Huge waves suddenly rose on the calm water surface. Is the "water monkey" making trouble again?

Above the quiet lake, a bird is wandering in the air. Here it is an absolute hunter, ready to enjoy its lunch in the water.

"Which fish should I eat?" It suddenly rushed to the surface of the water, aiming at the underwater prey, but before it touched the water, layer after layer of waves rose from the water, almost swallowing it. It had no choice but to abandon its armor and flee, bidding farewell to its lunch.

Why are there waves under the calm water? Is it the legendary "water monkey" causing trouble? In fact, these waves are the masterpiece of the weak fish school, the prey's counterattack against fate.

Waves of fish follow waves of fish

Under the birds' sharp beaks and powerful claws, the schools of small fish under the water are like an open buffet. This competition for survival is unfair from the beginning, but it does not mean that the small fish can only sit and wait for death - tens of thousands of delicious small fish unite together, they can stir up waves like an invisible big hand and reverse the situation.

The members of the school are Poecilia sulphuraria, a small omnivorous freshwater fish less than 5 cm long that lives in sulfur springs in southern Mexico and is currently listed as critically endangered by the IUCN.

A wild flower killifish that looks like it's good for biting | iNaturalist NZ

Wild flower killifish are famous for their love of "team building" and making waves, but people have never known why they do this. By squatting by the river, researchers finally solved the mystery - it turns out that whenever birds that prey on wild flower killifish appear above the water, hundreds of thousands of fish will work together to stir up huge waves. At this time, there are up to 4,000 fish per cubic meter of water, and each wave they create can last 3 to 4 seconds, wave after wave, and the waves raised by the school of fish will repeat for up to 2 minutes.

Can such a "fish wave" drive away predators?

The wild flower killifish has two natural enemies, the kingfisher and the flycatcher. The two predators have different characteristics. The kingfisher will jump into the water to catch fish, causing a large wave of fish, while the flycatcher will use their beaks to skim the water surface and kill small fish.

The living environment of the wild flower killifish. On the left are two of its bird predators: the kingfisher (top) and the flycatcher (bottom); on the right are a school of wild flower killifish (top) and their activities when making waves (bottom) | References [1]

The researchers found that once the fish created a wave, the kingfisher and the flycatcher hunted less frequently, and they waited twice as long between prey. The fish-induced wave obviously posed a problem for them: the kingfisher and the flycatcher had to switch the starting point of the attack according to the direction of the fish wave, and find the right dive angle, which greatly reduced their success rate.

The waves created by schools of wild flower killifish not only reduce the possibility of being eaten by natural enemies, but are also beneficial to birds: the waves are warning the birds "we have discovered you". Birds learn to decisively stop and wait quietly for the next opportunity. This is also a good way for them to save energy.

Fish survival tips

Group action is not only the survival secret of wild flower killifish, but also the most common way of survival for fish.

More than 50% of fish live in groups when they are young, and more than 25% of fish choose to live in large groups when they grow up. Migrating schools of adult sardines can even reach 7 kilometers long, 1.5 kilometers wide, and 30 meters deep.

When faced with a predator, sardines form a swirling "bait ball" to scare off the predator | Ed Bierman / Flickr

But wouldn't it be easier for predators to spot thousands of fish gathering together? In fact, the optical properties of the ocean environment are very different from those on land. Light is absorbed and scattered by substances in the seawater, resulting in an extremely short visible distance in the sea. Therefore, whether it is a single fish or a large group of fish, the shortest distance for predators to spot them is the same.

A large group of fish swimming together reduces the chances of individual prey. The visual stimulation caused by a large number of moving targets makes it difficult for predators to allocate their attention to individual prey. This is called the "confusion effect" - in simple terms, there are too many foods in front of them, and predators are "dazzled".

The study found that when largemouth bass (Micropterus salmoides) hunt Mississippi jawfish (Hybognathus nuchalis), if there is only a single prey, the largemouth bass will catch it quickly. As the size of the jawfish school increases, the frequency and success rate of largemouth bass hunting will gradually decrease.

Forget about the predator, you're dizzy too | Brocken Inaglory / Wikimedia Commons

The warning effect of a school of fish is also much better than that of a single fish. This is called the "many-eyes effect". There is strength in numbers. When more fish act together, more pairs of eyes pay attention to the surrounding environment, and predators will be discovered more quickly. When the warning task is distributed to each fish, the time an individual "stands guard" can be reduced, which means more time to eat.

So many eyes watching! | James Watt / Wikimedia Commons

In addition to fighting off natural enemies, group living also helps fish find food and reproduce.

Studies on carp have shown that whether it is goldfish or minnows, when the size of the school increases from 2 to 20, the speed at which the school finds food will be greatly accelerated. Some fish need to migrate to a fixed location to mate or lay eggs, and migrating in groups can help them find the right route faster. Moreover, mating in groups is a "large-scale blind date", otherwise it is difficult to find the right opposite sex in the vast ocean.

