Dive Dry with Dr. Bill

#186: Littorina Lust

Nothing could be finer than to be a Littorina in the morning... (with apologies to Gus Kahn who wrote the lyrics and Al Jolson who made the song famous). Now why in the world would this marine biologist be singing the praises of a tiny little snail so common on the rocks of the upper intertidal and splash zones along our coast? In fact these snails, commonly called periwinkles, live farther from the sea than some residents of Malibu! However, they have one of the most active sex lives of any species I know (but not in the Biblical sense). And we all know how important mating is to the success of any species, and to our scientific understanding of what a species is! Good biologists should be obsessed with sex, and I like to think I'm one of the best!

The scientific name Littorina means "shore dweller." Humans who frequent our rocky shores often encounter thousands of these small gastropods either crawling in tidepools where it is wet, or clinging tightly to the rocks where it is dry. These tiny snails are usually no more than 1/2" or 2/3" in length depending on the species. If there is more than one species of periwinkle in an area, they often occupy different levels above the tide zone. Those higher up are often better adapted to a "terrestrial" existence than the ones found closer to the water's edge. The ones in the lower zones often hide under algae to remain damp when the tide recedes. By occupying different zones, they reduce any potential competition between them.

There are several different species in this group of snails, but the two most common are the eroded and the checkered periwinkle. Both live from the far north, Oregon and Alaska respectively, to Baja California. As mentioned, they live high up in the intertidal zone or in the splash zone above it. During periods of low tides and calm seas, they are "high and dry." To avoid dessication, the periwinkle cements the opening of its shell to the rock with mucous. This allows it to remain moist inside the shell as well as expend little energy in holding the shell onto the rock.

The gills of these snails are adapted to breathe even when they are exposed to air for several months. They can also remain submerged in sea water for long periods of time. Temperature changes are tolerated easily so they can bake in the hot afternoon sun, or be cool during winter nights. Despite these high tolerances, they are often found grouped in shaded crevices or pockets in the rock surface which trap water. Apparently the young and smaller adults stay closer to the water while larger adults remain high up on the rocks. There one finds few microscopic algae, so the periwinkles feed largely on diatoms, blue-green algae and some green algae. Fortunately these snails are capable of fasting for weeks if they have enough water. Periwinkle species that live lower in the intertidal zone may also feed on the larger algae present there.

Like most snails, the periwinkles graze on the algae using a rasp-like structure known as a radula to scrape it from the rock. In the process the rock gets eroded away at reported rates up to an inch every 40 years! This is comparable to the effect of all the other erosive processes combined. Of course all this scraping means that the periwinkle must continually grow its scraping radula as it gets worn down during feeding.

In turn periwinkles are eaten mostly by predatory snails. Their nasty relatives drill little holes into the periwinkle's shell and digest the soft parts. Small hermit crabs often pick up these shells to serve as temporary homes until they grow too large for them. If you see a periwinkle moving across a tide pool at more than a snail's pace, it may well be a hermit crab instead! If a periwinkle falls from its high perch into the lower intertidal, it may be eaten by crabs or starfish that can't climb to where the snails are normally found but appreciate a "gravity fed" meal! I wouldn't suggest humans make a periwinkle stew since they are often intermediate hosts for various parasites.

The tentacles seen on the heads of snails are often used to follow mucous trails laid down by other snails. Without those tentacles they are said to run into things just like the bumper cars in an amusement park, or our rental golf carts in the hands of less than capable drivers! At least they travel at very slow speeds so liability insurance isn't necessary.

Often periwinkles are paired up in non-wedded bliss as you can see from the individuals in the image accompanying this column. These gastropods are not hermaphrodites, so the sexes are separate. Males of a similar species on the East Coast are said to create sexual pheromones (chemicals) which attract both males and females. In the eroded periwinkle of our shores, the female releases a chemical that triggers the males to mate. However, researchers have found that the male can't tell whether he is trying to mate with a female or another male until he makes contact (oops!). If two or more males try to mate with the same female, they will end up fighting over her... pushing one another with the leading edge of their shells.

Females lay their eggs near or below the water's edge, most likely to ensure they remain moist and viable. The individual eggs are encased in a disk-shaped package and these packages are stacked upon one another in clusters of 700 to 2,000 eggs each. Egg laying peaks at dawn and at dusk, and is often synchronized with other females in the immediate area. Larvae hatch anywhere from two to nine days, probably depending on temperature. They will then drift in the plankton and later settle out to begin their more sedentary adult lifestyle. Of course they are hardly sedentary from a mating perspective!

© 2006 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," or "Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Littorines doing what they do best... mating.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia