Information from Bill Rudman of the Sea Slug Forum:
heterobranch eggs can develop in a number of different ways. The most primitive way, and probably the most common is what is called planktotrophic in which the fertilised egg begins to divide and develops into a shelled veliger larvae with large wing-shaped velar lobes. This veliger larvae hatched out of the egg capsule and swims away, feeding on one-celled algae for a time before settling out of the plankton and turning into a microscopic crawling slug. A second type of development is called lecithotrophic and in this case a swimming veliger larvae emerges from the egg capsule, but does not feed in the plankton, and spends only a few hours to days in the plankton before turning into a small slug.

The third development type is called direct development because the embryo turns into a crawling slug within the egg capsule and hatches out as a small crawling slug. There is a whole range of direct developers from those which shpw no real sign of a veliger larvae in the grwth of the embryo to others which go through a recognisable veliger stage in the egg capsule before dropping the shell and turning into a slug all within the egg capsule.

I previously referred to Nerida Wilson's paper where she reports the development of Glossodoris vespa as ametamorphic direct which means that the embryo doesn't develop into a recognisable veliger larvae in the egg capsule before turning into a crawling slug. 'Ametamorphic direct' is a term given to species in which there is little sign of a veliger stage. However the A structures in your drawing are velar lobes, so they are quite a bit larger than I would expect in a species with ametamorphic development. However they don't seem to have long cilia along the edge so we can probably call them vestigial. Labelling development types is difficult because there are always species which straddle the border between one type and another.

Vestigial veliger stage anatomy (above)
A - velar lobes
B - larval kidney - a temporary structure lost after hatching
C - larval shell - in direct developer it is often reduced to a semispherical cap
D - body - visceral mass (digestive gland, stomach and intestine)
E - larval foot

Special Thanks to:
- Bill Rudman, Sea Slug Forum, Australian Museum, Sydney Australia
- Jeffrey Goddard, Marine Science Institute, University of California US
- Sunshine Coast University for the use of thier microscope, Olympus Dissecting Microscope SZ61
with an Altra 20 camera using Analysis Starter software. The highest magnifications were 45 X magnification
(i.e. 4.5 X with the objective lens and 10X with the eyepiece.






Day 3


Day 5


Day 8


Day 10


Day 12


Day 12 Photo Ross Eason


Day 12 Photo Ross Eason


Day 14


Day 17


Day 19


Day 21


Day 24


Day 27 Photo Ross Eason


Day 27


Day 28


Day 29


Day 29 Photo Ross Eason


Day 29


Day 29 Photo Deborah Milham-Scott


Day 29 Photo Deborah Milham-Scott


Day 29 Photo Deborah Milham-Scott


Day 29 Photo Deborah Milham-Scott - shell


Day 29 Photo Deborah Milham-Scott - shell