Only the soft orange-yellow gonads, sold as roe, are eaten. Japanese restaurants serve them raw as sushi and sashimi; Chinese cooks also stir them into fried rice.
Sea urchin is a common seafood, popular in Japanese cuisine.
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海 means sea and 胆 means gall bladder - because the rounded, spiky shape of a sea urchin was thought to resemble a gall bladder. It's a vivid anatomical metaphor from the ocean floor.