Dr. Chris Barnhart described the world of gastropods in his Trail of the Missouri Snail presentation at our Master Naturalists meeting. In it, he answered my long-standing questions about "right handed"snails.
Counting macroinvertebrates in a stream is one factor in measuring water quality. When Stream Teams collect macro invertebrates to assess the stream quality, we count the number of left and right handed snails. Right-handed snails are very sensitive to pollution and need well oxygenated water to survive. If there is a high percent of left-handed snails, your stream is in trouble.
How do you determine "handedness" is a creature that has only one foot (poda)?
Counting macroinvertebrates in a stream is one factor in measuring water quality. When Stream Teams collect macro invertebrates to assess the stream quality, we count the number of left and right handed snails. Right-handed snails are very sensitive to pollution and need well oxygenated water to survive. If there is a high percent of left-handed snails, your stream is in trouble.
How do you determine "handedness" is a creature that has only one foot (poda)?
You can tell the difference by holding the shell so that its tip is upward and the opening toward you. If the opening is to the right of the axis of the shell, the snail is termed dextral -- that is it is right-handed. (snail on the right) This type of snails are very sensitive to pollution. They need plenty of oxygen to survive. If the opening is to the left of the axis of the shell, the snail is termed sinistral -- that is it is left-handed. (Snail on left)
Right-handed snails are also known as Prosobranch, or gill-breathing snails. They possess a chalky plate called an operculum on top of their foot, which they use like a door to close the shell opening. The length or width of the shell ranges between 2 to 70 mm. The great majority of shells are black, brown, tan and gray in color. Most gastropods are vegetarians, eating algae, and dead and decaying plants. Most often snails can be found in waters less than 3 meters deep in more calmer parts of the stream. (from Ohio.edu)
Left-handed snails do not have gills and get air from the surface. Since they are not dependent on oxygen levels in the water, they are able to tolerate poorly oxygenated and polluted water. The English word sinister comes from the Latin word sinestra, which originally meant "left" but later took on meanings of "evil" or "unlucky". When you find left-handed snails in your stream, the implications are indeed sinister.
Left-handed snails in the ocean may have an advantage over right-handed snails. They are much more uncommon than the right-handers in many environments. Research reported in Why are some snails left handed showed that crabs which eat the more common right-handed snails have claws especially fitted for opening their coil, and have difficulty manipulating the left-handed snail.* They tend to ignore the left-handed snail to the snail's eventual benefit. A similar phenomena has been reported in snail eating snakes in The Evolutionary Biologist. If you want to dig deeper into the snail shell, try this in Pubmed. Most species are composed entirely of individuals that are one or the other type. In exceptional cases, populations may differ in their handedness, or chirality, but within a single population, all individuals tend to be alike. The mechanisms are spelled out in Pubmed.
* NPR story on Southpaw Snails 'Dodge' Right-Handed Crabs
You can get into counting snails with your local stream by joining a Stream Team.
