Secrets of a mollusk egg

I found a few of these interesting embryos in a plankton tow taken off a dock in Charleston, OR in January 2013. Under a dissecting microscope, I could see the embryo itself was greenish and about 250 microns across, but it seemed to be surrounded by a halo several hundred microns in diameter. When I sucked the embryo into my pipet, I could see the “halo” contacting the inside walls of the pipet. As it turns out, the “halo” is a jelly layer, a characteristic of some embryos that, among other things, helps protect it from microbes (Hellberg et al. 2012). Oregon Institute of Marine Biology research scientist George von Dassow identified the specimen as a gastropod mollusk (snail) embryo.

A closer look under a compound microscope revealed many fascinating things about this embryo. In the first photo, you’ll notice two small, clear cells at about 4 o'clock. These cells, called polar bodies because they mark the animal pole, are essentially the waste products of meiosis. In many invertebrate phyla, meiosis II is not completed until after fertilization, so the resulting polar bodies remain inside the chorion (the membrane surrounding the embryo inside the jelly).

In the next photo, you should notice two things: first, there seems to be a hole in the chorion at about 10 o'clock (a penetration point of sperm), and second, a sperm is hanging out just inside the chorion at about 7 o'clock. Other similar penetration points were visible in other parts of the chorion, indicating that multiple sperm were able to enter the chorion but did not fertilize the egg (because the embryo looks healthy). Polyspermy is actually a big problem for invertebrates, especially those that are free-spawning and have external fertilization (Franke et al. 2002). Polyspermy causes morphological abnormalities in development, so complex physiological mechanisms exist to prevent it. A close coupling exists between the species-specific surface proteins on sperm (e.g. lysin) and egg (vitellin envelope receptor for lysin), so interspecific hybridization is also prevented (Hellberg et al. 2012).

After a few days, the embryo had developed into a gastropod veliger larva about 400 microns long. The reticulated pattern on its shell suggests this veliger belongs to the turban snail genus Calliostoma, of which there are 5 species in Oregon (Goddard 2001). The larva uses the compound cilia of its velum (ciliated structure at 4 o'clock) to feed and move throught the water column.

Franke ES, Babcock RC, Styan CA. (2002) Sexual conflict and polyspermy under sperm-limited conditions: In situ evidence from field simulations with the free-spawning marine echinoid Evechinus chloroticus. American Naturalist 160(4): 485-496

Goddard JHR. 2001. Mollusca: Gastropoda. In: An Identification Guide to Marine Larval Invertebrates of the Pacific Northwest. Edited by Alan Shanks. OSU Press, Corvallis.

Hellberg ME, Dennis AB, Arbour-Reily A, Aagaard JE, Swanson WJ. (2012) The Tegula tango: A coevolutionary dance of interacting, positively selected sperm and egg proteins. Evolution 66(6): 1681-1694