Wednesday, August 17, 2011

moon snails

Moon snail shells are all over the beach. So are the sand collars the females build to house the eggs.  They're among the coolest gastropods (yeah, they eat with their foot) on the beach. They're fearsome predators of other mollusks. They can move around under the sand to stalk clams in their burrows.

I've never seen one with the foot extended. In fact, I mostly find empty shells or sometimes just the operculum  washed up in the wrack. The empty shells are popular with hermit crabs.

Moon Snail Shell

I did find one last week that was in its shell with the operculum tightly closed.

Moon Snail -- Operculum Tightly Closed

Operculum Washed up against a Rock

This is the time of year when I find lots of the sand collars, especially on certain areas of the beach near Emerson Rocks or near Sandy Point.
Part of a Sand Collar

The sand collars are made up of the snail eggs sandwiched between layers of mucus coated with sand . They look fairly solid, almost like rubber gaskets sometimes, but there's more to them than is immediately visible.

Sand Collar Closeup
The sand collar forms around the female's foot so it takes on the shape of that particular snail's foot. It's not a complete circle because the female has to leave herself an escape route. Basically she starts laying eggs and the sand sticks to the mucus. The collar keeps forming until she stops laying.
Sand Collar with Eggs Showing
The eggs pass through some larval stages while still attached to the sand collar. I'm assuming that what I saw and photographed yesterday (above) were eggs, but I'm not familiar with all the larval stages they go through. There were a ton of collars with eggs showing -- I don't know if gulls (of which there were hundreds) interrupted the egg-laying process or some other factor made the eggs visible.

More Partial Collars with Eggs
The other extremely interesting thing about moon snails is that they are a top predator. They eat clams -- even big ones -- by drilling a hole in the shell. They have drill appendage called a radula that they use to make the hole. They also secrete some kind of acid to help soften the shell for drilling. Once they've drilled the hole, they squirt digestive juices into the clam and suck out the meat.

Clam Shell with Countersunk Hole Drilled by Moon Snail
Yes, I had to look up the past tense of countersink.

Oh, and the main predators of moon snails are gulls.

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