Mollusks

Infrequently Asked Questions

Answered by Tim Pearce, PhD, Assistant Curator, Section of Mollusks Tim Pearce 

Where I can find edible land snails? 

Are slugs related to leeches? 

Are snails ever found away from a water source? 

How many species of snails are there? 

What do snails eat? 

Why do I see more slugs in my garden than snails? 

What is the biggest mollusk? The biggest snail? 

Are all snails gray?  

Do snails need to find new shells to grow bigger? 

How do you tell male and female snails apart? 

How do snails know which way to whorl the shell? 

Why did the snail cross the road? 

Why are slugs slimy? 

 

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 Q: Where I can find edible land snails? I've resided in Pennsylvania 7 years and I have yet to see a snail in the area!  

A: The easiest place to find edible land snails might be a French restaurant. If you want to find your own, read on.

As far as I know, any land snail is edible. Although I don’t know any land snails that are poisonous, I do know one species that tastes very bad (Anguispira alternata, the tiger snail), and the digestive glands of some slugs are bad tasting (so just remove them!). Most species of land snails are very small (less than 1/8 inch or 3 mm), so eating them would be a challenge. But (despite being a vegetarian) I have eaten some of the native snails with shells 3/4 inch (20 mm) diameter.

Maybe you want to know where to find the snails that are commonly eaten as escargots. Those species are from Europe. Some of them have been introduced into the United States, but most of them are considered pests. For example, Helix aspersa, one of the most commonly eaten land snail species, is a very abundant garden and agricultural pest in California. I think Cepaea nemoralis is also eaten; it is another introduced species and not generally considered a garden pest. There are populations of C. nemoralis in eastern New York, and a few populations in Pennsylvania in the Philadelphia area.

Be sure to get permission from the land owner. Note also that transporting living land snails is now regulated by the federal government (APHIS, http://www.aphis.usda.gov/plant_health/permits/organism/snails_slugs.shtml). Also, if you collect the native species, be careful not to over collect. The native species generally reproduce more slowly than the introduced species, so over collecting could wipe out a population.

You might try eating slugs. In an evolutionary sense, slugs are snails that have reduced their shell and internalized it (yes, most slugs have a small internal plate-like shell). Slugs tend to have more slime, and a bad-tasting digestive gland. To remove the slime, I use half vinegar half water, much as you would when cleaning snails. The digestive gland occupies approximately the posterior third of the body, so you can cut a slit in the skin near the back and peel the skin forward to expose the digestive gland, then cut it off, and use the rest of the slug as you would a snail. Oh, you might also remove the shell, which is located in the back part of the mantle, which is the saddle-like part on the back. If you develop a taste for slugs, your gardening neighbors will be more than happy to let you collect all the slugs from their gardens!

 Q: Are slugs related to leeches? 

A: Since all living things are related at some level, yes. But despite having similar body forms, slugs and leeches are not very closely related. Slugs are in the Phylum Mollusca, which also includes snails, clams, and octopus. Leeches are in the Phylum Annelida, which also includes the earthworms and polychaete worms. Mollusca and Annelida are sister phyla, both in a group called Trochozoa, since they both have a larval stage called a trochophore. So, they are as related as any other closely related phyla. Another way to look at the relatedness is to compare other members of their phyla. A slug is more closely related to other mollusks such as snails, scallops, and squid than it is to a leech. The similar external body shapes of slugs and leeches is not a good clue about how related they are. As a guide, humans and starfish are members of closely related phyla, but look nothing alike! We use internal features and larval development to recognize that they belong to different phyla. So, yes, slugs and leeches are related, but the relationship is not very close, and their similar form does not reflect relatedness.

 Q: Are snails ever found away from a water source? 

A: Yes, there are land snails in the desert! Desert snails sleep for many weeks, waiting for rain. When the rain arrives, they quickly wake up, mate, eat, lay eggs, and by the time the water dries up, they are in place ready to sleep some more. Snails have been known to stay alive and dormant in relatively dry conditions for 4–6 years. Sometimes I wonder, though, if that is really living. What kind of life is that if you spend 95% of your time asleep?

 Q: How many snail species are there? 

A: I can answer this question at many levels. Counting all gastropods in the sea, in freshwater, and on land, a good estimate is about 80,000 species. For just the land snails, we estimate about 35,000 species in the whole world. In North America north of Mexico, there are about 1,000 species of land snails. In Pennsylvania, there are about 125 species of land snails, and we find more species now and then, so the number keeps increasing slowly.

 Q: What do snails eat? 

