Japanese love seafood. Fish, crab, cuttlefish, octopus, clam, squid, shrimp, abalone, scallop and etc.
On the other hand, do not eat so much freshwater fish, as like Czech people who eat trout and carp.
May be the exception is the eel. Eel is one of the expensive fish in Japan but Japanese are eating eel for a long time.
The most popular dish of eel in Japan is grilled eel on rice. However, there are two names for this meal. One is called Una-don and the second one is called Una-ju. What is the difference?
Una-don comes with a relatively large bowl, called donburi. It is bigger than the usual rice bowl but smaller than the ramen bowl. Una-ju comes with a rectangular shape container made of Japanese lacquerware or plastic. Usually Una-ju comes with large size of grilled eel and therefore Una-ju is expensive than Una-don. Then, Una-ju and Una-don are provided at restaurants which are specialized in eel(you can not find Una-ju at sushi bar or ramen bar). May be it is served at restaurant chains but the eel should be precooked and the quality is not so good.
By the way, how to cook Una-ju are different between Tokyo and Osaka. Tokyo style is usually cut from the back of the eel but Osaka style is cut from the belly. Then, Tokyo style is grilled without any sauce for a while and then steamed. Finally, the eel is dipped into a Kabayaki sauce(soy sauce and sugar) and grilled again. On the other hand, Osaka style is grilled directly with the sauce.
Tokyo style is considered less greasy due to the fat of the eel are got ridden during the steaming process. Then, people of Tokyo disliked to cut from the belly due to it reminds of Harakiri of the samurai. Tokyo was the city of samurai but Osaka was city of merchant.
Anyway, when ordering Una-ju or Una-don, please consider the time. It takes a lot of time to prepare the grilled eel!