Sea urchins are a delicacy around the world. Here's why the orange tongues inside an urchin are so valuable.

Considered a delicacy in several parts of the world, sea urchin, or uni in Japanese, is prized for its unique flavor. But the cost of the gonads — the orange tongues found inside an urchin — is steep. Just one 200-gram box of premium gonads can cost $100. In Japan, some sea urchins can sell for five times that. Sea urchins are one of the few seafoods still hand-harvested by divers today. In recent years, masses of them have taken over the seabeds of California. So why are there so many urchins? And how, despite this seemingly huge supply, are they still so expensive?