A: There are many different species of snails with many different diets, so that is a hard question to answer. It is kind of like asking what mammals eat. Most land snails are herbivores or detritovores, meaning that they eat plants or decaying material. Some land snails are carnivorous and will eat other snails or other small creatures. Haplotrema concavum is a snail-eating snail that lives in Pennsylvania. If you are a gardener, you know that slugs, which are basically snails with their shell on the inside, are voracious at eating your tender vegetables and flowers. The snails that eat decaying material are probably getting much of their nutrition by digesting the bacteria and fungus that are also breaking down the decaying material.

 Q: Why do I see more slugs in my garden than snails? 

A: I can almost guarantee that in eastern North America, all the slugs in your garden are non-native species from Europe. Species that are successful invaders tend to have fast reproductive rates, probably explaining why there are so many of them in your garden. For some reason, slugs seem to be better invaders than snails. In Pennsylvania, more than half our slug species are non-native, but only about 3% of our snail species are non-native. There are some serious agricultural pest snails in the world, but fortunately, they haven’t gotten to Pennsylvania.

 Q: What is the biggest mollusk? The biggest snail?  

A: The biggest mollusk would have to be the giant squid, more than 20 m long. The largest modern snails are sea snails, for example, Syrinx aruanus (Linnaeus, 1758), is one of the longest. Among land snails, the giant African snail (Achatina spp.) are some of the largest, but some species in the Megalobulimidae of South America are actually larger, more than 25 cm long. I know you didn’t ask, but you might be interested to know about the smallest land snails. Some of the smallest land snails in North America include Punctum minutissimum (Lea, 1841) and Guppya sterki (Dall, 1888). Even smaller land snails in the family Diplommatinidae live on some tropical islands.

 Q: Are all snails gray? 

A: As the philosopher Hegel would say, at night all snails are gray. Snails are actually a variety of colors when alive. In land snails, the periostracum is a protein layer on the outside of the shell that usually contains the color. After death (and sometimes before death), the periostracum will erode off revealing the calcium carbonate shell, which is white or gray. Many snails seem to be  in colors that blend with the environment, offering protection from predators that use vision to find prey. Brown is a common color for land snails. Some tree snails, such as Liguus in Florida and Polymita in Cuba, are brightly colored, often with bands. Sea shells are usually more brightly colored and patterned. In most cases, we don’t know why they have those colors, but there is lots of speculation!

 Q: Do snails need to find new shells to grow bigger? 

A: Snails are attached to their shells (like your muscles are attached to your bones) so they cannot come completely out of their shells. Unlike hermit crabs, who need to find a larger shell when they want to grow, a snail simply grows its shell larger when it needs a larger shell. It secretes more shell material at the edge of the aperture and continues the spiral around. Some snails change the form of the aperture when they reach maturity, changes that usually signify the end of growth.

 Q: How do you tell male and female snails apart? 

A: Almost all of the land snails in Pennsylvania are hermaphrodites, meaning that they are both male and female at the same time. The two with separate sexes are Hendersonia occulta (Say, 1831) and Pomatiopsis lapidaria (Say, 1817) and are not very commonly seen. So, for the vast majority of the land snails and slugs that you see, you would be right most of the time if you proclaimed them to be hermaphrodites.

 Q: How do snails know which way to whorl the shell?  

A: By far most of the snails in the world coil clockwise (called dextral coiling)...yes, even in the southern hemisphere. To determine the coiling direction, start at the beginning of growth and trace which direction the shell spirals. Another way is to hold the shell with the spire (the beginning of growth) upwards, and the aperture (the part the snail’s body emerges from) toward you; the aperture will be on the right (dextral) in most cases. Genetics certainly plays a role in shell coiling direction, and whole species (and even whole families) of left-coiling snails exist. But rarely, a left-coiling individual appears in a normally right-coiling species. Those occurrences might be developmental accidents rather than genetics. Research is ongoing on this exciting topic. For more information, type “snail” and “chirality” in your Internet search engine.

 Q: Why did the snail cross the road? 

A: To get to the shell station, of course! Actually, the snail didn’t cross the road. Baur & Baur (1990, Are roads barriers to dispersal in the land snail Arianta arbustorum? Canadian Journal of Zoology. 68:613–617) showed that roads are effective barriers to snail migration. In fact, they found that even a three-foot-wide dirt path was an effective barrier to snail migration. We can expect roads to have serious conservation implications for snails because barriers can fragment snail populations.

Q: Why are slugs slimy? 

A: Slugs use mucus in a variety of ways. For locomotion, they lay down a layer of sticky mucus to attach to the substrate, then a layer of slippery mucus so they can slide over it. To slow down evaporation, they have special mucus on the back. Some slugs can produce foul tasting mucus when disturbed, which helps to prevent predation.