gill cover; operculum
Anatomical term for the bony flap covering and protecting the gills of fish.
If a fish's gill cover is open, it may not be fresh.
I checked the position of the operculum on the anatomical diagram.
Common mixed kanji-kana spelling; the reading えらぶた is standard.
Full kanji spelling using the rare character 鰓; also read さいがい in some contexts.
Katakana-hybrid spelling sometimes seen in informal or technical writing.
Full katakana spelling; may appear in biological contexts or emphasis.
Compound of えら (gills) and 蓋 (lid, cover). The kanji 鰓 is a specialized character for gills, but the mixed kana-kanji spelling is more common.