Sea Slug feeding and babies

February 16, 2000
From: Andrew McDonald

Hello Dr Rudman,

We have three dark brown with white spots sea slugs ranging from 50 to 150 millimetres long in an unheated room temperature salt water tank. They have large vertical fan like wings on each side which they like to flare and use to float and drift in the air bubbles from our air pump and under gravel filter. The sea slugs share the tank with about a dozen black fish (silver with zebra stripes), three lizard fish (speckled green and white bottom dwelling hunters) and a dozen prawns.

We have been feeding the sea slugs light green sea lettuce which they ravenously devour with their vertical mouths. I would be grateful if you could advise me on whether there is anything else they will eat and how often should they be fed?

So content are our sea slugs that they have produced six masses of strings of yellow eggs which are slowly turning pink. Could you please tell me if this colour change is good and how does one care for sea slug babies?

Locality: Victoria / New South Wales border, East Coast of Australia

Many Thanks

Andrew McDonald

McDonald, A., 2000 (Feb 16) Sea Slug feeding and babies. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Andrew,
I am pretty sure that your sea slugs are Sea Hares. If you go to that page you will find plenty of information on them and links to other pages on the Forum with stuff on Sea Hares.

Some species flap their the wing-like 'parapodia' to swim, but from your description, your animals are really just drifting in the currents. I suspect the species you have is Aplysia juliana which ranges in colour from pale brown to black and often has whitish patches on the body. Most sea hares produce a purple ink if they are disturbed. A. juliana is one of the few species that does not, so if your animals do produce purple ink let me know and I'll rethink my identification.

On food. Most species change their algal food during their life cycle. Your animals are apparently thriving Ulva - the Green Sea Lettuce, so I would leave them on that as other algae often foul aquarium water by excreting toxic chemicals. If you run out of sea lettuce one correspondent to the Forum said they used ordinary lettuce to feed sea hares.

The change in colour of the eggs (or rather the developing larvae/embryo) is quite normal. You will know when the larvae hatch because you will have swarms of tiny veliger larvae swimming in the aquarium and probably getting stuck in the surface film. Without special equipment it is unlikely that you can keep the free swimming larvae alive and feeding in aquarium conditions. How long will your adults live? In the wild they are only likely to survive a few months and definitely leass than a year. In aquaria things tend to live a bit longer but if they do die it's probably because of their age not the way you are looking after them.

Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (Feb 16). Comment on Sea Slug feeding and babies by Andrew McDonald. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Aplysia juliana

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