Fish-like? Non-fish? amphioxus!
Of the more than 40,000 species of vertebrates that live globally, fish account for about half. The oceans, which cover about 70% of the Earth's area, provide vast space for aquatic animals. After 400-500 million years, different species with different forms and adaptations to different ecological habits have evolved. Some of them resemble fish instead of fish, such as whales. Some are fish and not like fish, such as seahorses. Some developed lungs, traveled from the ocean to land, and evolved into amphibians, such as the total fin fish. Some are still wandering on the path of low evolution, such as sturgeon. Today I am talking to you about Wenchang fish that is like a fish called a fish but not a fish.
Wenchang fish is smooth and tender, delicious, with a protein content of up to 19.05% and contains 8 essential amino acids[1], which was once an important fishing object in the fishery. Fishermen along the coast of Xiamen, Fujian Province, China have had the tradition of fishing Wenchang fish since ancient times. Local fishermen have been passed down from generation to generation, and every year around the birthday of the legendary god Wenchang Emperor in southern Fujian, this fish will appear, so it is called "Wenchang fish". Folklore about Wenchang fish dates back to the Tang Dynasty. Wenchang fish, which has been barking for so long, how come it is not a fish anymore?
Zoologists define fish as a lifelong, gill-breathing, fin-finned vertebrates. Wenchang fish resembles a fish, living in the water all its life, breathing with gills, having odd fins, and its body temperature also changes with changes in water temperature, but it has no spine, and its body structure and various physiological functions are still very different from fish.
First, Wenchang fish do not have a spine
The cord is the precursor to the spine, and only a few inferior vertebrates retain the cord for life, while most vertebrates retain the cord only in the early stages (or embryonic stages) of animal growth, and are gradually replaced by a long string of vertebrae separated by each. Fish have a chordochal cord early in embryonic development, which later calcifies into a spine. Wenchang fish have no vertebrae, and support the whole body is a primitive, unsected chord that extends all the way to the front of the body (the back of the fish's early embryonic development also only reaches the back of the head). Therefore, Wenchang fish is not a vertebrate, which is the most critical reason why it does not belong to fish.
Nerve cord neural tube notochord chord body-wall muscles body wall muscle intestines
Oral cirri mouth tentacle pharyngeal gill slits pharyngeal fissure gonad gonad reproductive glands atrial cavity gill cavity atriopre gill cavity hole anus
Anatomical diagram of Wenchang fish| literature[3]
Second, Wenchang fish does not have body gills
Gills and gills are the respiratory organs of fish in the water, and the gill fissures open directly on the surface of the body on both sides of the pharynx. Wenchang fish also breathe with gills, and the gills on both sides of the pharynx are similar to the round-mouthed species of vertebrates (lamprey eels), except that the pharyngeal gill fissure does not open directly on the surface of the body, but is surrounded by the gill cavity formed by the skin and muscles.
Third, the fins of Wenchang fish have little to do with movement
Wenchang fish do not have pairs of pectoral and ventral fins like most fish. There is a low dorsal fin fold in the center of the body, with fin and caudal fin, the caudal fin is thatched, mainly plays a balancing role, and has little to do with movement, so Wenchang fish is not strong in free movement. In addition to odd fins such as dorsal fins, fish also have pairs of even fins (pectoral and ventral fins), which make the fish more balanced and swimming, and can swim freely in the water.
Fourth, Wenchang fish has no real mouth
The body of Wenchang fish is translucent, flattened, pointed at both ends, and the muscle segments are obvious, which is in line with the morphological characteristics of general fish. It has no real mouth, and the ventral surface at the front of the body has a funnel-shaped mouth, surrounded by dozens of tentacles to protect it from large particles falling into the mouth. The mouth of the fish is much more complex than that of the Wenchang fish, and the jaw that can bite up and down has appeared, which greatly enhances the ability to predation.
