Katsuobushi are the shavings of bonito or skipjack tuna that have been boiled, smoked and dried, while introducing Aspergillus glaucus - a mold that facilitates fermentation in the months long aging process. The result resembles petrified wood, and the shavings  are commonly used along with kombu to flavour dashi (the Japanese stock), or are seen 'dancing' atop takoyaki or okonomiyaki.