Wednesday, May 22, 2013

Bowling for Calliostoma

Calliostoma ligatum is a gastropod found on the rocky intertidal of the Pacific coast. Calliostoma are dioecous (i.e. have separate sexes) and, like most other archaeogastropods, free-spawn their gametes. Conveniently, C. ligatum can be easily induced to spawn in the lab if mildly harassed. 

The two individuals pictured here were collected intertidally at the South Cove of Cape Arago near Charleston, OR. I placed them out at room temperature in small bowls filled with sea water, set the bowls on my desk to allow the water to warm up, and flipped the snails upside-down. When they regained their footing (takes about a minute or two) I would flip them again. I repeated this for about fifty minutes (it usually only takes as little as half an hour) after which the snails began to spawn. The first picture shows a male actively releasing sperm, hence the cloudy water in the bowl.

This is a female which has recently released a mass of large 220 µm greenish eggs loosely connected by a jelly. She is flipped on her back, so one can see the bright orange foot rimmed with brown, the operculum, two cephalic tentacles and four pairs of epipodal tentacles. 

The third picture shows an egg two hours after fertilization. You can see here that C. ligatum eggs are surrounded by an egg envelope (a distinct membrane near the surface of the egg), and a thick bilayered jelly coat. Note the two tiny clear cells (at 6 o’clock) trapped inside the egg envelope. These are polar bodies, the tiny sister cells of the oocyte, which contain the DNA discarded during meiosis. As in many other marine invertebrates meiosis in Calliostoma is completed after fertilization. Presence of the polar bodies is a sure sign of fertilization. Note a single needle-like spermatozoan still trapped in the inner layer of the egg jelly (at about 1 o'clock). Clearly it did not make it!

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