Saccular otolith mass asymmetry in adult flatfishes

A dimensionless measure of otolith mass asymmetry, w, was calculated as the difference between the masses of the right and left paired otoliths divided by average otolith mass. Saccular otolith mass asymmetry was studied in eight flatfish species (110 otolith pairs) and compared with data from a previously published study on roundfishes. As in the case of symmetrical fishes, the absolute value of w in flatfishes does not depend on fish length and otolith growth rate, although otolith mass and the absolute value of otolith mass difference are correlated with fish length. The values of w were between 02 and þ02 in 964 % of flatfishes studied. The mean S.E. value of w in flatfishes was significantly larger than in standard bilaterally symmetrical marine fishes (‘roundfishes’), respectively 0070 0006 and 0040 0006. The most prominent distinction is the existence of downside prevalence of saccular otolith mass in flatfishes, which contrasts with no right–left prevalence in roundfishes found in a previous study. In the right-eyed flatfishes (Soleidae), the left saccular otoliths are heavier than the right otoliths. In the left-eyed flatfishes (Bothidae and Citharidae), the right saccular otoliths are heavier than the left otoliths. Not all flatfishes, however, fit in this design: 118 % of flatfishes studied had the heavier saccular otoliths in the upside labyrinth and 54 % of flatfishes had no otolith mass asymmetr