Side-Gilled Seaslug

Berthella plumula

Berthella plumula is an oval-shaped sea slug that can reach a length of up to 60 mm. Its body exhibits a cream to orange coloration, often marked with reticulate patterns. The head is flat, and a prominent oral veil is situated between the propodium and the mantle. Notable features include protruding, enrolled rhinophores and acid glands in the skin that secrete sulphuric acid for defense. This species can be confused with Berthella stellata, which is smaller and has a distinctive white mark on its back, and Berthellina edwardsii, which is generally larger and more red in color. This sea slug inhabits rocky coasts in the north-eastern Atlantic, Mediterranean Sea, English Channel, and North Sea, typically found in the infralittoral zone at depths of up to 30 meters. It is often located under rocks in intertidal regions. As a slow-moving predator, Berthella plumula feeds on colonial ascidians of the genus Botryllus and Oscarella sponges, scraping its radula against rocky surfaces to obtain its food. Berthella plumula is hermaphroditic, allowing individuals to reciprocally fertilize each other by exchanging sperm. The reproductive season occurs in spring, and the species lays tube-shaped eggs in spiral formations. Encounters with this sea slug during dives can be intriguing, as divers may observe its unique shape and coloration while it slowly navigates its rocky habitat. The conservation status of Berthella plumula is not evaluated.