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The Common Sea Squirt, very often called Sea Cucumber is neither a coral, seaweed, shellfish or whatever.
It is an animal of its own class.
in its natural habita, already a prey to many marine predators, it has become rare because of the extensive catch by humans.
Its natural colour is whitish out of the water while (see pic above) Hoya rasied by humans are of a deep orange colour.
We are just in the middle of its season, May.
They are mainly raised in Miyagi Prefecture while natural ones are caught in Iwate Prefecture.
You have to cut it open to reach its edible part.
It can be eaten raw, slightly boiled or pickled.
It is said to be the rare sea animal combining the four tastes: sweetness, saltiness, sourness and acidity.
For a better view of its insides. It is called sea squirt, because it is mainly filled with sea water which can be expelled at will.
Ipersonally appreciate it as sushi nigiri, although it is a bit of an acquired taste!
SHIZUOKA SUSHI & SASHIMI: The other Jewels of Japan 
Hey, just wanted to clarify that sea squirts are not actually sea cucumbers! Sea cucumbers are a completely different animal, related to sea stars, sea urchins, and other echinoderms. Sea squirts are a kind of animal called a tunicate and are more closely related to craniates than to echinoderms.