Kaisendon is a donburi; rice bowl containing various raw seafood - sashimi. Rice of the kaisendon is not same as the sushi rice which contains rice vinegar and sugar, but normal white rice. However, especially in sushi restaurant serves sushi rice for its kaisendon.
Also kaisendon has two types. The first one is sashimi style raw fish are on the rice. This type can be seen in mostly in sushi restaurant or place where serve sushi. The distinction is that there is only one sashimi per fish. Which means there are no two slices of tuna but just one. On the other hand, you can enjoy many types of sashimi per bowl.
The second type is one or two raw fish on the bowl. May by maximum three types of seafood. This type can be seen in diners and restaurants which are specialized in kaisendon or do not offer sushi. The major seafood used for this type are sea urchin, squid, tuna, salmon roe and scallop. However, usually tuna will be either minced or marinated in soy sauce and other seasonings.
Then, there are two ways to eat. The first type is to mix the wasabi and soy sauce in a small plate and pour over to the seafood. The second type is putting the seafood to the mixes wasabi and soy sauce as you eat sushi.
Kaisendon is popular in northern part of Japan, northeast Honshu and Hokkaido. Especially kaisendon is the signature dish in Hakodate, Hokkaido. There is a kaisendon street which many diners offer only kaisendon.