Black-margined Nudibranch
Doriprismatica atromarginata
Four-lined Polycera
Polycera quadrilineata
About
This small sea slug wears a different outfit almost everywhere it turns up. It ranges from plain white to bold black and yellow stripes. That makes it one of the most varied sea slugs a diver will find on a northern reef.
Also known as: fourline nudibranch
How to tell it apart
It can be confused with the crowned nudibranch, another sea slug. Look for a scatter of raised yellow spots down the middle of its back. The crowned nudibranch does not have these.
Appearance
The four-lined nudibranch is a soft, smooth-bodied sea slug with a white or grey base colour. Many animals carry black, yellow, or orange stripes running down the back, though some show no stripes at all. Four, or sometimes six, yellow finger-like horns sit on the head. The feathery gills and the paired sense organs on the head, called rhinophores, are see-through white with yellow tips.
Habitat
Found on shallow reefs and rocky shores across the northeast Atlantic, from Scandinavia south into the Mediterranean, and separately off South Africa. It lives among bryozoans, the lacy, coral-like colonies it eats. It tucks under rocks from the intertidal zone, the strip covered and uncovered by the tide, down into deeper water.
Did you know?
For more than two centuries, divers and scientists treated every four-horned nudibranch in the northeast Atlantic as this one species. A 2020 genetic study revealed they were actually two separate species hiding under the same name, and split off a new one called Polycera norvegica.
Log it in Into the Blue
Describe what you saw and we name it, then keep every species you find in your Oceandex.
