Fish that spit

When Swiss-born ophthalmologist Dr. Edmund Landolt proposed a new type of symbol for testing visual acuity in 1888, he probably would never have dreamed that one day it would be used to help explain how fish are able to feed on insects.

But that is exactly what Dr. Shelby Temple at Bristol University (Bristol, UK), and a team at the University of Queensland and the University of Western Australia have done. They have modified Dr. Landolt's so-called "C test" to discover the resolving power of the eyes of a family of fish known as archerfish.

The archerfish themselves are rather unusual creatures. They have a special way of hunting for food that involves spitting jets of water at tiny aerial insects high above the water's surface. Because sound and smell do not cross the air-water interface, these fish must depend on their visual capabilities to find, identify and accurately spit at their prey.

To discover how visually acute such fish are, the researchers first trained them 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.

"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," says Dr. Temple.



To test the archerfish's resolving power, the size of the letters were decreased in steps. The scientists then compared the behavioral results from their experiments to the fishes' predicted acuity based on measurements of the photoreceptor density in their retinas.

The results, published in the journal Vision Research, show that archerfish are one of the most visually acute freshwater fish. They are able to resolve between 3.23 and 3.57 cycles per degree (0.155-0.140° of visual arc) with the part of their retina that looks up and forwards, which is not surprising given their interesting foraging strategy.

If Dr. Landolt were alive today, I'm sure he'd undoubtedly be amused to learn how his test had been repurposed, and impressed to see how effective it was at helping biological scientists such as Dr. Temple determine the visual acuity of animals other than human beings.

More information on the research is available on the Bristol University web site here. The researchers' technical paper "A comparison of behavioural (Landolt C) and anatomical estimates of visual acuity in archerfish (Toxotes chatareus)" is available here.