Sudachi

Citrus sudachi:

The Sudachi is a natural hybrid between the Yuzu and another citrus, perhaps the Tachibana orange.

Much sought after for it's use in Japanese cooking the fruit is bitter and highly aromatic, about the size of a small mandarin, but rounder. It is a main ingredient of Ponzu sauce.

The Sudachi is also on the list of Citrus that are hardier than most, so another one for the hardy Citrus experimenters.