[Smart Farmers] The Wandering Earth from an Agricultural Perspective: Having Survived Several Extinction Events, This “Living Fossil Fish” May Become Hope for Doomsday

Editor’s Note:

The recent hit of "The Wandering Earth 2" has aroused deep thought behind the sci-fi blockbuster: the doomsday scenario assumed in the movie is far-fetched, but if the earth reaches the "doomsday", what will humans eat? Popular Science China Smart Farmers has launched a series of popular science articles, welcome to follow.

As a fishery worker, I have been dealing with aquatic life for many years. I often develop feelings for some beautiful and special aquatic life. Especially when I am doing aquatic science popularization, I often say that fish have expressions, but everyone seems confused. It was not until a few years ago when I went to the Toronto Aquarium in Canada and showed everyone a smiling fish that I became more convincing. The fish I want to talk about is the stingray, the protagonist of this article.

Image provided by the author

In the topic of "If the earth wanders, what will we eat?", I included manta rays in the "hope" list. On the one hand, the smiling manta rays give us confidence and hope. More importantly, it is the prosperous family and indomitable spirit of the manta rays that flew from ancient times.

The ancient "flying" stingray

Manta rays belong to the class Chondrichthyes, and are a general term for a group of elasmobranch fish with ventral gill openings and a thick tail. According to research, 180 million years ago, manta rays were the same species as sharks, but in order to adapt to life on the seabed, they hid their bodies in the sand on the seabed for a long time, and gradually evolved into their current appearance. Manta rays are recorded in many ancient Chinese books. The description of manta rays in the Classic of Mountains and Seas is: "It looks like a carp, with a fish body and bird wings, with gray patterns and a white head and red beak. It flies at night, and its sound is like a phoenix."

There are many species of rays, with about 380 species in the world. There are 38 species in my country, with the largest number of species in the order Rayiformes, 28 species; 8 species in the order Electric Rays; and only 2 species in the order Sawfishes. In addition to being distributed in the South Pacific and the northeastern coast of South America, rays are distributed in shallow waters in all temperate and tropical regions of the world, and are also distributed in the South China Sea and the southern part of the East China Sea in my country. The huge manta ray and the electric ray that can discharge electricity are both rays; they swim by flapping their wide pectoral fins, which looks like flying in the water. Some species are famous for their ability to jump from the water to high altitudes. For example, the line board ray, which is the largest type of ray, has a pectoral fin of 8 meters when it is spread out, and loves to fly on the sea. As stated in the "Compendium of Materia Medica": "The rays come from the south of the sea, and the large ones are about a foot long, with wings that are as long as the tail. They are also called flying fish. When they fly on the water in groups, sea people catch them, and there will be strong winds." The common gray ray can grow to over 2 meters in length.

The picture of the "flying" stingray comes from the Internet

Stingrays are near-shore bottom fish, often lying quietly on the seabed or buried in sand and mud. Due to their special body structure, stingrays need to use a special closed-mouth breathing method when lying on the seabed to avoid inhaling mud and sand as much as possible. When breathing, water is inhaled through the pipes on the top of the head and finally flows out through the gill slits on the ventral side. Stingrays sometimes crawl on the seabed or swim near the sea. They swim mainly by waving their pectoral fins back and forth. They move slowly, hiding during the day and coming out to forage at night. Their main delicacies are small crustaceans, polychaetes, mollusks, small fish and cephalopods. Young stingrays feed on animals living on the seabed, such as small crabs and crayfish. When they grow up and become strong, they will take the initiative to hunt squid and other mollusks. Stingrays have a very sensitive sense of smell, and when hunting, they mainly rely on smell to hunt.

Like most fish, stingrays are oviparous animals, but their eggs are special. The eggs are covered with a quadrilateral keratin shell, which can stick to the attached objects and develop with the help of the protrusions at the four corners. Therefore, it has the beautiful name "mermaid's purse". Moreover, fish mothers do not lay eggs all at once, but usually lay them in batches every few days, and only lay 2 eggs each time.

Fighting fish

Don't think that some rays are always smiling and are honest fish that will be bullied. You are wrong. Since rays can survive from ancient times to the present and have experienced the extinction of the five major creatures on the earth, their survival skills are extraordinary. The eyes and water blowholes of rays are on the top of their heads, and the mouth, nose and gill slits are on the bottom. These are gradually evolved by rays to adapt to bottom-dwelling life. They usually hide in the sand, waiting for small crabs and shrimps to approach them and suddenly attack them for a good meal. There are many weapons on rays that can be used for fighting, such as their teeth like mortars that can grind anything, especially the back with a poisonous thorn, which can be used to kill prey and defend against enemies. Rays can use this weapon flexibly and quickly, and quickly bounce their tails, even animals in front will be stabbed. Swimmers must be extra careful not to step on them when wading in the sea.

