A comparison of behavioural (Landolt C) and anatomical estimates of visual acuity in archerfish (Toxotes chatareus)

Archerfish forage by shooting jets of water at insects above the water’s surface. The challenge of detecting
small prey items against a complex background suggests that they have good visual acuity, but to date
this has never been tested, despite archerfish becoming an increasingly important model species for vertebrate
vision. We used a modified Landolt C test to measure visual acuity behaviourally, and compared
the results to their predicted minimum separable angle based on both photoreceptor and ganglion cell
spacing in the retina. Both measures yielded similar estimates of visual acuity; between 3.23 and
3.57 cycles per degree (0.155–0.140 of visual arc). Such a close match between behavioural and anatomical
estimates of visual acuity in fishes is unusual and may be due to our use of an ecologically relevant
task that measured the resolving power of the part of the retina that has the highest photoreceptor density
and that is used in aligning their spitting angle with potential targets.