A long, slim aeolid up to 10 cm (4 in), its back and sides carrying dense rows of finger-like cerata in brown, blue and purple, with banded rhinophores held up at the front. Older animals are darker than young ones, and the difference is not age but cargo.
It farms. The slug eats hydroids, and from them it takes the single-celled algae that live inside them, moving the cells into its own digestive branches inside the cerata and keeping them alive there. Those algae photosynthesize, and the sugar goes to the slug: a well-stocked adult gets a large part of its energy from sunlight and can survive long periods without eating at all. That is why it sits in the open in bright light with its cerata spread, and why the browner animals are the older ones.
The second theft is the stinging cells. Like other aeolids it moves undischarged nematocysts from its hydroid prey into the tips of its own cerata and fires them at anything that bothers it. Handled, this species can raise a welt on thin skin - one of very few slugs here that can sting a person at all.
The name changed recently. What every guidebook calls Pteraeolidia ianthina turned out to be several species, and the animal in this ocean is semperi.
- long slender body carrying dense rows of finger-like cerata along both sides
- cerata brown to blue-purple, often with pale bands near the tips
- banded rhinophores held high, body raised off the bottom in the light
- length up to 10 cm
