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Goldfish use visual cues to judge how far they have traveled

The manner in which mammals obtain information about their position in space by creating an internal map of the environment is relatively well understood. Research has shown that the same system is used by birds and reptiles, but the neural circuits that enable spatial navigation in fish have not yet been investigated. There are around 30,000 species of bony fish, found inhabiting various ecological niches worldwide, and understanding the cues and neural mechanisms they use to move around in space would allow scientists to understand the evolutionary origins of spatial navigation in vertebrates. 

To investigate whether fish have similar spatial navigation systems to terrestrial species, researchers from the University of Oxford started by testing whether goldfish (Carassius auratus) can accurately estimate distance traveled – an ability that is central to successful spatial mapping.

The study, led by Dr. Adelaide Sibeaux from the Department of Biology, University of Oxford, involved training nine goldfish to swim a distance of 70 cm within a narrow tank that had a pattern of 2 cm vertical stripes along the sides. They swam the distance along the tank and then a trainer signaled (by waving) to the fish and they turned around and swam back to the start, where they were rewarded with food. Once the fish had learned this skill, the trainer no longer signaled the turning point but left the decision about when to turn around up to the fish. The researchers assumed that a fish would turn back once it estimated it had swum the target distance. 

The results, published in the Proceedings of the Royal Society B Biological Sciences, showed that after 405 trials, the average distance the goldfish swam before turning around of their own accord was 74 cm. The researchers also found that the fish continued to swim the approximately 70 cm distance even when their start position was shifted 20 or 40 cm forwards along the length of the tank. This provided robust evidence that goldfish are pretty good at judging distance.

In order to understand the method used by the goldfish to assess distance travelled, the researchers experimented with different surround patterns fixed to the outside of the tank. When the pattern was changed from vertical stripes to black and white checks, also of 2 cm width, the fish continued to judge the target distance of 70 cm accurately. The researchers deduced that these two patterns conveyed the same spatial information to the fish and this enabled them to make accurate judgements of distance.

However, when the background was altered to a pattern of vertical stripes every 1 cm (doubling the frequency of spatial information), the goldfish overestimated the distance they travelled by 36 percent. This meant they turned before reaching the target distance (47.5 cm on average). Similarly, the fish turned after a shorter distance (65 cm on average) when the surround pattern was one of horizontal stripes. The modification of the visual information displayed in the background significantly affected the distance travelled by the goldfish, providing evidence for the use of optic flow as a cue for distance estimation. The researchers deduce that goldfish estimate distances by visually streaming the apparent motion patterns of objects in the environment (called ‘optic flow’) as they swim past.

According to the researchers it is possible that the goldfish used number of tail beats to give a clue about distance travelled, just as humans use strides. The number of tail fin beats was correlated with distance travelled in general, although the fish swam more slowly when confronted with some of the patterns. This approach would also have to assume that goldfish are pretty good at counting. 

Another alternative explanation was that goldfish assess the time elapsed and use that to assess whether they have achieved the target distance. The results, however, showed that time taken to swim 70 cm showed twice as much variability as was recorded for the distance judgements. The fish do not always swim at the same speed, at times travelling more slowly. This would make it unlikely that the goldfish used time elapsed to guide them in making a distance judgement. 

Many terrestrial species are known to use optic flow to estimate distance, but goldfish appear to process the information differently. Animals, including humans, ants, wolf-spiders, and honey bees, estimate distances by measuring how the angle between their eye and surrounding objects changes as they travel. Goldfish, on the other hand, appear to use the number of contrast changes experienced as they move along.  The only other research on fish navigation has also shown that Picasso triggerfish (Rhinecanthus aculeatus) are able to estimate travelled distance accurately using local optic flow as a form of odometer.

“We present robust evidence that goldfish can accurately estimate distance and show that they use optic flow to do so,” said Dr. Sibeaux. “These results provide a compelling basis to use goldfish as a model system to investigate the evolution of the mechanisms that underpin spatial cognition in vertebrates.”

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By Alison Bosman, Staff Writer

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