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Blue Planet II

These are the world’s smartest fish

By Karl Gruber

Life in the ocean is hard, just ask any fish. If they could answer, they would tell you that they spend half their time escaping a bigger fish and the other half going after a smaller one.

But thanks to this constant struggle for survival fish have evolved some amazing cognitive abilities that easily make them some of the smartest animals around.

Fish parenting

The Ocean is a fish-eat-fish kind of world. But one species has learned not to eat their own kids, even when they are just a tiny embryo.

The mangrove rivulus or killifish (Kryptolebias marmoratus) is a well-known cannibal, it will eat its peer’s eggs whenever they get a chance.

But, unlike other cannibal fish, the killifish would never eat their own eggs. According to a new study this clever fish is able to recognise their kin even if they are still tiny embryos. This is the first time such an ability has been found between adults and embryos in a fish.

"These little mangrove fish have a remarkable ability to discriminate between their own offspring and those of a relative. They will take embryos into their mouths and spit them out if they turn out to be their own,” says Patricia Wright, at the University of Guelph, who led the new study.

“It takes a lot of sensory ability to "taste" a subtle difference like this - we think these are pretty smart fish," she adds.

How do they do it? According to Wright, chemicals released to the water by the eggs might do the trick.

“We don't really know- but the fact that the adult fish "tastes" the egg first suggests that a chemical signal may be involved," she adds.

Fish are no strangers to the use of chemicals for other purposes. Chemical cues, for example, help fish recognize their own species and even single individuals.

But some fish have moved a step further, relying in little more than their eyes.

Fish recognition

The East African cichlid fish Julidochromis transcriptus, a tiny fish no more than seven centimetres long, is able to recognize unfamiliar individuals just by looking at their eyes.

This stripped little fish lives hidden among rocks in Lake Tanganyika, one of the world oldest and largest freshwater lakes. According to a recent study when another fish comes around, a simple look at the patterns around the eyes of the newcomer reveals if it is a friend or a stranger. Similar results have been found for another species living in this lake. The cichlid fish Neolamprologus pulcher uses face colour patterns to identify different individuals.

Another fish able to identify individuals by their faces is the Japanese rice fish (Oryzias latipes). A recent study showed that this little fish has evolved a complex way to deal with faces, similar to the way human process face patterns.

Humans and primates can easily identify any objects, even if they are upside down, but when it comes to faces, things get more complicated.

“The neural pathway used for discriminating faces is different from other objects in mammals, and when faces are upside-down, our brain considers them as non-face objects and we cannot discriminate them as fast as right-up faces,” says Mu-Yun Wang at the University of Tokyo, Japan.

And it seems like the brain of the Japanese rice fish works this way too.

“Medaka fish also delays face recognition when the faces are upside-down, and it is possible that they also have specific brain region for processing faces, just like us humans,” Wang says.

“As research efforts continue, we are finding out more and more about the cognitive abilities of fish, and learning that there are many cases where the abilities of fish rival other vertebrates,” says Alex Jordan at the Max Planck Institute Department of Collective Behaviour in Konstanz, Germany.

“The long-held idea of a three-second memory for fish will slowly recede under the weight of evidence from studies like these as time goes on,” he adds.

But face recognition is just one of the many skills fish have.

Social behaviour

Scientists think social behaviour may be an important factor influencing the development of advanced cognitive abilities, Jordan explains. “The highly social species of cichlids in Lake Tanganyika appear to have developed higher cognitive capacities than non-social species,” he says.

Jordan is currently examining how the frequency and diversity of behaviours displayed by different species are reflected in their ability to recognise others, to resolve conflicts, and even the effects this has had on the architecture of their brains.

For example, several fish species have learned that working in a large group is a good way to fence off predators. Several species of damselfish, the Slender Hardyhead (Dascyllus marginatus) or the sea goldie (Pseudanthias squamipinnis) all form school of fish ranging from just a dozen individuals to millions of fish all moving in unison. Like in this video, shot by Simon Gingins, a postdoctoral Fellow in the lab of Iain Couzin, also at the Max Planck Institute in Konstanz.

In these massive gatherings fish become a kind of superorganism, one that can respond to threats from a predator or find prey more efficiently.

Another good example of advanced social behaviour occurs in French grunts, says Culum Brown, at Macquarie University in Australia.

“They hide during the day in small groups among echinoderms and as the sun goes down the make a small migration to their preferred feeding patch.  The path they take there and back is passed through the group members via social learning,” he says.

“In many ways, it’s much like kids learning the route to the grocery store by repeatedly travelling the path with their parents,” he adds.

Tools of the trade

The use of tools to solve everyday problems is considered an advanced behaviour in animals, and it is well-known for groups like primates and birds. But fish are no strangers to tools.

The California sheephead (Semicossyphus pulcher), for example, use rocks as makeshift anvils to crush open preys like urchins or clams. Quite a feat, considering fish have no limbs to help them around, says Robert Dunn, doctoral researcher at San Diego State University & University of California, Davis.

Other species, like the blackspot tuskfish (Choerodon schoenleinii) have also been seen cracking open clams with the help of rocks.

“I was really amazed when I first saw these fish smashing urchins on rocks. I had deployed underwater cameras for a separate experiment, and observing this behaviour was a real surprise,” Dunn says.

A male California sheephead uses rocks to open sea urchins on the La Jolla kelp forest, in Southern California, USA (Credit: Robert P. Dunn)

These are just a few examples of some of the amazing feats fish can do, says Brown.

“Fish also exhibit advanced degrees of cooperation with one another, even between different species. They also have insanely good spatial learning and migration capabilities. There is good evidence of fish forming cognitive maps and taking short-cuts or having spatial learning in three dimensions. They can even count and use the same system as we do for tracking and comparing sets of objects, Brown says.

Teamwork

In an incredible feat of strength and problem solving, the saddleback clownfish (Amphiprion polymnus) has been filmed moving objects across the sea floor for the very first time by the Blue Planet II crew.

Like many species of clownfish, the saddleback lays its eggs on a hard object and while other clownfish find suitable locations (such as shells) on the sea beds or reefs where they live, things aren’t quite so simple for the saddleback.

Saddleback make their home among the venomous tentacles of the ‘carpet anemone’ (Stichodactyla gigantean) sheltering them from predators. However, it also means these clownfish must live out their lives in the open sand; away from the reef and its readily-avaliable egg-laying surfaces.

With no hard surfaces close by, the saddleback must think outside the reef. Using all their strength, and a bit of teamwork, they are able to push objects up to 10 times their own weight up to a distance of two metres.

Featured image by Jason Isley.

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By Karl Gruber