Pseudocentrotus depressus is a species of sea urchin in the family Strongylocentrotidae, commonly known as the red sea urchin or Aka-uni in Japan. It is a marine echinoderm native to the northwest Pacific Ocean, particularly along the coasts of Japan, Korea, and parts of eastern China, where it inhabits rocky subtidal environments. The genus name Pseudocentrotus derives from the Greek pseudo- meaning “false” and kentron meaning “spine/sea urchin,” distinguishing it from other spiny sea urchins, while the species epithet depressus is Latin for “flattened,” describing its relatively low, flattened test (shell).

Historically and in modern culinary practice, Pseudocentrotus depressus is harvested primarily for its gonads, known culinarily as uni, which are considered a delicacy in Japanese cuisine. It is consumed raw as sashimi and sushi, salted and preserved, or incorporated into sauces and rice dishes. Commercial aquaculture of this species has developed in Japan to ensure consistent quality and flavor, with feeding regimes designed to enhance sweetness and texture. It is strongly associated with southern Japanese culinary traditions, especially in coastal regions near Kyushu, and is also used in Korean seafood cuisine.

Nutritionally, the species contains significant bioactive compounds including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), echinochrome A, a polyhydroxylated naphthoquinone pigment with antioxidant properties; ovothiol, a sulfur-containing antioxidant; and various carotenoids such as β-carotene. These compounds contribute to its nutritional value and potential health benefits, particularly in reducing oxidative stress and supporting cardiovascular health.