Mesocentrotus nudus, commonly known as the northern purple sea urchin (distinct from the Pacific and Japanese species), is a marine echinoderm in the family Strongylocentrotidae. It is native to the cold temperate waters of the northwest Pacific Ocean, particularly along the coasts of northern Japan (Hokkaido and northern Honshu), the Korean Peninsula, the Russian Far East, and parts of northern China. The genus name Mesocentrotus derives from Greek meso- meaning “middle” and kentron meaning “spine,” referring to taxonomic distinctions within sea urchins based on spine morphology and skeletal features. The species epithet nudus is Latin for “naked” or “bare,” likely referring to aspects of its test (shell) or spine coverage noted during its original classification. It inhabits rocky coastal substrates and kelp forest ecosystems, where it grazes on macroalgae.

Historically and in modern times, Mesocentrotus nudus has been harvested for its edible gonads, which are considered a delicacy in Japan and Korea and are also marketed in China. As a food source, Mesocentrotus nudus is valued for its gonads, known in Japanese as uni. The edible portion consists of five lobes with a creamy texture and a flavor profile that balances briny, sweet, and umami notes. It is most commonly consumed raw as sushi or sashimi, but may also be lightly salted, preserved, or incorporated into rice bowls, soups, and contemporary seafood dishes. The quality and flavor of the gonads depend strongly on seasonal reproductive cycles and diet, especially access to high-quality kelp. In Japan, uni from northern waters is considered the most prized of all uni and carefully graded for color, texture, and sweetness.

Mesocentrotus nudus contains omega-3 fatty acids such as eicosapentaenoic acid (EPA), carotenoids such as echinenone and β-carotene, vitamin E (tocopherols), and free amino acids such as glycine, alanine, and glutamic acid that contribute to sweetness and umami taste. As a benthic grazer, Mesocentrotus nudus can bioaccumulate environmental contaminants depending on local water quality, but when sourced from clean coastal environments it is regarded as a nutrient-dense and culturally significant seafood in northern East Asian cuisines.