Have you seen markings in the sand that look like these? Do you just ignore them or explore further? Well—you SHOULD explore!
If you dug into one of the circles, this is what you would find: a purple olive shell, Callianax (Olivella) biplicata.
These marine snails originally got their name from looking like olive pits. They are about 1” long and can be almost white, purple, as shown, to a dark purple-brown. They range from British Columbia to Baja, California and are found on many sandy beaches along our coast here in Central California. When found in the rocky intertidal, the snail is usually replaced by hermit crabs, although how the crabs manage to get into the small slit is beyond me.
The snails will usually stay just under the sandy surface and you can see their tracks if they are actively moving around. These tracks are mounded, like a mole, rather than on the surface.
The snail itself has a white or cream-colored foot that often extends up over the shell. The foot is very muscular and is what allows the snail to burrow and move rapidly under the sand. At the front end of the shell one can often see a white tubular siphon. When a snail is traveling, this siphon is extended above the surface of the sand, possibly to determine water depth. The highly polished shell also allows the snail to move easily under the sand without restriction.
Studies have shown that these snails actively move according to light and dark. They remain under the sand and inactive during the day and at dusk move up to the surface to hunt and feed. Although seemingly carnivorous, feeding on decaying marine snails, they are probably omnivorous, as they have been seen feeding on kelp and other detritus on the surface. They also seem to move up and down the beach depending on the tides.
The sexes are separate in Callianax and mating seems to occur throughout the year. Eggs are placed separately on rocks, or shells in a transparent capsule. Females are known to lay over 4000 eggs over a 6-week period! Hatching occurs in 10-28 days into the veliger (larval) stage, which swims to the bottom and stays there while maturing and developing the shell. The snail is said to have a life span of 8-15 years.
Many organisms prey on Callianax: octopus, cone snails, moon snails, some sea stars and birds, and probably also crabs and fishes. The Native Americans used the shells for ornament, money and probably ate the snails themselves. Many olive shells have been found in the local middens. A report on the use of olive shell beads by the local Chumash can be found here: http://www.californiaprehistory.com>rep0018.