Scientists Give Archerfish An Eye Exam
Brett Smith for redOrbit.com — Your Universe Online
One of the world´s most unique hunters, archerfish spit a stream of water at their small insect prey — knocking it down into the water where the fish snaps it up. Since the water acts as a barrier for the fish´s senses of smell and sound, the researchers were looking to see just how well the fish could spot prey that is often flying erratically just inches above the water.
To check the fish´s visual acuity, the Commonwealth team settled on using a variation of the Landolt C test, which is typically used to asses visual acuity in the military. The test requires subjects to discern between an ℠O´ and a ℠C´.
“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,” said Shelby Temple, co-author of a report on tests that was recently published in the journal Vision Research.
The letters were presented to the fish in decreasing sizes to see just how small of a gap the fish could discern. The scientists also took measurements of the photoreceptor density in the fishes´ retinas and calculated their visual acuity based on these measurements.
The results of the experiments show that archerfish have one of the highest visual acuities for freshwater fish, at 3.5 cycles per degree. By comparison, zebrafish have a visual acuity of 1 to 2 cycles per degree, while humans and other primates have a visual acuity of about 60 cycles per degree.
“This huge range across animals is mainly because of the optical clarity of the media in which they see and the distances to their targets,” Temple said. “While birds and land animals see through air and look for objects at distances of tens to hundreds of meters, fish and other aquatic animals have to see through water which at best gives only tens of meters of clarity,” said Temple.
“In the case of many freshwater fishes visibility of their water may be less than a few centimeters,” he said. “Accordingly, archerfish have lower acuity in the part of the eye that looks down into the murky water and the highest acuity in the part of the eye that looks up and forwards.”
Temple´s previous research found archerfish see colors differently in different parts of their eyes. This pigment segmentation has a specific functionality as visual pigments in the upper part of the eye, which sees light from below, are tuned to the murky brownish waters where they fish live. The lower, forward-looking part of the archerfish´s eye has tri-chromatic color vision, which is similar to human vision.
In combination with their higher acuity that appears to be in this same part of the eye, archerfish are clearly equipped for sharpshooting prey out of the sky.