TEMPERATURE AND MYOGENESIS IN EMBRYOS OF THE ATLANTIC HERRING CLUPEA-HARENGUS

The development of axial muscles has been investigated in spring-spawn ing Atlantic herring (Clupea harengus L.) reared at 5, 8, 12 and 15 de grees C, In 1994, around 90 % of embryos hatched after 28 days at 5 de grees C, 16 days at 8 degrees C, 9 days at 12 degrees C and 8 days at 15 degrees C, The somites were formed from cells of the paraxial mesod erm in a rostral to caudal direction, starting at the neural plate sta ge, Somites were added at rates ranging from one every 3 h at 5 degree s C to one every 52 min at 15 degrees C, A small number of myoblasts, located adjacent and lateral to the notochord, elongated to span the s omite to form mononuclear myotubes, The majority of muscle fibres were formed by the fusion of 2-5 myoblasts to produce multinucleated myotu bes that subsequently differentiated into either superficial or inner fibre types, The timing of myogenesis with respect to somite stage and the initial appearance of the gut, pectoral fin buds and pronephric t ubules was found to vary with development temperature. For rostral myo tomes, the synthesis of contractile filaments and myofibril assembly w ere first observed at the 42-, 38- and 27-somite stages at 5, 8 and 12 degrees C, respectively, The differentiation of myotubes into morphol ogically recognisable muscle fibre types first occurred at the 62-somi te stage at 5 degrees C, at the 48-somite stage at 8 degrees C and as early as the 40-somite stage at 12 degrees C, Cell proliferation exper iments with 5-bromo-2'-deoxyuridine showed that another population of myoblasts was activated on the surface of muscle fibres just prior to hatching, Development temperature also affected muscle cellularity; th ere were 43 % more inner muscle fibres in the myotomes of 1-day-old la rvae reared at 12 degrees C than at 5 degrees C (P<0.02).