Antarctic fish muscles — structure, function and physiology

Abstract

The structure and function of swimming muscles in Antarctic fish is reviewed, the emphasis being on the highly endemic sub-order Notothenioidei. Adult stages of the vast majority of species swim at low speeds using large pectoral fins (labriform locomotion). This is supplemented with sub-carangiform swimming in pelagic juvenile stages and in the adult stages of some other species. The thrust for sustained activity is provided by the recruitment of slow muscle fibres. Slow muscle myofibrils typically occur in columns one fibril thick entirely surrounded by mitochondria. The resulting high volume density of mitochondria (30–60%), and short inter-mitochondrial spacing, is thought to represent an adaptation which serves to compensate for the detrimental effects of low temperature on enzyme reaction and diffusion rates. Sub-carangiform swimming is used to achieve burst speeds associated with prey capture and/or predator avoidance. Burst speeds require the recruitment of fast twitch fibres in the myotomes. In many demersal species the energy supply for burst swimming largely comes from phosphogen hydrolysis, and the capacity of fast twitch fibres for anaerobic glycogenolysis is severely reduced. Antarctic fish are characterized by delayed maturation, slow growth and low metabolic rates. The fast myotomal muscles of adult stages often contain few fibres less than 80 μm diameter, fibres 200–450 μm diameter forming the major size class in numerous species. It is therefore likely that hyperplasia ceases at a relatively early stage in development and that subsequent muscle growth involves hypertrophy of existing fibres. Studies of the contractile properties of isolated muscle fibres suggest that low temperature limits maximum swimming performance in Antarctic fish. Kinematic data suggest that this is most noticeable for juvenile stages, which have higher maximum tail-beat frequencies than adults.