Onchidella borealis Dall 1872

Biology/Natural History: Often found in as a group of several. Graze on diatoms from rocks and algae at low tide. On mechanical stimulation, the marginal tubercles or papillae produce secretions that repel potential predators such as seastars, especially Leptasterias hexactis, which retreats from an encounter. Hemigrapsus nudus will eat dead individuals but not live ones. Lays a gelatinous mass of 6-40 encapsulated eggs. Hatch as young juveniles.

Crevices, mussel beds, kelp holdfasts such as sea cabbage and Laminaria and red algae such as Odonthalia floccosa.

Geographical Range: Alaska to San Luis Obispo county, California

Depth Range: Intertidal. This species spends considerable time above the water, especially in caves. This may be related to the fact that it has a lung instead of gills.

This small, inconspicuous opisthobranch-like mollusk has a smooth dark dorsum which may be greenish, gray, brown, or black and sometimes has dots or streaks. It has no cerata. The foot is cream or tan. It has no shell. The margins of the dorsum have short blunt tubercles or papillae, often colored lighter than the rest of the dorsum. The animal has eyes on short stalks and has a pneumostome opening to an internal "lung" on the posterior end just dorsal and posterior to the anal opening. It is NOT closely related to the limpets and has no shell despite the common name. Length to 1.5 cm.

How to Distinguish from Similar Species: , are unclear)

Onchidella borealis is a species of air-breathing sea slug, a shell-less marine pulmonate gastropod mollusk in the family Onchidiidae.