Do fish drink water? Do fish pee? Do fish have a memory of only 7 seconds? Even more interesting is...

Expert of this article: Lin Zhideng, PhD in Zoology, East China Normal University

There are many types of fish, each with its own characteristics.

However, when it comes to fish, many people have a common impression - "fish's memory is only 7 seconds."

In real life, people often use the phrase "fish memory" to describe a person with a bad memory. But is this really the case?

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In addition, some friends may be curious about fish: Fish live in water, do they drink water? Do they pee? How can we keep fish alive longer? Read on to find the answers.

Fish's memory is only 7 seconds?

It is widely believed that fish have a memory of only 7 seconds, but in fact, the memory of fish is much longer than 7 seconds.

In the 1960s, American scientists conducted a study on fish memory using goldfish as the subject. The researchers emitted a beam of light from one end of the fish tank to the other, and after 20 seconds, released an electric shock at the other end of the light source, and repeated this operation many times. Later, they found that the goldfish formed a memory of the electric shock, and when they saw the light again, they would quickly swim to the end where the light source was released to avoid the electric shock, and the goldfish could remember this "light-avoidance" behavior pattern for up to a month.

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For another ornamental fish, the paradise fish, researchers have also found that they have super memory. They can remember "strange neighbors" they have seen before, and this memory can last at least several months.

In addition, many scholars have used zebrafish as a model animal to further prove that fish have long-term memory. Researchers have found that after training, zebrafish can learn to distinguish feeding time based on the color of signal lights and learn how to walk through a maze.

In fact, we can also see in real life that fish have a long memory. When tourists approach the fish pond, the ornamental fish in the scenic area will actively swim towards the tourists to try to get food. In addition, a study published in the journal Nature Communications in recent years found that zebrafish can also pass on memory to their offspring through DNA methylation (epigenetics).

Do fish drink water? Do they pee?

According to their living environment, fish can be divided into freshwater fish and saltwater fish. They all need to absorb enough oxygen from the water to meet their own metabolic needs.

Fish mainly complete the breathing process through the coordinated action of the mouth, oropharynx and gill cover. During this process, a large amount of water will enter the fish body from the mouth. This process can be simply defined as the process of fish "drinking water". Due to the great difference in salinity of the water environment in which freshwater fish and marine fish live, in order to maintain the stability of their own osmotic pressure, they have formed different ways of "drinking water".

For freshwater fish, the salt concentration in their bodies is higher than the fresh water outside, so a large amount of fresh water will continuously penetrate into the fish's body. It can be said that freshwater fish are "fed" with water. In order to maintain the stability of their own osmotic pressure, freshwater fish need to excrete a large amount of water that has penetrated into their bodies. Their kidneys continuously excrete excess water and absorb salt at the same time, and finally excrete it through the bladder and cloaca.

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In contrast, marine fish live in seawater, and the salt content in their bodies is lower than that in the seawater outside, which causes the water in their bodies to be constantly lost. In order to maintain the stability of osmotic pressure, marine fish must actively "drink water" to replenish their own reduced water. Marine fish also need to excrete excess salt and metabolic waste from their bodies, and they also need to urinate, but the amount of urine is much lower than that of freshwater fish.

Generally speaking, both marine and freshwater fish "drink water", but freshwater fish "drink water" passively, while marine fish "drink water" actively. In addition, during the evolution process, some fish have developed special ways to maintain the stability of their own osmotic pressure. For example, sharks can retain urea to make the osmotic pressure in their bodies slightly higher than that of seawater. The metabolic waste in sharks is mainly discharged through the skin. Sharks do not have bladders, so sharks do not "urinate".

How long is the life span of a fish?

There are currently more than 20,000 species of fish discovered worldwide, and there are large differences in the lifespan of different species of fish.

The fish with the shortest lifespans found so far include killifish, tiger fish, and silverfish. Although they are not born and die in the same day as mayflies, they only live for about one year from birth to death. Guppies and zebrafish have a slightly longer lifespan, generally surviving for about two years. The average lifespan of arowana is 6 to 10 years.

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Sturgeons are a type of fish that matures over a long period of time, and their life span can reach 40 years. For ordinary sharks, they can live for 30 years, while the life span of whale sharks can reach 70 to 100 years.

Of course, there are fish that live longer than whale sharks, and that is pike, which can be called the "oldest centenarian" among fish, with a lifespan of up to 200 years. The long lifespan of pike is related to its living in the cold regions of the northern hemisphere.

What should I pay attention to when raising fish?

As the saying goes, raising fish starts with raising water, and water quality is particularly important. The natural water used at home usually has a high chlorine content. Some ornamental fish are sensitive to chloride ions, which may damage the fish's gills. Therefore, it is best to fully aerate natural water or let it stand for a few days before use.

The supply of oxygen in the water is also very necessary to prevent fish from dying from lack of oxygen or long-term hypoxic stress. In addition, the filter device needs to be cleaned and replaced regularly to ensure the cleanliness of the water.

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Since fish are temperature-changing animals, it is best not to change more than 1/3 of the water each time, so as to ensure that the water temperature is relatively stable to prevent the fish from "catching a cold." Of course, adequate nutrition is also necessary, but attention should be paid to feeding in moderation. Too much leftover bait will cause the water quality to deteriorate.

In summary, clean and stable water quality and adequate nutrient supply are important guarantees for fish to live longer.

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