Emerald Sea-Slug – Aldisa smaragdina

The Emerald Sea-Slug is a species in the class of the Gastropoda and thus belongs to the phylum of the Mollusca. Aldisa smaragdina was first described in 1982 by Ortea, Pérez and Llera.

I did not find a common name in any source. The smaragdina refers to the greenish-white gills, which is why I call them simply Emerald Sea-Slug.

Emerald Sea-Slug - Aldisa smaragdina gastropoda mollusca species diving canary islands atlantic ocean
Emerald Sea-Slug – Aldisa smaragdina


Aldisa smaragdina has an orange to intense red colouring. Many small warts spread over their oval bodies. Just behind the rhinophores, there is a band of white dots on both sides. The tips of the gills, which can be retracted, are also white. Between rhinophores and gills are two crater-shaped, darker circles. These give the emerald snail a perfect camouflage on Phorbas fictitius (aka Anchinoe fictitius), a sponge from which it feeds.

Its maximum size is 3cm.

Habitat and Distribution

Aldisa smaragdina lives on rocky ground and the sponge Phorbas fictitius. Nothing is known about the maximum depth, but it is associated with that of Phorbas fictitius. This occurs down to 60m deep.

When diving in the Canaries, we can rarely observe the Emerald Sea-Slug. If you see Phorbas fictitius, you should take a closer look!

Its distribution area extends over the Canary Islands, Cape Verde, the Azores, Madeira, the Spanish, Portuguese and Moroccan Atlantic coast and the Mediterranean Sea.


Aldisa smaragdina feeds on Phorbas fictitius. Through the two small craters on their back, they are perfectly camouflaged on the sponge.

I have seen Aldisa smaragdina only night active.

The Emerald Sea-Slug is a hermaphrodite, with the individuals fertilizing each other. Larvae hatch from the eggs, which feed on plankton and live pelagic. Later, they experience a metamorphosis to the sea-slug.







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