Janthina lays egg capsules and attaches them to the underside of its float. In the photo below, it's easy to see the bubbles making up the float on the left, and the egg capsules (filled with embryos!) on the right:
A close-up reveals the small white embryos inside the clear capsules. One book estimated that Janthina umbilicata has about 80 embryos per capsule. What do you think?
We kept this snail in the lab, and here's what the embryos looked like today (4 days later):
They've grown quickly! Their cap-shaped shells are an intriguing shade of brownish-red:
The embryos are still very small (the egg capsules themselves are only ~2 mm long). This is the best view I could get:
Eventually, the larvae will emerge from the capsules as free-swimming veligers. But I haven't been able to find information on development time within the egg capsules, or the duration of the larval stage once free of the capsule. For example, how long does it take until the swimming larvae metamorphose into floating snails and start living at the surface of the ocean? When do they create their first float? (How many bubbles are in their first float? When do the shells become purple? Does the color change happen when they start eating Velella?)