While doing field work on the Mendocino coast earlier this week, Eric encountered a large concentration of egg capsules on the underside of a plastic washer that had been in a mussel bed (see below).
The white eggs capsules were deposited by a Leather Limpet (Onchidella borealis). There's one adult Leather Limpet in the upper left corner of the photo. [The yellow capsules are flatworm eggs.]
Leather Limpets aren't true limpets (they're pulmonates), but they look like limpets without shells. Adults are ~6-10 mm long and live in the mid-high intertidal zone near algal holdfasts and shaded crevices. Adult coloration is highly variable. Many are yellowish-green, but others are whitish and some are deep maroon. They graze on diatoms and microalgae.
In the photo above, there are two Leather Limpets (one red, one white), along with one true limpet (lower right) and one chiton (upper right).
A few years ago, Eric found some Leather Limpet egg capsules in various stages of development. We took some photos under a microscope.
Leather Limpets are direct developers. The larvae complete their development within the capsule over ~30-40 days and then emerge as tiny juveniles ~1 mm long.
The capsules look a bit like bubbles. Note that each larva is developing within an individual chamber.
This is a late stage larva.
A juvenile is hatching and crawling over the outside of the capsules.
This next picture always makes me laugh. I can't help wondering if the juvenile inside (not yet hatched, but with two dark eye spots) is looking at the crawling juvenile and wondering what it's like on the outside.