Phallusia mammillata is a solitary marine tunicate of the ascidian class found in the eastern Atlantic Ocean and the Mediterranean Sea.

The animal feeds on the planktonic particles that it filters from the incoming seawater by passing it through a mucous net.

[3] Phallusia mammillata is one of a small number of ascidians that accumulate the element vanadium in their blood cells.

[4] It is unclear why the tunicate should do this, but concentrations of up to 350 mM have been found, some ten million times higher than in the surrounding seawater.

The eggs hatch into free-swimming, tadpole-like larvae, which within a few days settle on the seabed and undergo metamorphosis into juveniles.