Scale formation in selected western North Atlantic flatfishes

Abstract

Patterns of scale formation (onset, completion and spatial pattern) were examined for five species of flatfishes in four families (Paralichthyidae: summer flounder Paralichthys dentatus, smallmouth flounder Etropus microstomus, Scophthalmidae: windowpane Scophthalmus aquosus, Pleuronectidae: winter flounder Pseudopleuronectes americanus and Soleidae: hogchoker Trinectes maculatus, to determine if the patterns are a useful indicator for the transition from the larval to the juvenile periods. In all species (except T. maculatus in which samples were limited), the ontogenetic pattern was very similar with onset of scale formation occurring on the lateral surface of the caudal peduncle, then spreading anteriorly along the presumptive lateral line, then laterally over the body, on to the head, and eventually on to the median fins. The timing of scale formation, relative to fish size, was late relative to other morphological and behavioural characters (i.e. fin ray formation, eye migration and settlement). The onset of scale formation, across all species, occurred at 9·0–27·0 mm total length (LT), at the same approximate size as eye migration and settlement. Completion of scale formation on the body occurred at 22–54 mm LT but completion of scale formation on the fins did not occur until 44–88 mm LT. Thus completion of scale formation in these flatfishes is apparently the last external morphological change to occur during the larval to juvenile transition and, as a result, is not completed until approximately one third (S. aquosus and P. dentatus) to one fourth (P. americanus) or about the same time (E. microstomus and T. maculatus) as the size at first reproduction. This character may have relevance to defining the end of the larval period and the beginning of the juvenile period in flatfishes and other fishes. In addition, the pattern of scale formation may be useful in enhancing understanding of systematics, functional morphology and habitat use.