Sea urchin is the spiny, globe-shaped echinoderm whose only edible part is the "uni" — the five orange-to-gold gonads spoked inside its shell. Creamy, custardy and melting on the tongue, uni is briny-sweet and intensely umami with a faint marine-iodine edge, prized above almost all other seafood in Japan, where it is eaten raw as sushi and sashimi. The colour, sweetness and price swing wildly with species, diet, season and sex, making it one of the most graded and connoisseurial of all foods.