Range and natural history
Its range extends from southern California to Peru. Schools typically gather near shore, where the fish eat crustaceans and swimming marine snails called pteropods. The body is compressed from side to side, with a gray-green back, silvery flanks and a dark mark behind the gill opening.
Separating the Pacific thread herrings requires more than spotting the dorsal filament. Gill-raker counts are important, and older accounts sometimes combined this species with other Opisthonema. Historical records under those names need checking before they are assigned to this species.
