Eye use during viewing a reflection: behavioural lateralisation in zebrafish larvae.

Development of visual lateralisation was studied in zebrafish larvae of an outbred strain when examining their own reflection in a mirror. There was significant left eye preference at all ages studied. A decrease in left eye use around 14 days, followed by a later increase at 21 days, parallels similar but differently timed shifts in the domestic chick. Age-dependent shifts in the likelihood of control by one or other eye system may be responsible. Larvae tested at 26 days of age with unfamiliar conspecifics of similar age also used the left eye (LE). Larvae of another strain (TupLF) also LE viewing of their reflection, although this was confined to the first or first and second minutes of the test rather than lasting through 10 min. The reflection differs from prior experience of fellows in the timing and character of its movements relative to those of the larva. Specialisation of the left eye system for the assessment of novelty, which is present in zebrafish, just as in many other vertebrates, probably therefore explains left eye use in the mirror test. Reasons why strains might differ in the length of time that they devote to assessment of the reflection are discussed.