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.
The colour of this nudibranch varies from bright red to pale orange-yellow with tiny black spots scattered on its dorsum. The mantle bears many minute tubercles and defensive spicules. There is a characteristic pale patch between the rhinophores. Adults may attain a body length of up to 15mm. Unlikely to be confused with other British dorids except perhaps with the red variety of Archidoris pseudoargus
R. rubra has been found around the Orkneys and at St. Abbs Head on the North Sea coast as well as on the south and west coasts of the British Isles. Further distribution from Norway south to the Mediterranean.