How many eggs do sea slugs lay?

October 4, 2000
From: MIKE




MIKE, 2000 (Oct 4) How many eggs do sea slugs lay?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Mike,
There is no simple answer to your question. In some species such as Vayssierea felis there can be as few as one or two eggs in an egg mass, while at the other extreme the Sea Hare Aplysia fasciata is estimated to lay as many as 25 million eggs(Thompson & Bebbington, 1969). The North American species Aplysia californica is estimated to lay 1.5 million eggs (Kandel, 1979).

If you have a look at the list of General Topics you will a number of them are to do with eggs. Have a look at them for a general background idea on the range of egg ribbons Sea Slugs produce and why there is such a difference in egg numbers. You could also use the Search Button and type in egg. That will give you a much larger list of pages to look at with pictures of Sea Slug eggs.

Basically there are three main strategies used by sea slugs. They can have very many very small eggs which hatch into free-swimming veliger larvae that spend some time in the plankton feeding before settling down. This is a dangerous life for the larvae, but because there are so many some usually survive. At the other extreme are slugs which have only a few large eggs which hatch out, not as free-swimming larvae, but as tiny crawling slugs. This relatively 'safe' way of producing off-spring is relatively expensive in energy so only a few eggs are able to be produced.

There is a sort of 'intermediate 'third method' where some species produce a reasonable number of quite large eggs. These eggs turn into free-swimming larvae which don't feed and only spend a little time in the plankton.

The number of eggs produced depends on many factors, including the size of the animal, how well-fed it is, and the type of larval development it has.

Best wishes,
Bill Rudman.

PS: Your email address is incomplete so I can't let you know I have posted this answer.

Rudman, W.B., 2000 (Oct 4). Comment on How many eggs do sea slugs lay? by MIKE. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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