Uni, Japanese for sea urchin, is for some an expensive gourmet delicacy while for others it is a rather revolting and funny tasting yellow gloop.
Uni is certainly an acquired taste and even for the editor it took a few tries to actually get used to, but the taste is certainly unique. The occasional visitor to Japan is most likely to come across uni on top of a bit of rice surrounded by dried “nori” seaweed at a “kuru kuru” sushi restaurant (the revolving conveyor belt style sushi restaurant which is the fast food of Japanese sushi and often not the gourmet experience eating sushi should be). European visitors to Japan tend to assume that uni is not eaten in Europe, but southern Italians will recognise it from (to my mind) a rather tasty spaghetti sauce.
The best sea urchin is said to come from Rishiri Island/利尻島, which is a very remote island just on the north east tip of Hokkaido. The uni around this island eat kelp – konbu/昆布 – which is plentiful around the island and, along with the cold water, is said to give a perfect environment to enrich the taste. Rishiri Island itself is a small, circular volcanic island similar in shape to Mount Fuji and very beautiful when viewed from the nearby Hokkaido coast.
The kanji for uni are used phonetically rather than being a compound referring to uni itself, but are rather attractive un/雲 means cloud while ni/丹 (also pronounced tan) is used for a sulphide of mercury which is red in colour so takes that connotation.
The picture you linked to this page from comes from a website that seems to have some of the best Japanese food photography on the web. The site is for a very expensive (USD300-USD800 or so per night) but very nice looking Japanese Ryokan.