Rostanga rubra is usually bright scarlet in colour and up to 2 cm in length. The body is domed and warty.The deep red coloration is derived from the red sponges Microciona atrasanguinea and Ophlitaspongia papilla on which it feeds. There may be scattered black spots dorsally and a characteristic yellowish patch between the rhinopores. Two finger-like oral tentacles project from the sides of the head. The mantle bears many short, blunt tubercles dorsally, together with defensive spicules. Up to ten simple pinnate gills may be present in a tight circle, which retract into a deep pocket or cup.Spawns in June and July.