Teaching a fish to walk, can you scientists be more boring?

It is said that fish tanks, like thermos cups, are standard equipment for middle-aged people. Whether it is out of appreciation or belief in "feng shui", the types of ornamental fish on the market are becoming more and more diverse. There is a "dinosaur king" from Africa, with a body length ranging from about 30 cm to 1 meter. Its official name is "polypterid", which also appeared in "Animal Crossing". The only family left in the order Polypteriformes is the Polypteridae. Polypterids with different scale colors are commonly known as tiger-striped dinosaur kings, golden dinosaurs, blue dinosaurs, etc. in the market, or are called nine-section dragons according to the number of dorsal fins.

Polypterus delhezi | ばぶじ～/ Wikimedia Commons

The reason why it is called "dinosaur" is related to the peculiar dorsal fin structure of this type of fish: multiple diamond-shaped dorsal fins extend all the way to the tail, with a number of about 5-18, while common fish only have one dorsal fin, and each fin ray is supported by sharp spines in front, with a ferocious shape, like the bone plates on the back of Godzilla's stegosaurus. Polypterus means "many (poly) fins (pterus)". Its English common name is "dinosaur eel". Although the slender body of polypterus is similar to that of eels, it is not a relative of eels.

Color variation of Polypterus senegalus | lienyuan lee / Wikimedia Commons

If you observe carefully, you can find the uniqueness of the bipterygium. It can use its pectoral fins and tail to support its body and make crawling-like movements at the bottom of the tank. Although the "Dinosaur King" is not a real dinosaur, it has real ancient characteristics.

Witnessing a vertebrate landing

There are two genera in the family Polypteridae: Polypterus and Calamoichthys (also known as Calamoichthys, commonly known as "grass rope dinosaur"), which are distributed in freshwater rivers and lakes in Africa from Egypt to the southern Congo Basin. They live in estuaries, swamps and other muddy bottom water bodies.

The biological community's attention to them can probably be traced back to the field research of French naturalist Étienne Geoffroy Saint-Hilaire in Egypt. In 1798, Napoleon Bonaparte organized 167 scientists, artists, and engineers to Egypt for a "scientific expedition", and Saint-Hilaire was one of them.

He studied the bi-finned fish found in the Nile River and found that the anatomical structure of the gills and pectoral fins of this fish was similar to that of mammals. Based on this, he proposed the "theory of homology", that is, "all animals have the same body structure such as head, limbs, and spine, even if the shape and size are different." This view was refuted by Georges Cuvier, another famous paleontologist of the same period. Cuvier believed that vertebrates and mollusks were fundamentally different and that "species were created according to the environment in which they live."

French cartoonist Grandville's drawing of two naturalists arguing, Geoffroy Saint-Hilaire (in ape form) and Cuvier (Saint-Hilaire: Are you polite?) | Wikimedia Commons

Based on previous research and debate, the view that "tetrapods evolved from fish" has gradually been generally recognized. The jawed bony fish include the sarcopterygians and ray-finned fish. About 390 million to 360 million years ago, the sarcopterygians successfully landed on land and evolved into today's terrestrial vertebrates. The teleost fish in the ray-finned fish continued to stay in the water and became the vast fish that now account for about 50% of the total number of vertebrates on Earth. To paraphrase the poem "Man and ape bid farewell to each other", perhaps it can be called "Man and fish bid farewell to each other".

Bipterygians are the first group to separate from the evolution of ray-finned fishes. Fossils show that their body structure has not changed much over hundreds of millions of years. Bipterygians in South America became extinct in the early Cenozoic Era, but populations in Africa survive to this day. Although they did not eventually live on land, they still retain some characteristics that allow them to survive in the water-land interface environment.

Juvenile bipterids have distinct external gill structures, similar to those of amphibians | hirokiDX / Wikimedia Commons

The lungs of bifin fish are simple, smooth sacs instead of alveolar tissue like tetrapods. They have no trachea and absorb oxygen from the air through the slit-like blowholes on the top of their heads, which helps them survive in waters with low oxygen content. They can even survive for a period of time buried in moist mud when the water dries up. In addition, the lungs of bifin fish also serve as swim bladders.

The scales of bipterygians are also different from the overlapping, imbricate bony scales of most bony fish. In fact, they are a unique type of hard scale (ganoid scale). The hard and thick scales are diamond-shaped, and adjacent scales are embedded in each other through concave and convex surfaces, like a three-dimensional puzzle, forming a closed protective layer on the outer surface of the body.

The "blowhole" structure is located at the back of the eye socket of cartilaginous fish and a few bony fish. During the evolution of tetrapods, this structure gradually evolved into a part of the auditory structure and became a channel connecting the inner ear and external sound sources | lienyuan lee / Wikimedia Commons

It's okay to go ashore and take a walk

In the experience of raising bifins, it is mentioned that the fish tank should be covered to protect it, otherwise the bifin may jump out of the fish tank and crawl hard on the ground. This scene is somewhat scary. The "crawling" of bifins relies on the alternating movement of the pectoral fins supported by muscles and endoskeleton, and the body is rotated around this axis, with the head raised, presenting an S-shaped movement trajectory. Its pectoral fin peduncle structure is composed of three wing-like bones, the outer side is rod-shaped, and the middle is plate-shaped.

Pectoral fin structure | David Starr Jordan / Wikimedia Commons

In 2014, Nature magazine published a study by the University of Ottawa in Canada. After the young bipterids were raised in a suitable out-of-water environment for a period of time, their bone structure and muscle tissue changed, which to a certain extent restored the evolutionary stage of the earliest tetrapod ancestors from fins to limbs. In February 2021, Cell published an analysis of the genome of primitive ray-finned fish by the Chinese Academy of Sciences and other units. The results showed that the area where the posterior basal fin bone and fin rays of bipterids connect corresponds to the synovial joint structure in tetrapod species. Bipterids retain valuable evidence of the evolution of tetrapods from aquatic to terrestrial. It is not an exaggeration to call it a "living fossil."

A bipterus struggling to crawl | Noah Baker / Nature (2014)

In the wild, bifin fish hide during the day and come out at night. In captivity, bifin fish are less active during the day and spend most of their time lying on the bottom sand of the fish tank in dim light, giving people the first impression of "no presence". In fact, it is a predatory fish, and small fish and shrimps in the same tank with it are prone to mysterious disappearance.

The vision of bifin fish is very poor, but it does not hinder their hunting. On the one hand, bifin fish have a keen sense of smell. The surface area of ​​the epithelial cells of the olfactory receptors can reach 3200 square millimeters. In addition, there are both liquid and gaseous olfactory receptors in its olfactory receptors, which can detect not only the smell in the water, but also the molecules in the air. On the other hand, bifin fish are one of the few bony fish with primitive electroreceptors, which can sense the electric currents emitted by other animals around them to hunt.

Polypterus endlicheri, commonly known as the tiger-striped dinosaur king | Giant water wolf dog carp / Station B

At present, the wild bipinnatus is still classified as of least concern, but in its native Africa, its habitat is gradually being developed to grow cash crops such as oil palm, and its living environment is also facing threats. As a "dinosaur king" that witnessed history, perhaps it should not just serve as a quiet background board in the fish tank.

This article comes from the Species Calendar, welcome to forward