Differentiation of swimming muscles and gills, and development of anaerobic power in the larvae of cyprinid fish (Pisces, Teleostei)

Summary

Histological, histochemical, morphometric and electrophoretic methods were combined to study the differentiation of the swimming muscles and of the gills in the larvae and juveniles of cyprinids during the first 3 months after hatching. The bodies of recently hatched larvae are always surrounded by a single layer of muscle fibres (“red layer”) which possess strong cytochrome oxidase (COX) activity but whose myofibrils do not show the arrangement typical of the fibres of adult red muscle. The contribution of the red layer to total muscle mass decreases from about 12% on day 3 to 4% on day 40 post-hatching, after which the red layer becomes indistinguishable from the developing mass of adult red muscles. The adult fibres seem to originate through splitting from the larval fibres. The inner muscle masses of recently hatched larvae also display conspicuous COX activity which, however, disappears gradually. The development of the gill surface follows a time course which during the first 40 days is a mirror image of that of the larval red layer of muscle fibres, i.e. increasing at about the same rate as the latter decreases. The most cathodic of the isoenzymes of LDH (M₄), an indicator of the glycolytic capacity of the white muscle of adult fish, develops only slowly in the larvae. In whole body homogenates, the dominant isoenzyme after hatching is the aerobic H₄-form, and it is not until 2 weeks or so that the anaerobic M₄ is the strongest fraction in electropherograms of total body homogenates. In the bleak, Alburnus alburnus (Linné, 1758), which spends the first 8–10 days after hatching on the bottom of the aquarium, the M₄ isoenzyme takes much longer to develop than in Rutilus rutilus (Linné, 1758) or Chondrostoma nasus (Linné, 1758) which start swimming within 3 days after hatching. All these findings suggest that in the early larvae swimming is almost entirely aerobic, being powered by the deep layers of muscle fibres. During this developmental phase the superficial red layer perhaps is the main respiratory organ.