A modified version of an eye test used to assess visual acuity in the military has been given to archerfish by scientists to help explain how these remarkable fish are able to accurately spit down tiny insects high above the water’s surface. Dr. Shelby Temple, now at the University of Bristol in the UK, and his team at the University of Queensland and the University of Western Australia used a modified version of the Landolt C test to discover just how fine a detail the archerfish could resolve. The researchers first trained the fish to spit at one of two letters – an ‘O’ or a ‘C’ – by rewarding them with food. Then they showed them small versions of both letters together and recorded which letter they spat at. Dr. Temple said: "This modified Landolt C test works because the only difference between the two letters is the gap in the ‘C’ so in order to tell the difference and spit at the right target to get their reward the fish must be able to resolve the gap." To test the archerfish's resolving power, the size of the letters were decreased in steps to see just how small they could go. The scientists then compared these behavioural results to the fishes' predicted acuity based on measurements of the photoreceptor density in their retinas. The results, published online ahead of print in the journal Vision Research, show that archerfish are one of the most visually acute freshwater fish, able to resolve approximately 3.5 cycles per degree with the part of their retina that looks up and forwards, which is not surprising given their interesting foraging strategy. Archerfish have a special way of hunting for food that involves spitting jets of water at aerial insects above the water’s surface.
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