The Wenchang fish has no heart, obvious head, and paired eyes
Fish have distinct heads, and the brain and sensory organs of the head show a high degree of specialization and concentration. Wenchang fish has no obvious head, no pairs of eyes, only one eye point that can feel the intensity of light. The neural tube above the notochord, the anterior end expands, forming a cerebral vesicle, which is just the prototype of the brain. Wenchang fish has no heart, only a large abdominal artery with the ability to contract, driving blood to flow from the back to the front, and the blood lacks hemoglobin and iron compounds, so the blood is colorless. Fish have a heart with one ventricle and one atrium, and the whole body is flowing with red blood, which can carry rich oxygen and nutrients, speed up the circulation of blood, and make the metabolism more vigorous.
In this way, although the shape of Wenchang fish resembles fish, its body structure and various physiological functions are still very different from those of real fish. So, although it is called a fish, it is not a real fish. At present, the taxonomic status that zoologists have given to Wenchang fish is the chordate phylum and cephalopoda. Although Wenchang fish cannot be included in the large family of fish, its identity is very special. Since the 18th century, biologists around the world have not stopped studying it, and it was once considered a mollusk, a low vertebrate, a jawless, etc. Contemporary molecular biology research results show that Wenchang fish is the closest extant invertebrate to vertebrate relative, a transitional type of invertebrate to vertebrate evolution, a distant ancestor of fish, and has an extremely important position in the evolutionary history of animals [4].
Wenchang fish is a very small marine animal, China's Xiamen species body length is only 4-5cm, the largest is the United States of California Wenchang fish, the body length is only 10cm. It has higher requirements for the living environment, and prefers to inhabit shallow, sandy seas with clear water, slow flow and looseness, with a water depth of 8-15m. The camp is semi-benthic and half-burrowing, and spends most of the day half of its body half-buried in the sediment, exposing only the front half of the body, relying on plankton brought by the water as food, mainly diatoms. Drill out of the mud and sand at night and swim briefly in the sea. Wenchang fish is dioecious, sexually mature at first age, and some species can spawn several times a year, with an average lifespan of about 4 years [3].
Wenchang Fish | Smithsonian Environmental Research Center/Flickr
There are not many extant species of this ancient taxa, and according to the World Marine Species List (WoRMS), there are only 30 species in 1 family and 3 genera[11], which are widely distributed in temperate and tropical oceans around the world. Since 1923, when Wenchang fish was reported in LiuWudian, Xiamen, China, Wenchang fish was found in Shandong, Hebei, Guangdong, Guangxi, Hainan, Taiwan, Macau, Hong Kong and other sea areas. It can be seen that the distribution of Wenchang fish in China is very extensive, from the Bohai Bay to the vast sea areas along the coast of Hainan, as long as there is a suitable sandy beach, Wenchang fish may be distributed [2]. China's Xiamen Liu Wudian is the world's largest Wenchang fish production area, once formed the world's only Commercial Fishing Value of Wenchang Fish Fishery, in the heyday of the 1930s, the annual output of up to 250 tons. In the 1950s, annual production was maintained at 70-150 tons, and production has decreased sharply since then. Due to wenchang fish's sensitivity to organic matter pollution and changes in substrate quality, natural disasters and human factors have damaged the liu wudian sea area, and since the 1970s, Wenchang fish resources have declined sharply, and even face the danger of extinction[5]. China attaches great importance to the protection of Wenchang fish, and has listed Wenchang fish from wild populations as national second-level key protected wild animals, and established Wenchang fish conservation areas in Xiamen, Fujian, Qingdao, Shandong, Changli, Hebei, Maoming, Guangdong and Leizhou[6]. In 2005 and 2008, two Wenchang fish stocking activities were carried out in the Huangcuo Sea Area of Xiamen, releasing more than 100,000 Wenchang fry [7-8].
The Wenchang fish, which resembles a small fish, is an inconspicuous small animal that is difficult to attract people's attention. Although it has gone through hundreds of millions of years without much morphological change, its genome reflects the genomic structural characteristics of the ancestors of vertebrates[4], and compared with other living organisms, Wenchang fish can provide clues to the early origin of vertebrates, and is an ideal population for studying the evolution of vertebrate gene families and the evolution of animal systems, playing a key role in the history of zoological research. We have a responsibility to protect it so that this 500 million-year-old living fossil continues to accompany humanity and continue to contribute to the study of the origin of vertebrates and the evolution of the human genome.
Author: Underwater World
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