Low Temperature Limits Burst Swimming Performance in Antarctic Fish

Abstract

The temperature of the Southern Ocean varies from −1.86 °C at high latitudes to around 5 °C towards the Antarctic Convergence. Seasonal variations in temperature are less than 0.2 °C close to the permanent ice shelf (Littlepage 1965), and only 2.5 °C in the northern Antarctic (Everson 1970). There is evidence that these stenothermal conditions have existed relatively unchanged for several million years (Kennett 1977). Antarctic fish have therefore become highly specialized to cold conditions, and their upper lethal temperatures are often only 5–6 °C (Somero and DeVries 1967). Shallow-water species synthesize a variety of glycopeptide or peptide antifreezes to prevent the growth of ice crystals in the blood down to −2.7 °C (DeVries 1988). The nature of other adaptations which confer cold tolerance are relatively poorly understood (Clarke 1983; Johnston 1990). For example, although brain tubulins from antarctic fish assemble in vitro to form microtubules at −2.3 °C, they have broadly similar isoelectric points and amino-acid compositions to their mammalian counterparts which are cold-labile (Detrich and Overton 1988). The rates of molecular diffusion and enzyme reactions slow markedly at low temperatures. Thus, in the absence of compensating mechanisms, physiological processes would be expected to proceed more slowly in antarctic than in temperate or tropical fish. The rate of embryonic development would appear to confirm this prediction; this is exemplified by the time from fertilization to hatching in Harpagifer antarcticus, which is around 100 days at 0 °C, compared with 36–48 h in warm temperate fish at 25 °C (Blaxter 1988; Johnston 1990). However, there is evidence that other processes, such as locomotion and respiration, show varying degrees of temperature compensation (Clarke 1983, 1987; Johnston 1990). During burst swimming, ATP utilization by the fast muscle fibres increases over 100-fold very rapidly. Since maximum speed is an important factor determining the success of prey capture and predator avoidance it is crucial to the survival of individuals, and presumably subject to high selective pressures. Larval stages are capable of much higher tail-beat frequencies and length-specific speeds than adult fish because of scaling effects (Blaxter 1986). Thus the escape behaviour of larvae is an ideal place to look for the limits to cold adaptation in polar species.