Living on or inhabiting gills; describing organisms that live on the gill surfaces of fish or other aquatic animals.
From branchia (gill) + -colous (suffix meaning 'dwelling in' or 'inhabiting'), following the pattern of 'arboricolous' (tree-dwelling). A specialized ecological term from 19th-century zoology.
Some parasitic copepods are so specialized for branchicolous life that they've lost eyes and limbs—they're blind little vampires perfectly adapted to live nowhere else but inside a fish's gills.
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