Introduction

Ectothermic organisms in the temperate zone exhibit phenotypic plasticity in reaction to changes in environmental factors such as temperatuie, producing what has been teimed, a norm of reaction over an environmental gradient (Schemer, 1993). Seasonal temperatuie fluctuations have long been known to produce physiological compensatoiy changes, known as acclimation responses, in a diversity of rate functions and a wide variety of species (see Bullock, 1955, for eaiiy review). Fiy and Hart (1948) caiTied out a classic study examining the eflfect of thermal acclimation on the swimming speed of goldfish {Carassius auratus L.). Acclimation extended the theimal range of activity, and there was a shift in the optimum temperature for performance, between high and low temperature acclimation. Increases in speed at low temperature following cold acclimation were at the expense of swimming peiformance at high temperature, compared to warm-acclimated fish. Following cold acclimation, the mechanisms underlying altered swimming peifoimance include, inter alia^ hypeitrophy of red muscle fibres (Johnston and Lucking, 1978; Sidell, 1980), increases in mitochondiial volume density (Johnston and Maitland, 1980; Egginton and Sidell, 1989; Hubley et al 1997), altered expression of myosin fight chain ÇMLC) isofoims (Crockford and Johnston, 1990; Langfeld et al 1991; Hhayama et al 1997), and altered expression of myosin heavy chain isofoims (MHC) (Gerlach et al 1990; Hwang et al 1991; Johnson and Bennett, 1995; Imai et al 1997) and associated increases in fast muscle myofibrillar ATPase activity (Johnston etal 1975; Heap etal 1986; Crockford and Johnston, 1990).

Not all temperate species of fish exhibit theimal acclimation. In fact, fish fiom habitats with laige, daily temperature fluctuations may exhibit reduced, or no, theimal acclimation. In the killifish {Fundultis heteroclitus), rapid daily fluctuations in water temperature of salt mai’sh habitats, have been associated with a lack of acclimation in myofibrillai" ATPase activity over a temperatme range of 5 to 25 °C (Sidell et al 1983). Over a broader test range of 10 to 35 °C, ATPase activity showed some acclimation,

although reduced in compaiison to goldfish. This was reflected in a similarly reduced acclimation response in escape perfoimance (Johnson and Bennett, 1995). It has been suggested that acclimation modifications aie avoided if the contractile complex has a low theimal sensitivity to acute temperatme fluctuations experienced by the organism (Sidell

et ah 1983). Fmtheimore, the high degree of short-teim temperatme vaiiation may mask

any stable cues for acclimation.

During ontogeny, seasonal shifts in temperatme and/or changes in habitat, can profoundly alter the theimal optimum for physiological processes in ectotheims. For example, the theimal optimum for growth increases dming ontogeny in lai"val plaice

{Pleuronectes platessa) (Hovenlcamp and Witte, 1991) and winter flounder {Pseudo- pleuronectes americaniis) (Buckley, 1982). In the dragonfly (Libellula pulchella),

matmation is associated with a more defined thermal sensitivity and an increase in optimum thoracic temperatme and upper lethal temperatme (Marden, 1995), paialleling the increase in thoracic temperatme associated with the more active lifestyle of adult stages (Mdxdenetal. 1996).

The evolutionaiy significance of acclimation, has been less rigorously examined than the mechanisms (Huey and Bemgan, 1996). Precht (1958) classified compensatoiy changes m physiological rate processes with thermal acclimation, as “perfect”, “partial”, “excess”, “inverse” or absent (“no”). Despite a cautionaiy note by Fisher (1958), that “when acclimations occur it does not follow that these necessarily have obvious survival value”, it has often been assumed that acclimation responses aie adaptive (Hazel and Prosser, 1974), and fitness is improved following a period of acclimation. This has become known as the “Beneficial Acclimation Hypothesis” (Leroi et al. 1994; Huey and Bemgan, 1996) and, until recently, has been proffered as the primaiy evolutionaiy explanation of acclimation. However, it has received recent criticism by Huey and Bemgan (1996), since, in reality, post-hoc adaptive stories can be created for any of Precht’s above-mentioned acclimation responses. According to Leroi et al. (1994), an acclimation response is only to be regaided as beneficial if it enhances diffeiential reproduction, also Imown as Dai*winian fitness. Fmtheimore, when specifically tested, the assumption has often been rejected (Leroi et al. 1994; Zamudio et al. 1995; Bennett and Lensld, 1997). Huey and Bemgan (1996) suggest examination of the evolutionaiy

significance of patterns of acclimation should use more rigorous a priori hypotheses based on the natmal history of the species. Using this rationale, a set of hypotheses concerning theimal acclimation, were tested in two fish species of the family Cottidae.

Maiine Cottidae are paitial residents of the littoral zone (Gibson, 1969). Both short-hom sculpin {Myoxocephalus scorpius L.) and long-spined sea scorpion {Taumlus bubalis Euphr.) are benthic, maiine teleosts of the temperate zone. In the noith-east Atlantic, short-hom sculpin have a distiibution between 45 and 78 °N, whilst long-spined sea scoipion occupy a somewhat lower clime, between 40 and 68 °N (Unesco, 1986) (Fig. 2.1). Around the British Isles, adult short-hom sculpin aie caught offshore (Foster, 1969; King et al 1983) between about 30-50 m (Unesco, 1986). In contiast, juvenile sculpin, and all stages of long-spined sea scorpion aie found in rock pools and in the shallow sublittoral zone (Foster, 1969; King and Fives, 1983; Unesco, 1986). Smface water temperatuies in St. Andrews Bay, Scotland, range fiom 3-5 °C in the winter, to 15- 18 °C in the summer (I. Murdoch, unpublished observations). Both sea scoipion and juvenile sculpin also experience acute changes in temperatme, paiticularly in the intertidal zone, hi the same bay, rock pools where these species aie found have mean water temperatmes in spimg ranging fiom 5.7 °C to 12.5 °C over a 24 horn: period (G. Temple, unpubhshed obsei"vations). Pool temperatmes have been found to be even more vaiiable dmmg the summer months (Morris and Taylor, 1983).

The short-hom sculpin and long-spined sea scorpion aie “ambush predators”, and engage in veiy little steady swimming. Despite other foims of predator avoidance, such as camouflage and spines, both species commonly employ fast-starts for escape. Measmement of peiformance has been proposed as a practical method of yielding infoimation on fitness and physiological compensation (Huey and Stevenson, 1979; Arnold, 1983). Several studies have shown or suggested a positive coiTelation between bm'st speed and sm*vival, which can have a perceptible influence on fitness (Taylor and McPhail, 1985; Swain, 1992; Watkins, 1996; Andiaso, 1997).

hi the present study the following predictions were made, using the escape response, as a coixelate of fitness: 1) in the short-hom sculpin, waim acclimation improves escape perfoimance at high temperatme, at the expense of peiformance at low temperatme, compaied to cold-acclimated fish, 2) escape perfoimance in the intertidal

Figure 2.1. A) Short-hom sculpin {Myoxocephalus scorpius L.) and geographic distribution in the north-east Atlantic. B) Long-spined sea scorpion {Taurulus bubalis Euphr.) and geographic distribution in the north-east Atlantic. C) High speed video recording of the first two half taü-beats and gliding stage of an escape response typical of both the short-hom sculpin and long-spined sea scorpion. Scale bars, 20 mm.