THE INTRASPECIFIC VARIATION

An Experiment To Investigate The Intraspecific Variation By Study Of Electrophorectic Banding Patterns Derived From Striated Muscle Samples In Herring

An Experiment To Investigate The Intraspecific Variation By Study Of Electrophorectic Banding Patterns Derived From Striated Muscle Samples In Herring

Introduction

The Atlantic herring (Clupea harengus L.) is an important component of pelagic ecosystems in the North Atlantic, North Sea and Baltic Sea (Parrish and Saville, 1967). The species has a complex population structure, with numerous races, tribes and stocks that differ in morphology and in ecological and behavioural characteristics (Parrish and Saville, 1967; Jørstad et al. 1991). Herring have transparent eel-shaped larvae that hatch at approximately 6-9 mm in total length, depending on the stock, and swim continuously to avoid sinking. The myotomes contain a single superficial layer of red muscle fibres surrounding an inner mass of approximately 250 largerdiameter white muscle fibres (Batty, 1984; Johnston, 1993).

The red muscle fibres of herring larvae express myosin light chains characteristic of adult white muscle (Johnston and Horne, 1994), and the inner fibres contain a distinct larval isoform(s) of myosin heavy chain (Crockford and Johnston, 1993). Mitochondria occupy 46 % and 26 % of fibre volume in the red and white muscles, respectively, in larvae reared at 15 °C (Vieira and Johnston, 1992), which suggests that both fibre types are recruited for continuous swimming. The larvae rely exclusively on cutaneous respiration until the gill filaments start to form at 20 mm, although haemoglobin is not present until towards the end of metamorphosis at 35 mm (De Silva, 1974).

Discussion

Studies with a wide range of fish larvae have documented numerous changes in muscle structure and function during ontogeny. For example, as development proceeds, the aerobic capacity of both muscle fibre types decreases (El-Fiky et al. 1987), and the superficial and inner muscle fibres gradually start to express myofibrillar proteins characteristic of adult red and white muscles, respectively (Rowlerson et al. 1985; Chanoine et al. 1992; Johnston and Horne, 1994; Martinez and Christiansen, 1994; Mascarello et al. 1995). In adult herring, the red muscle fibres are multiply innervated and the white muscle fibres are focally innervated (Bone, 1964).

Electromyographic studies with adult Pacific herring (C. pallasii L.) have shown that only the red muscle fibres are used for continuous swimming. Following the recruitment of the white muscle, there is a rapid onset of fatigue, leading to a marked division of labour between the fibre types (Bone et al. 1978). Temperature has a profound effect on the rate and degree of expression of the developmental programme in fish embryos (Johnston et al. 1996). For example, myofibrils are observed at later somite stages in the rostral myotomes of herring embryos incubated at 5 °C than in those incubated at 12 °C (Johnston et al. 1995). At hatching, the composition of myofibrillar proteins (Crockford and Johnston, 1993), the volume density and spatial distibution of muscle fibre organelles, and the number and size distributions of muscle fibres (Vieira and Johnston, 1992; Johnston, 1993) have all been shown to ...