Aplidium californicum is a compound tunicate forming sheets, mounds or slabs on rocks and other hard substrates.

The tunic is jelly-like in consistency, 1 to 3 cm thick and a shiny yellow, orange, reddish-brown or a translucent white colour.

The individual zooids are brown or buff, 6 mm long and arranged in oval or elongate systems.

[2][3] Aplidium californicum is common on the west coast of North America from British Columbia south to Baja California, Mexico and the Galapagos Islands.

Phytoplankton and other small organisms get trapped in mucus threads secreted by the endostyle.