Features of the Changes in External Morphology and Axial Skeleton in Juvenile Salmonid Fishes (Salmonidae) Associated with Smoltification

Changes in external morphological characters and relative lengths of vertebral centra from different regions of the vertebral column are analyzed during smoltification in wild juveniles of five salmonid fish species: Atlantic salmon Salmo salar, brown trout S. trutta, mikizha Parasalmo mykiss, coho salmon Oncorhynchus kisutch, and northern Dolly Varden Salvelinus malma. The changes in the body proportions and external morphology are similar in different salmonid species, but the patterns of differentiation of the vertebral column’s postanal part are different. In Atlantic salmon, all vertebral centra of the postanal part are subject to elongation; in mikizha and brown trout, a small number of the centra are elongated only in the anterior region of the postanal part; in coho salmon, the centra are elongated in the posterior region of the postanal part; in Dolly Varden, the centra are elongated in the middle region of the postanal part. Thus, despite observed universal changes in external morphology associated with smoltification in the family Salmonidae, the development of future marine migrants' phenotypes is species-specific due to different growth of various groups of vertebral centra in the vertebral column’s postanal part. The possible reasons for the species diversity in the growth of various groups of vertebral centra are discussed.