Rhopalaea birkelandi is a species of sea squirt (a type of tunicate) belonging to the family Diazonidae. Tunicates are marine invertebrates that filter-feed in ocean waters. Like other tunicates, Rhopalaea birkelandi has a sac-like body enclosed in a gelatinous outer tunic. It has two siphons—one for drawing in water (incurrent siphon) and another for expelling it (excurrent siphon). Commonly inhabits coral reefs, rocky substrates, or areas with strong water currents, where it can efficiently filter-feed on plankton and organic particles.