Identification
This elegant nudibranch, reaching lengths of up to 30mm, is a master of understated beauty. Its translucent, buff-colored body is adorned with numerous "cerata"—the finger-like respiratory plumes covering its back. To distinguish *Aeolidiella alderi* from its cousins, look for the "white ruff." The first few rows of cerata are dusted with a dense, opaque white pigment, creating a distinct collar effect behind the head. Unlike the similar *A. glauca*, which has a more uniform, muddy-grey appearance, *A. alderi* looks as though it has been lightly dipped in powdered sugar.
Habitat & Range
A true prize for the coastal explorer, this species is found along the Atlantic coasts of Europe, from the British Isles down to the Mediterranean. It is strictly a shallow-water specialist, inhabiting the lower intertidal and sublittoral zones. You won’t find it in open water; instead, peer under stable boulders and within crevices in sheltered rocky shores where its prey resides.
Behaviour
*Aeolidiella alderi* is a cryptic, largely nocturnal creature. During the day, it tucks itself into the shadows of the "under-stone" world to avoid desiccation and predators. If you are lucky enough to find one, you may also spot its egg mass: a delicate, white, concertina-like spiral ribbon firmly attached to the underside of a rock, resembling a tiny piece of discarded lace.
Diet
This species is a specialized carnivore with a very specific palate. It feeds almost exclusively on sea anemones, particularly the Daisy Anemone (*Cereus pedunculatus*) and members of the genus *Sagartia*. It uses a specialized radula to rasp away at the anemone’s column, often living in close proximity to its "victim" for a constant food supply.
Fascinating Fact
The "ruff" isn't just for show—it’s part of a biological heist! Like many aeolids, *A. alderi* steals the stinging cells (nematocysts) from the anemones it eats. Instead of digesting them, it transports these live weapons to the tips of its cerata (cnidosacs) to use for its own defense. Any fish bold enough to take a bite gets a mouthful of the anemone’s own stingers!