Sand-encrusted capsules

This is a follow-up to the Tinted Wentletrap post on 26 June 2012.

I had wondered if there were egg capsules next to one of the snails.  I went back to take a closer look, and here's what I found.

 

Sure enough, they were egg capsules!  Tinted Wentletraps lay small egg capsules (~1 mm across) that are encrusted with sand and strung at intervals along a thin, elastic strand (see next photo).

 

One of the capsules was cracked open and you could see tiny white embryos inside:

 

 

Amy Breyer described these capsules in her Master's thesis at Sonoma State University in 1981 (Observations on the reproduction, feeding, and ecology of the wentletrap, Epitonium tinctum).  She found between 17-60 eggs in each case, with larger females producing larger capsules and more eggs per case. 

Impressively, she was able to watch the females producing egg capsules — first releasing a mucous-coated capsule, then attaching it to the thread, and finally molding sand grains (collected in a pocket within the snail's foot) around the capsule before it hardened.

It takes about a week for the embryos to hatch and emerge as larvae from the capsules.  This stage (called a veliger) then swims in the plankton before eventually undergoing metamorphosis into a juvenile snail.

Here's one more view of these unique capsules:

 

You may be wondering, why does the snail bother to coat the capsules with sand?  Although I don't think it has been studied yet, Amy suggested several hypotheses: camouflage, defense against predators, or protection from physical factors (wave action, desiccation, temperature changes).  Which do you think it is?  Or do you have other ideas?