In many fish, both production of new muscle fibres and neurogenesis continue into juvenile
life. To test the hypothesis that new motoneurons are produced to supply the expanding
muscle target we used the seabream (Sparus aurata), which shows a many-fold increase in
the number of fibres in lateral muscle during posthatching juvenile development. A motor
nerve branch innervating a segment of epaxial lateral white muscle was identified, and the
type and number of its axons were measured in fish of several larval and post-larval ages.
Contrary to expectation, total axon number was greatest in the larval fish (114.3\ub122.6);
unmyelinated axons were found only in the larval nerves, and the number of myelinated axons
increased only modestly over the ages examined, from 58.5\ub112.4 in larval fish to
77.5\ub17.3 in post-larval juveniles. We conclude that in seabream the larval nerve still includes
axons of motoneurons destined to die during the normal developmental phase of targetdependence
in addition to those axons which will survive into juvenile life, and that the definitive
number of motoneurons is already present in the larval fish before the main increase
in muscle fibre number occurs