After successful mating, the females lay eggs together, and the eggs they lay gather together. The hatched offspring can form a group again and start a new life cycle.

Group: 1+1>2

The amazing thing is that a school of fish is not simply a collection of fish, but an organism in itself, in which each individual fish is like a cell.

When the predator approaches, tens of thousands of wild killifish suddenly rise up as a whole, stirring the pond. As if directed by an imperceptible signal, they suddenly move forward and turn back in the blink of an eye, as if they themselves are a giant hand, rising and falling.

Fish schools have different forms in different states: (1) moving; (2) resting; (3) eating; (4, 5) surrounded by many predators; (6) alerting the surroundings; (7, 8) avoiding a single predator | References [3]

This massive movement is so spectacular that one can't help but wonder how they can do it so uniformly. Kevin Kelly wrote about the scene of animal group movement in Out of Control:

The movement of a single bird or a single fish, however fluent, cannot give us the same shock as the dense procession of a million mullets swimming in a row… Thousands and thousands of fish swim like a giant beast, breaking through the waves. They are like a whole, bound by an irresistible common destiny. Where does this unity come from?

The reaction speed of a school of fish is even faster than that of a single fish. This high-speed reaction is the accumulation of individual decisions. The fish in the school that first sees a bird or is attacked by a bird will be the first to dive and escape. Most fish have a pair of neurons called Mauthner cells, which allow them to make an escape reflex within a few milliseconds when they encounter external stimuli - quickly turning, wagging their tails, and running away.

If attacked, the killifish will almost instantly dive, creating a wave on the water surface | Juliane Lukas

The dive response is a signal that a predator is approaching. When the fish around it see their neighbor flee, they will immediately make a fleeing reflex, and the fish further out will follow closely. Like a falling domino, the fish dive vertically downward one after another, naturally creating waves on the water surface - deterring birds is actually a side effect.

Bird flocks have a similar mechanism.

Starlings always fly in groups. When they encounter predators, they turn and dive together. The team changes like waves to avoid the attacks of predators. However, this is not an organized and disciplined collective activity. Each starling in the group only pays attention to the actions of the seven birds around it. What they do is very simple - follow their neighbors and keep their speed the same. Each bird adheres to this simple principle, but the signal changes with the movement of each bird and can be spread quickly throughout the flock. It takes less than 0.5 seconds for the turn signal to spread to a flock of 400 starlings.

Starling flocks change like waves | odysea.gr

In addition to the common schools of fish and birds, there is another type of cluster on Earth that is closer to an "organic whole" - the tube jellyfish.

The siphonophore looks like a spectacular giant chain, but if you look closely you'll find that it's made up of countless individuals, like Lego bricks, each of which is called an "individual."

Each member is an independent individual, but has a clear division of labor. Some are responsible for eating, some are responsible for hunting, and some are responsible for swimming, just like the large and small cells on a giant. The individual also has excitable epithelial cells that can use electrical signals to quickly transmit information and coordinate work.

In Australia, researchers have even discovered giant tube jellyfish that are over 100 meters long and are a huge entity composed of tens of thousands of individuals.

A siphonophore eating a fish. It looks like an individual, but it is actually a colony of countless individuals | C. Anela Choy, Steven HD Haddock and Bruce H. Robison

The transparent members of the siphonophore Marrus orthocanna can contract, allowing the siphonophore to move. | Kevin Raskoff

A group is not a simple aggregation of individuals. Even if each member of the group takes the simplest actions, the complexity and flexibility that the collective can ultimately achieve is far beyond the imagination of the individual.

Perhaps the charm of nature is that it allows seemingly weak species to evolve the wisdom to survive in groups, facing environmental changes as a larger whole, thereby giving each individual an advantage in survival.

References

[1]https://www.cell.com/current-biology/fulltext/S0960-9822(21)01654-7

[2]https://www.scientificamerican.com/article/fish-do-the-wave-to-ward-off-predatory-birds/

[3]https://www.researchgate.net/publication/264977013_Patterns_and_mechanisms_of_schooling_behavior_in_fish_A_review

[4]https://asa.scitation.org/doi/full/10.1121/10.0007485

[5]https://www.jstor.org/stable/23736019

[6]https://article.xuexi.cn/articles/index.html?art_id=7622498619809851921&study_style_id=feeds_default&source=share&share_to=copylink

[7]https://www.sciencedirect.com/science/article/abs/pii/S0003347286802081

[8]https://link.springer.com/article/10.1007/BF00300175

[9]https://www.nature.com/articles/s42003-021-02407-4

[10]https://rss.onlinelibrary.wiley.com/doi/full/10.1111/j.1740-9713.2008.00288.x

[11]https://www.nature.com/articles/nphys3035

[12]https://en.wikipedia.org/wiki/Shoaling_and_schooling

[13]https://www.sciencedirect.com/science/article/pii/0301008280900222

Author: Cat Tun

Edit: Flip

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