Wenchang fish, also known as Bai's Wenchang fish, slugs, double-pointed fish or sea spear, is a precious marine animal, and because of its extremely special morphological structure, both the characteristics of invertebrates and vertebrates, is a typical representative of the transition type of invertebrate evolution to vertebrates, so it is highly valued by the biological community at home and abroad.
The shape of the Wenchang fish is very strange and interesting, its body shape is very small, the left and right sides are flat, the two ends are pointed and thin, the whole body is spindle-shaped, and the general body length is only 4 to 5 centimeters.
It is different from fish, the head has not yet differentiated, there is no obvious head, there is no obvious brain and eyes, ears, nose and other sensory organs, so it is also called headless. At the front of the body there is a small black spot, called the eye spot, which can feel the intensity of light. It has no scales on the surface of the body, and the body segments are very obvious: through the pink translucent body surface, it can be seen that there are 63 to 66 muscle nodes alternately arranged in a "V" shape parallel arrangement in the body, overlapping left and right, and the number of these muscle nodes is one of the important features of the classification of Wenchang fish.
Vertebrates include fish, amphibians, reptiles, birds and mammals, and can be inferred from fossils found in different formations, as well as comparative anatomy and embryological findings. However, due to the fact that animals lacking a hard endoskeleton are not easy to form fossils preserved in the formation, no fossil evidence of the transition type between invertebrates and vertebrates has been found so far.
Fortunately, the living animal Wenchang fish, whether in terms of life habits, embryonic development, external morphology and internal structure, is a prototype with both the characteristics of some invertebrates and some vertebrate characteristics, so it is a transitional type between the two, filling the gap between the two, and becoming an indispensable "bridge" in the entire history of biological evolution. In this regard, Darwin, the founder of biological evolution, believes that "this is the greatest discovery that provides the key to revealing the origins of vertebrates".
The formation process of the notochord, neural tube, pharyngeal gills, mesoderm, body cavity, etc. of Wenchang fish is similar to that of vertebrates, and the structure of some internal organs such as kidney ducts and reproductive glands is similar to that of invertebrates, so studying its internal structure and the situation of various stages of embryonic development has a great enlightening effect on understanding and explaining some basic theoretical aspects of animal phylogeny.
Wenchang fish is produced in the shallow seas of Fujian, Guangxi, Guangdong, Hainan, Shandong and Hebei coastal areas in China, especially in Xiamen, Fujian, Liu Wudian is the most famous. It is also found in Japan, Sri Lanka and Eastern Africa.
Wenchang fish has very strict requirements for the natural environment, requiring high transparency of seawater, clean water quality, water depth, water temperature, pH, salinity should be appropriate. Environments such as estuaries with large amounts of fresh water flowing into the river and sludge on the seabed with high acidity will cause them to die.
Wenchang fish used to be an important economic animal in China, with a high yield. However, with the construction of a large-scale seawall and the reclamation of the seabeds, the hydrodynamic conditions of the sea area have changed, and the sandy sedimentary environment on which Wenchang fish depend has been seriously damaged. In addition, factors such as barrages, the great development of oyster farming on shallow beaches, a large amount of sand extraction on the seabed, offshore pollution and indiscriminate fishing have also aggravated the deterioration of the living environment of Wenchang fish, making Wenchang fish resources increasingly depleted and endangered.
Fortunately, in recent years, the habitat of Wenchang fish has been found in the sea area of Huangcuo and Xiaodeng Island in Xiamen, indicating that Xiamen is still an important producer of Wenchang fish in China. As early as more than a decade ago, the 5,000-hectare Xiamen Wenchang Fish Nature Reserve was established to avoid repeating the mistakes of Wenchang fish resources being destroyed.