Marine gastropod escaping its chorion

On Aprl 26^th, 2010 at 11:25 AM, I started a culture of a marine gastropod (snail) Calliostoma ligatum. When I looked at it 3 days later, the veliger larvae were still in their chorions (egg envelopes). While in the chorions, veligers beat long cilia on their velum really fast, then stop for a moment to take a breather, and then continue moving. By beating the cilia on their velum really quickly against the chorion, they were able to deform the chorion. This eventually ruptured the chorion and the larvae hatched! The first picture shows a veliger larva (complete with a shell and a foot, like a miniature snail) in the process of deforming its chorion with its velum. As you can see, the chorion is flattened on the side where it contacts the velum, while the rest of the envelope is still more or less rounded.

Eight days after fertilization, I looked at my Calliostoma culture again. At this point, most of the embryos were dead or abnormal. Larvae can be so temperamental! There were some larvae resting at the bottom of the dish that looked normal and moved their cilia. While under a cover slip, they used cilia on their velum to move around, and they moved FAST! Those I could observe had two eyespots and two tentacles forming in the apical area. At metamorphosis, the velum will degenerate, and the miniature snail will start crawling using its foot. This picture shows the veliger on its side, with one of the eyespots facing us. The velum is out on this picture (top). You can also see the foot (left) and the operculum (trap door) attached to it. At this time, I witnessed one of the other juveniles moving on its foot, — all I could see was the shell waddling around the slide.

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