When Nudibranchs hatch, are they just smaller versions?

Dear Alisse,
A good question. I haven't had time to prepare a page on this topic but there is quite a bit of information elsewhere on the Forum on the topic. Molluscs (clams, snails, slugs, squid etc) basically develop from an egg to a free-swimmimg shelled larval stage, which we call a veliger larva. As with all living things, there are some variations to that plan and Nudibranchs (and other opisthobranch sea slugs) show quite a bit of variation in how they develop from an egg to a crawling slug. There are three main development types and we can make a bit of a guess at what type a particular species will have by looking at the size of the egg.

As a general rule each species has its own 'typical' egg diameter - very small eggs almost certainly leading to planktotrophic veliger larvae which spend a long time feeding and growing in the plankton. At the other extreme very large eggs usually indicate that the larva is going to develop in the egg capsule, without a free-swimming veliger larva stage, to hatch out as a miniature crawling slug. We call this direct development. Between these two extremes are eggs which develop into lecithotrophic larvae (free-swimming non-feeding larvae) which spend a very short time, sometimes only a few minutes, swimming around before settling to the bottom and metamorphosing into a crawling slug. There are exceptions to this rule but it gives us an indication about the possible development for any species we are interested in.

As to egg size ranges, an English scientist, Tom Thompson, produced a range of egg diameters based on nudibranchs of known development types which we can use to get an idea about the type of larvae a species will have.
• Planktotrophic: 40-170 microns
• Lecithotrophic: 110-250 microns
• Direct development: 205-400 microns
[1mm = 1000 microns]

There are many places in the Forum where you will find photos and information on this topic:
• Larval Development of Aplysia oculifera
• Have a look at the Flabellina amabilis settlement page showing a planktotrophic larvae settling out of the plankton and turning into a slug.
• Have a look at this page showing differences between the larvae of two species.
• Have a look at the messages on the Hypselodoris zebra page for great photos of a lecithotrophic larvae settling and turning into a slug.

I haven't at present got any photos of a direct developing larva but hopefully the ones I've listed above will keep you going. If you are not sure how to use the Forum just click on anything underlined, or use the buttons at the top and bottm of each page, and you will be moved to another page
Best wishes,
Bill Rudman