Uni: Sea Urchin, うに、雲丹、海胆。Class Echinoidea var. genera.
The gonads of both male and female sea urchins (so either ovaries or testes, which by the way look almost exactly the same in the case of sea urchins), usually called "sea urchin roe" or "corals," are culinary delicacies in Japan (and many other parts of the world). These gonads can occupy two-thirds of the sea urchin's body. Japan imports large quantities from the United States, South Korea, and
other countries, and consumes 50,000 tons annually, amounting to over
80% of global production. Sea urchin roe can retail for as much as ¥40,000 per kilogram.
The orange-yellow ovaries are a true top delicacy. They are loaded with
tasty amino acids and IMP, and also contain 15 to 25 percent fat. Uni is
served raw as sashimi or on nigirizushi, with soy sauce and wasabi. Uni
can also be preserved in salt (shiokara), and it can be mixed with egg yolk as a topping for grilled seafood (uniyaki).
Uni has not always been that popular: until fairly recently, uni were detested in Japan because they eat kelp. So kelp harvesters destroyed sea urchins. But sometime after the 17th c. , first in japan's Hokuriku area and then elsewhere, people came to appreciate the taste if uni so much that they started harvesting sea urchins instead of destroying them.
Today, the uni caught in the waters around Hokkaido are considered the best in the world. They come in two types: murasaki uni (purple uni, with long spines) and bafun uni (dark brown with shorter spines - bafun means "horseshit," not because of the taste, but the form; in fact, bafun uni are considered as the most tasty of all). The hardier species is murasaki, which takes up about 90% of all uni.
[Paracentrotus lividus, a regular sea urchin
(Euechinoidea, infraclass Carinacea)]
In the water, sea urchins are spiny balls, a sort of globular hedgehogs with a round and spiny hard shell. About 950 species live on the seabed of every ocean. Sea urchins move slowly, crawling with tube feet, and also propel themselves with their spines. Algae and kelp are the primary diet; the predators who in their turn eat sea urchins are the sea otter and the starfish, the wolf eel, and... human beings.
Photos "Creative Commons" from Wikipedia.
Some information about uni was gleaned from The Meaning if Rice by Michael Booth, and The Zen of Fish by Trevor Corson.