The dorsal light reactions of larval and metamorphosing flatfish

Before metamorphosis, plaice and turbot show dorsal light reactions generally similar to those of other species, suggesting that their balance mechanisms are also similar, and that their utriculi are functional. The reactions are unusual in one respect in that larvae of both species tilt away from light incident from below the horizontal. In turbot, during metamorphosis, the null position (that taken up when illumination is from directly above) becomes gradually inclined to the right until the larva is horizontal at the end of metamorphosis. There is evidence that the null position of the static system is reset to an inclined position during this change. In plaice, during metamorphosis, the shift in null position is smaller, in the opposite direction, and at the end of metamorphosis there is a rapid change to a horizontal position. The null position of the static system does not appear to be reset. In plaice the influence of light on balance increases during metamorphosis, possibly due to it affecting the brain or the otolith organs. This is not seen in turbot, but light has a strong influence on the balance of this species both before and during metamorphosis. At the end of metamorphosis both plaice and turbot appear to utilize only static information from the sacculi and lagenae. Light could be seen to influence balance in swimming turbot, but not in plaice.