What kind of animal is a sea urchin?

Sea urchins, also known as prickly pots and sea hedgehogs, are invertebrates living in marine shallow waters under the phylum echinoderm. they can be divided into two subclasses: regular sea urchins and irregular sea urchins, with a total of 22 orders, 225 genera and more than 900 species, distributed on the seafloor from the intertidal zone to a depth of several thousand meters, mostly concentrated in the rocky seafloor or sandy seafloor in the coastal zone, or widely distributed, or limited to specific sea areas, different from species.

Sea urchin is an ancient creature in the ocean and is closely related to starfish and sea cucumbers. According to scientific research, sea urchins have existed on the earth for hundreds of millions of years. There are many species as far away as Paleozoic and Mesozoic, and as many as 5000 species of sea urchins have been found. Due to the vicissitudes of life, fossils of sea urchins have been found in the Xizang Plateau of China. Many extinct species are marker fossils of the Paleozoic and Mesozoic.

The main characteristics of sea urchins are spherical, disk-shaped or heart-shaped, wrist-less. The endoskeleton heals each other to form a strong shell. Most species have complex chewing devices in their mouths, called Aristotelian lanterns, which have teeth and can chew food. The digestive duct is long tube-shaped, curved in the body, and feeds on algae, hydra and worms. Polyandrogynous, it passed through the sea urchin larva (long wrist) in ontogeny, then metamorphosed into juvenile sea urchin, and it took 1-2 years to reach sexual maturity.

Sea urchins live in the oceans of the world, with the largest number of species in the Indian Ocean and the western Pacific Ocean, ranging from shallow water to 7000 meters of deep water, all living in the bottom or sediment. Sea urchins like high-salinity sea areas, so there is little or no distribution near rivers and low-salinity waters. China is mainly distributed in the coasts of the Yellow Sea and Bohai Sea, Liaodong Peninsula and the northern part of Shandong Peninsula, southward to Zhejiang and Fujian shallow seas, Zhoushan Islands and Taiwan Strait. Now it has been artificially cultured and the eggs are edible. Japan uses eggs as the raw material for making Yundan (sea urchin egg paste).

Sea urchins are naturally timid and run away at the sight of an enemy, but sea urchins can't move quickly. Exercise with food intake, if food is abundant, it may only move 10 cm a day, and when food is scarce, it can move 50 cm a day. Exercise is carried out by transparent, small, numerous and sticky tube feet and thorns, which are similar to starfish and can hold on to rocks, while thorns at the bottom lift the body of sea urchins to help sea urchins move at will. Sea urchins eat a wide range of food. Carnivorous sea urchins feed on undersea worms, molluscs or other echinoderms, while the main herbivorous food is algae, and there are also sea urchins that feed on organic debris and animal carcasses.

The vast majority of sea urchins are hermaphrodites with no difference in appearance, and some species are hermaphrodites. The gonad is located in the inner step zone of the gallbladder and is very long when it matures. It hangs in the body cavity and is composed of two layers of body cavity epithelial cells, muscle and connective tissue. Each gonad has a short reproductive duct, which passes through the reproductive plate to open the reproductive pore in vitro. Sperm and eggs are fertilized in sea water and ovulate several times a year. The breeding season is from June to mid-July.