快猫短视频

Triggerfish have an impressive ability to estimate how far they have swum, and scientists think that understanding this may shed light on the how all animals with a backbone evolved the ability to navigate spaces.

There is growing evidence to suggest that ray-finned fish, known as teleosts and which comprise 96 per cent of all freshwater and marine fish species on the planet, have a region of the brain that works like our hippocampus does.

This part of the brain is critical to the way we navigate, and 鈥� if analogous in fish 鈥� could mean that spatial memory first evolved 400 million years ago when these fish, mammals and birds shared a common ancestor.

If this is true, teleosts would navigate as these other vertebrates do. But despite many years studying how invertebrates such as bees and ants estimate distances, scientists know very little about how ray-finned fish do.

To understand more, Cecilia Karlsson at the University of Oxford and her colleagues trained Picasso triggerfish (Rhinecanthus aculeatus) to swim a specific distance and then return home for food.

Read more: Fish recognise friends and foes through their unique faces

They trained five fish to swim 80 centimetres down a pipe with striped walls from a set starting spot to where there was an infrared motion detector. When the fish reached the motion detector, they triggered lights above the aquarium to switch on, at which point food was dropped into the aquarium back at the home area.

Once the fish had learned that swimming 80 centimetres to the motion detector turned on the aquarium lights and also led to the appearance of food, the researchers tried a different experiment. They moved the motion detector further away 鈥� 1.3 metres down the tube. The fish still swam 80 centimetres down the tube before turning back to claim their food reward, even though they hadn鈥檛 swum far enough to turn on the lights.

In other words, the fish weren鈥檛 simply swimming down the tube and responding to the lights going on as a signal to return home for food. Instead they realised they had to travel a specific distance to receive food, no matter whether the aquarium was lit or remained in darkness. On average, the fish always turned around after travelling 80.3 centimetres.

Studies on humans and rodents have found that they rely on tallying the number of steps taken to track distance either partially or fully. However, when the team analysed the fin beats of these triggerfish, they found that these were variable and couldn鈥檛 explain how the fish were able to estimate the distance so precisely.

Grid cells

The time taken for the fish to complete the task was also too variable for them to be using time to gauge distance. This led the researchers to conclude that ray-finned fish may have special neurons for spatial navigation, similar to the grid cells that mammals have and that fire at regular intervals while moving around. Karlsson and her colleagues published their findings online, but declined to discuss their findings with 快猫短视频 because the study is now under peer review.

Culum Brown at Macquarie University, Australia, says the paper is a breakthrough in studying how fish estimate the distance they travel: 鈥淚t鈥檚 not an easy problem to solve,鈥� he says.

This paper eliminates some possible explanations, such as fin beats and time, although there may be other techniques that haven鈥檛 been tested yet, he says. For instance, if the fish have some ability to count, they may be judging distance by noting how many stripes on the pipe walls they pass, he says.

鈥淕rid cells may well be a universal vertebrate tool which may have its evolutionary roots in fishes,鈥� says Brown. 鈥淗owever, a lot more work needs to be done in this area to pinpoint the exact mechanism.鈥