Onchidella borealis is a small, inconspicuous opisthobranch-like mollusk. The dorsum has a leathery appearance. The colour may be dark greenish, gray, brown, or black and sometimes has dots or streaks. It has no cerata and no shell. The foot is cream or tan.
Onchidella borealis has stalked eyes (short stalks) and has a pneumostome opening.Onchidella is an air breathing sea slug.
Its alimpet-like sea-slug, but this species is not closely related to the limpets.Onchidella borealis graze on diatoms from rocks and algae at low tide.limpet-like sea-slug