Fish Can Recognize a Face Based on UV Pattern Alone

Two species of damselfish may look identical -- not to mention drab -- to the human eye. But that's because, in comparison to the fish, all of us are essentially colorblind. A new study published online on February 25th in Current Biology, reveals that the fish can easily tell one species from another based entirely on the shape of the ultraviolet (UV) patterns on their faces.

Although scientists have long known that some animals have UV vision, the new findings suggest that this sense can be keener and perhaps more useful as a "communication channel" than had been anticipated, according to the researchers.
"Researchers have been assuming for a long time that UV vision is not very good -- and that it is only useful for detecting the presence and absence of UV light, or objects in front of UV bright backgrounds," said Ulrike Siebeck of the University of Queensland in Australia. "The exciting thing is that we can show that these fish can tell the difference between intricate UV patterns -- something that was not expected based on previous assumptions."
In fact, researchers had some good reasons to doubt the precision of UV vision. The short wavelengths of light that characterize UV are prone to scattering in air and water. And even animals that can see in the UV range usually don't have all that many UV cones, or photoreceptors, in their eyes. But apparently nobody told that to the damselfish.
In the new experiments, Siebeck's team presented the very aggressive fish with two intruders, representing different species that vary in appearance only in their UV patterns. Those initial choice tests showed that the fish always attacked one species over the other. But, when the researchers took away the fishes' ability to see in UV, that preference between species disappeared.
The researchers next transferred the two species-specific UV patterns onto otherwise blank pieces of paper. They trained the fish to swim up to and nudge one of the patterns by offering food rewards. When the fish were later presented with both patterns, they still selected the pattern they had been trained on.
Put together, the two lines of evidence support the notion that the UV patterns are both necessary and sufficient for the fish to tell the two species apart.
The ability to see in this visual field is likely quite convenient for the fish, Siebeck said. "If you think about it in simple terms, fish have to be inconspicuous if they want to go undetected by their predators and prey, but at the same time, they have to be conspicuous if they want to attract the attention of potential mates, for example. Using UV patterns to do this is a clever way to maximize both at the same time -- they are still inconspicuous to predators but very conspicuous to other fish with UV vision."
The researchers say the new findings now call for more detailed investigation of UV vision in damselfish and other UV-sighted animals, to find out just how well animals can see in this range, and over what distances. The researchers are also testing whether fish can tell different individuals -- as opposed to whole species -- apart based on fine-scale variation in their UV facial patterns.
The researchers include Ulrike E. Siebeck, University of Queensland, Brisbane, Australia; Amira N. Parker, University of Queensland, Brisbane, Australia; Dennis Sprenger, University of Tubingen, Tubingen, Germany; Lydia M. Mathger, Woods Hole Oceanographic Institution, Woods Hole, MA; and Guy Wallis, University of Queensland, Brisbane, Australia.

Seeing Blue: Fish Vision Discovery Makes Waves In Evolutionary Biology

Small classes in early grades improve test scores in later grades for students of all achievement levels, but low achievers get an extra boost. That's the finding of a study on the long-term effects of class size in the November issue of the American Journal of Education.

The study suggests that reducing class size in early grades provides a dual benefit: It raises achievement for all students through middle school, while also closing the persistently large gap between high- and low-achievers, say authors Spyros Konstantopoulos from Michigan State University and Vicki Chung from Northwestern University.
According to the study, small classes—13 to 17 students—are most effective when they are consistent from kindergarten through third grade. Students in consistently small early classes had substantially higher test scores in grades four through eight than students who had been in larger classes. Students at all achievement levels benefited, but low achievers showed stronger benefits in reading and science.
Exposure to a small class in third grade alone also had some long-term benefits, the study found. But those benefits were smaller, and about equal for high and low achievers.
"One year of exposure in small classes is not enough [to influence the achievement gap]," Konstantopoulos said. "It appears that class size reduction is most effective for all students and low achievers in particular, if it is implemented early in elementary grades and for multiple years."
The study used data from Project STAR, a major longitudinal study of class size and its effects on more than 11,000 students. Previous research using the Project STAR data has found that small classes in early grades have positive long-term effects on average student achievement. But whether small classes could help close the achievement gap in later grades had remained an open question.
Since Project STAR provides no qualitative data on teacher practices, it's hard to say exactly why small classes benefit low achievers, the authors say.
"One hypothesis is that teachers in small classes are more likely to identify low achievers and hence are more likely to provide instruction designed to benefit these students," Konstantopoulos said. "Alternatively, in small classes there is a higher likelihood for low achievers to interact with teachers and be more engaged in learning."
Regardless of the mechanisms, the study suggests that initiatives to shrink class size, if widely implemented, could play a role in reducing the achievement gap. It also reinforces the findings of previous studies that show strong and lasting benefits of small classes.
"[I]t is remarkable that an intervention that is easily defined and implemented can have important lasting benefits at least to the end of middle school for all students," Konstantopoulos and Chung write.
Journal reference:
Konstantopoulos et al. What Are the Long-Term Effects of Small Classes on the Achievement Gap? Evidence from the Lasting Benefits Study. American Journal of Education, 2009; 116 (1): 125 DOI: 10.1086/605103
Adapted from materials provided by University of Chicago Press Journals, via EurekAlert!, a service of AAAS.