Morphometry of the stone loach, Barbatula barbatula: do mensural characters reflect the species' life history thresholds?

Growth variability in 23 mensural characters was examined in 387 specimens of stone loach, Barbatula barbatula, from England. The standard length (SL) of the specimens ranged from 15.3 to 115.4 mm. We tested the hypothesis th at body proportions change abruptly, rather than gradually, at certain inte rvals of ontogeny by fitting linear, quadratic and split linear curves to p lots of each variable against SL. Based on patterns of allometric growth, t wo groups and two subgroups of mensural characters have been found. Three c haracters were best explained by a linear regression, indicating isometric growth. Eight characters were best explained by a quadratic curve, indicati ng gradual allometry. The remaining 12 characters were best explained by a split regression, indicating mainly isometric growth with abrupt allometry occurring at a specific SL (breakpoint). The first shift in morphometric va lues (a transformation of the head; breakpoints in three characters) occurr ed at 26-35 mm SL, the second (a change in fin shape and size as well as bo dy form; breakpoints in six characters) at 36-47 mm SL. The coincidence of shifts in body morphology with those in microhabitat use (between the respe ctive size classes) suggests that thresholds (though not as sudden as those between embryo and early larva steps) do occur during this interval of sto ne loach life history. We suggest that the larva period ends with the compl etion of the first shift in relative growth (i.e. not later than at 35 mm S L, depending on individual variability), and that the second shift in morph ometric values reflects a threshold between the first and the second step o f juvenile period. The importance of changes in external morphology decreas ed as the fish grew bigger and older.