Fish that can generate electricity

In the ocean, there are three types of fish that can generate electricity. Interested people rank them according to the size of the discharge: the electric ray, the "undersea electric shocker", the electric catfish, the "generator" in the water, and the electric eel, the "high-voltage line" in the water. The electric ray ranks first, belonging to the suborder of electric rays, characterized by a pair of large electric organs between the head and pectoral fins, which can generate strong currents to defend themselves or kill prey. The skin of the electric ray is smooth and scaleless, and the head, trunk and pectoral fins form a round disc, with a short and thick tail. There are 20 known species of electric rays, mainly in tropical and temperate waters around the world. There are many species, most of which live in shallow waters, but the deep-sea electric ray genus can also live in deep waters below 1,000 meters. The largest species can weigh up to 90 kilograms.

Electric ray pictures from the Internet

But don't be afraid, electric rays move slowly and feed on fish and invertebrates. They are harmless to humans if not touched.

Why? Because in fact, the power generation of electric rays is also an instinct for them to protect themselves. There are many kinds of electric rays in the world, and their power generation capacity varies. The voltage of the African electric ray is about 220 volts, the voltage of the medium-sized electric ray is 70-80 volts, and the smaller South American electric ray can only generate 37 volts at a time. Although the electric ray can discharge 50 amperes of current and the voltage reaches 60-80 volts, it is known as the "living power station" in the sea. The electric ray can discharge 50 times per second, but after continuous discharge, the current gradually weakens and disappears completely after 10-15 seconds. It can only recover the discharge capacity after a rest, so this kind of power is not continuous. Therefore, some people ask, can electric rays be used as generators? The answer is no! However, the discharge characteristics of electric rays inspired people to invent and create batteries that can store electricity. The paste filling between the positive and negative electrodes of the dry batteries used in daily life is inspired by the gelatinous substance in the electric ray generator.

In ancient Greece and Rome, doctors often placed patients on electric rays or asked them to touch electric rays that were discharging electricity in a pool to treat rheumatism, epilepsy, etc. On the coast of France and Italy, you may also see some elderly people with rheumatism looking for electric rays as a folk remedy on the beach after low tide.

Interestingly, at a time when there has been bad news about species disappearance, a new species has been discovered in the electric ray family, which brings us joy. This new species is called the super suction electric ray, which is a member of the largest known electric ray family, the Monopteridae. Interestingly, because the video footage of the electric ray foraging taken by scientists shows that this fish can catch prey in the water like a vacuum cleaner, so scientists named it after the Electrolux vacuum cleaner.

Manta rays are widely distributed in the three oceans and are also found along the coast of China. However, the annual output of manta rays in the world is not high, only about 160,000 to 200,000 tons. The meat is edible and has a good taste. It can be eaten fresh or made into salted and dried products. Manta ray liver is used to make cod liver oil, which also has certain economic value.

However, you should be careful when enjoying stingrays, as some species of stingrays are now endangered and cannot be caught, let alone eaten. For example, the smiling fish at the beginning of this article is called sawfish, which is listed in Appendices I, II and III of the Convention on International Trade in Endangered Species of Wild Fauna and Flora. But some are still edible, such as the ray, which is a secondary economic fish in the Yellow Sea and the East China Sea, and its meat is still very delicious!

The picture of the ray comes from the Internet

In short, although the family of manta rays is prosperous, with many brothers and sisters, and some new members have been discovered, it shows that the genes of this family are still very strong. However, we still need to pay attention to protecting them. First, among their many brothers and sisters, there are also unprosperous branches that have been listed as endangered species; second, their overall production is not high and their reproductive capacity is not strong, unlike some fish that can lay hundreds or thousands of eggs at a time. Therefore, in order to hope that they can still provide us with delicious food when the "doomsday" comes, please pay attention to and protect this fish that "flew" from ancient times.

Produced by: Popular Science in China to Benefit Farmers

Producer: Guangming Online Science Department

Author: Liu Yadan, Researcher, former Assistant Secretary-General of the Chinese Society of Fisheries and Chief Scientific Communication Expert of China

Planning: Wu Yuetong, Xie Yun