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Phylum Mollusca > Class Gastropoda > sea slugs
Slugs: nudibranch, sea hare or sap-sucking slug?
How to tell them apart?
updated Jul 2020

All these slugs belong to Phylum Mollusca but come from different Orders.
They are often mistaken for one another.

Order Nudibranchia
Sea hares
Order Anapsidea
Sap-sucking slugs
Order Sacoglossa
In most only the rhinophores are obvious, although in others, the oral tentacles are also visible (they look like a moustache). Usually two pairs of large tentacles are visible, a pair of flappy oral tentacles, and a pair of rhinophores. Usually only one pair of rhinophores visible.
Some have flowery gills on their backs. But these can be retracted into the body. And some nudibranchs don't have these external gills. Some nudibranchs also have a body made up of two flaps. The gills are enclosed in the body mantle. In many, the body is made up of two flaps. No external gills. In many, the body is made up of two flaps.
Nudibranchs have no external shells. Some sea hares have internal shells. Some sap-sucking slugs have external shells.
All nudibranchs are carnivores. Most eat immobile animals such as sponges, bryozoans and ascidians. Sea hares eat seaweed and tend to be seasonally abundant in synchrony with blooms of their food source. Sap-sucking slugs eat seaweed too and pierce the cells to suck out the juices.

More comparisons

Some nudibranchs don't have
flowery gills on the back.

Some nudibranchs have a pair of long
oral tentacles as well as rhinophores.

Some nudibranchs are large
and resemble stones!

These are NOT nudibranchs or sea hares or sap-sucking slugs

Cowries often cover their shells with a mantle
and are thus mistaken for slugs.

how to tell apart hairy slugs and snails
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