Plural of branchia; multiple gills or gill-like structures in aquatic animals.
From Latin 'branchiae' (plural of branchia), maintaining the Latinate plural form common in anatomical and zoological terminology.
The branchiae of a single fish can have thousands of delicate filaments creating enormous surface area—if you unfolded all of one fish's gill tissue, it could cover several square meters.
Complete word intelligence in one call. Free tier — 50 lookups/day.