How do fish avoid danger? What are some ways to defend against enemies?

Produced by: Science Popularization China

Author: Life Pulse Team

Producer: China Science Expo

What should the fish in the water do when encountering danger?

Please use your imagination to fill in the "Don't come over here" emoticon.

Indeed, being a fish is so difficult!

(Photo source: Veer Gallery)

There are various physical and chemical factors that affect the survival of fish in their living environment, and there are different plants and animals. In order to survive and reproduce, fish not only need to have the ability to hunt for food, but also evolve many mechanisms to resist adverse physical and chemical factors and the invasion of other individuals.

So, how do fish defend themselves against external threats?

What are defensive behaviors?

Almost all defense mechanisms have behavioral components. The object of defense is often an individual animal of the same or different species, such as male individuals resisting attacks from other male individuals during courtship, prey resisting attacks from predators, using warning coloration to scare off predators, etc. Defense mechanisms against predators are divided into intrinsic defense mechanisms and secondary defense mechanisms.

Primary defense mechanisms refer to behaviors that prevent predators from getting close to them, including hiding, hiding, and displaying warning colors. Secondary defense mechanisms refer to behaviors that allow animals to escape after encountering an enemy, including retreating, escaping, intimidating, feigning death, and distracting the attacker.

(Photo source: Veer Gallery)

Displaying warning colors, cryptic body colors, color changes, and fleeing are purely defensive behaviors, purely for the purpose of protecting oneself. However, many defensive behaviors are expressed in the same way as offensive behaviors, such as the strong jaws of catfish, the long beaks of needlefish, the sharp upper jaw swords of swordfish, and the powerful electric currents of electric rays, etc. The methods and places of use can be the same or different.

In other words, if you really can't avoid me, I'm going to bite you!

Fish's natural defense

There are three forms of inherent defensive behavior in fish, including:

**Hidden life: **Live in caves or crevices, such as the bearded catfish hiding in the dark places and caves at the bottom of the water body during the day; the yellow eel likes to hide in caves at the bottom of the turbid muddy water during the day; the anglerfish hides in the mud and sand on the seabed; the stargazer buries its body deep in the sand, only its small and active eyes are exposed. From above, the mouth crack looks like a crack on the sand surface. When it is still in the sand, it is difficult to find its existence; the damselfish and the double sawfish often live between the tentacles and in the body cavity of the sea anemone, the young silver pomfret can hide well under the jellyfish, and the submerged fish parasitizes in the body of the starfish. These small fish use the tentacles of the coelenterates to drive away the fierce fish to achieve the effect of protecting themselves. The remora relies on the large suction cup on the back of the head to attach to the belly of sharks, whales, turtles and ships to avoid attacks by enemies.

(Photo source: Veer Gallery)

Concealment: In the natural environment, most fish have a body color that is consistent with the surrounding environment to conceal themselves or confuse enemies or prey. Some fish, such as flounder, change their body color so that no matter what environment the fish is in, its body color always remains consistent with the color of the environment. This species with multiple concealment properties (also known as cryptic polymorphism) can effectively protect itself.

Display of warning coloration: Dangerous or unpalatable species often have bright, eye-catching colors that are very inconsistent with the environment, which is used to warn predators and remind them not to disturb them. For example, the beautiful colors of pufferfish are a manifestation of warning coloration.

(Photo source: Veer Gallery)

Secondary defenses in fish

Secondary defense behaviors mainly manifest in five forms, including:

Retreat: Retreat is the most common and basic secondary defense behavior. When any fish encounters danger, the first reaction is to retreat. Some small fish swim into the crevices of rocks, and some enter the water grass. Seahorses and pipefish stay in the water grass. To avoid enemies, pufferfish often hide among eel grass and seaweed, with their heads facing down, their mouths pressed against the bottom, and their fins moving in the water. Their slender bodies, waving fins and green stripes make predators mistake them for seaweed or eel grass leaves. Although retreat is an effective defense method, it often affects other activities of the fish, such as feeding, and it is impossible to know whether the predator has left and the danger has been eliminated.

(Photo source: Veer Gallery)

Escape: includes fast swimming and jumping out of the water. When encountering an enemy, flying fish will swim quickly in groups, using the speed and momentum of swimming to glide out of the water. When encountering danger, fish such as silver carp will jump out of the water to escape.

Intimidation: Individuals that are slow to flee or in danger of being caught sometimes take intimidating postures or actions. For example, when a porcupinefish encounters danger, in addition to swallowing air to swell its body, all the spines on its body surface stand up, forming a large ball of spines to scare off the enemy.

Feigning death: Many predators only attack active prey, so motionless prey often escapes. For example, pufferfish feign death and then suddenly escape after a while.

Butterfly fish

(Photo source: Veer Gallery)

Distract the attacker's attention: Some fish have a large round eye spot on the upper back of the body, which makes the enemy mistake the tail for the head, causing the attacker to misjudge the direction of attack. Some butterfly fish have a black band on the head that runs through the eyes, and a large eye spot on the back of the body. The real eyes are often difficult to distinguish, but the clear and obvious large round spot looks very much like an eye. This kind of fish often moves with its tail first, swimming slowly backwards for a short distance. When frightened, it moves its head first and swims quickly forward. The enemy often mistakes the tail for the head, thus misjudging the direction of the butterfly fish's movement.

Conclusion

The defensive behaviors introduced above all refer to the actions taken by individual fish. Defensive behaviors can also be social behaviors. Living in groups is also a form of defense. Fish often gather in large groups. When encountering predators, although some of the fish in the school are preyed, the total number of individuals preyed in this way is less than the sum of the number of individuals preyed in a dispersed state. Moreover, it is not easy for predators to lock on and chase a target in a large school of fish. In addition, in a school of fish living in a group, if any individual finds danger and gives an alarm signal, all members can flee together.

When fish perform defensive behaviors, they often affect other behaviors. Effective defense is often carried out in coordination with attack, which is an adaptive behavior in the evolution of species. In the process of species evolution, only individuals that can effectively attack and defend can survive and reproduce.

Editor: Sun Chenyu