Krill is a crustacean with a chitin exoskeleton, which is divided into three parts: head, chest and abdomen. The first two parts * * * are the head and chest. Most krill exoskeletons are transparent. Krill has complex compound eyes; Some krill can change color to adapt to the light in different environments [7]. They have two tentacles and some feet on the chest, which are called pectoral feet or pereiopod (so named because most of these feet are under the chest; Different genera and species will have different numbers of thoracic limbs. All krill have five swimming feet, called pleopod or "swimmerets", which are very similar to common freshwater lobsters. Most adult krill are about 1 to 2 cm long, while some species of krill can be as long as 6 to 15 cm. Krill can be easily distinguished from other crustaceans, such as using external protruding gills to distinguish krill from real shrimp. Many krill use filtration: they use their pectoral limbs to filter food from seawater. This filtration method is very suitable for krill with phytoplankton as its staple food, especially when eating unicellular algae diatoms. However, most people agree that most krill are omnivorous and a few are carnivorous, and will catch small zooplankton and young fish. Except deep-sea krill, other krill can emit light. They have an organ called bare shore, which can emit light. Their light is caused by enzymatic chemiluminescence reaction, in which fluorescein (a pigment) is activated by luciferase. Studies have pointed out that the fluorescein of many krill is a kind of fluorescent tetrapyrrole, which is similar to dinoflagellate, but not exactly the same. Krill itself cannot produce substances, but it can be absorbed from food containing dinoflagellate. The luminous organ of krill is a complex organ, which can focus light and be rotated by muscles. The precise operation of this organ has not yet been clarified by human beings; They are likely to have mating motivation, which may be one of the ways of social interaction. Some researchers believe that krill may use fluorescence as camouflage, and use light to brighten their shadows into the surrounding environment, making it more difficult for hunters below to notice them. Most krill are social animals; Due to different regions and species, the size and density of these races will be different. Antarctic krill has been recorded as many as 10000 to 30000 per cubic meter of water [1 1]. Ethnic group is a way of defense, allowing hunters to choose individuals in the group, which can reduce the chance of being hunted. Krill usually has a vertical migration route. They will dive to the deeper bottom during the day and float to the shallow water near the water surface at night. When they dive into deeper water, their activities will decrease [12], which seems to reduce the chance of being hunted and save more energy. Some krill populations (such as Antarctic krill, Pacific krill, Hansen krill, Pseudeuphausia latifrons or Thysanoessa spinifera) also feed and breed in the water during the day, but this is more dangerous because it makes them vulnerable to predators.