Conger eel

Squid (ika) or conger eel (anago) are usually coated with a sweeter, thicker glaze called tsume.
Tsume is a reduction sauce made with the broth used to simmer squid and conger eel or other seafood. At first it has almost no viscosity, but after reduction it becomes a gleaming thick sauce like honey.
The sauce gets its name for the Japanese word for reduction, nitsume.