Marbled tuna (shimofuri) is the part in which it looks like the fat has fallen like a frost and melts like snow on your tongue then leaving only the umami taste.
The lined fatty tuna (Jabara) is the part that has white fat lines running through it. When you first take a bite, surge of aroma and umami flavor fills your mouth. After that, the potent fatty umami fills your mouth. The way the fat melts in your mouth differs between Jabara and marbling, so which you like better depends on preference.
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Revision date: May 4, 2021