The species studied were the short-horn sculpin (Myoxocephalus scorpius L.) and the long-spined sea scorpion (Taurulus bubalis Euphr.). Fast-start escape performance was measured as a proxy of fitness, and maximum body-length-specific speed (Umax) and acceleration (Amax), and angular velocity (max) and cumulative turning angle (CTA) were quantified. Fish were acclimated to 5, 15 and 20 degreesC and filmed using high-speed cinematography at 0.8, 5.0, 15.0 and 20.0 degreesC. The following hypotheses were tested: (1) improvements in escape performance following acclimation to high temperature in adult short-horn sculpin that live offshore would involve a trade-off in performance at low temperature, (2) the escape response of the sea scorpion, which experiences large daily temperature variation, would exhibit low thermal sensitivity and be unchanged by acclimation between 5 and 15 degreesC, and (3) in short-horn sculpin, the ability to acclimate maximum speed thermally is acquired during ontogeny in parallel with the offshore migration of the late juvenile stage. At 20.0 degreesC, in adult short-horn sculpin, Umax and Amax were 110 % and 55 % higher, respectively, in 15 degreesC- than in 5 degreesC-acclimated fish. No evidence was obtained for improved fast-start performance at 0.8 degreesC or 5.0 degreesC following cold acclimation. In the long-spined sea scorpion, acclimation to 5 and 15 degreesC did not improve Umax or Amax compared with fish acutely exposed to these temperatures, although acclimation to 5 degreesC increased max (P=0.005). When tested over the most extreme thermal range found in the field, all variables were improved at a test temperature of 0.8 degreesC in 5 degreesC- compared with 15 degreesC-acclimated sea scorpion. Acclimation therefore appeared to be beneficial in some instances in both species. How this affects relative fitness is uncertain. The scaling of Umax with acclimation to 5 and 15 degreesC was examined in both species over the test range 5.0-15.0 degreesC. Temperature acclimation did not affect scaling relationships of Umax in long-spined sea scorpion ranging in total body length (L) from 45 to 160 mm. At a test temperature of 15.0 degreesC, the scaling of Umax for short-horn sculpin ranging in total body length from 43 to 270 mm changed from aL-0.98 in 5 degreesC-acclimated fish to aL-0.50 (where a is the proportionality coefficient in the regression equation) in 15 degreesC-acclimated fish (P<0.01). In short-horn sculpin, therefore, the ability to modify escape performance with temperature acclimation was found to vary during ontogeny, potentially paralleling a migration from a more variable to a more stable thermal environment.
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