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Snail Vs Slug: What’s The Difference?

In the past, we’ve covered whether a snail is an insect. There, we discussed gastropods, their behaviors, habitats, and their lovely shells, which sometimes aren’t a welcome sight among gardeners. However, one thing we need to discuss is the difference between snails and slugs. Snail vs slug – what’s that all about?

There are a few things to think about within this particular topic. For instance, are there differences in size? Are the shells the same among snails and slugs? We know there are land snails and freshwater snails. But are there distinct habitat differences between most snails and slug species?

We’ve determined that they both have that stomach foot – remember they belong to the class Gastropoda which comes from the Greek words gastros (stomach) and poús (foot). They’re both parts of the phylum Mollusca. We know they have the ability to completely destroy gardens quickly. Slugs and snails are very sensitive, slow-moving animals and they have many similar features. 

Among all these common features it seems difficult to tell these pests apart. Here, we’ll parse out the key variations between these two slow-moving creatures.  

What’s a Slug?

Slug
Slugs have an internal shell hidden inside their bodies. Source: AlexandreRoux01

Let’s start with a quick run-down of basic snail and slug features. So what is a slug? Slugs are gastropod mollusks (like snails) but they don’t have an external shell. They live in places similar to snails, in areas that are dark and moist. They love eating plants and vegetation. Sometimes they eat insects. They leave a trail of slimy mucus just like snails. They have a muscular foot, with epithelial cilia that assist in movement, and slender tentacles that sit atop their heads and around the mouth. 

What’s a Snail?

Snail
Snail shells can be quite pretty. Source: fdecomite

You might already know that a snail is a lot like a slug. But one main difference is the snail’s shell that is carried on its back. They are gastropod mollusks just like slugs. Their homes are dark, dank, and full of foliage and insects to munch on. They too have a slimy trail indicating where they’ve been. And they have similar anatomy to slugs in the form of the muscular foot and slender tentacles where they see and eat. 

What’s the Difference?

Snail vs slug
When comparing snail vs slug, there are similarities as well as differences. Source: me’nthedogs

Both snails and slugs have similar features. They have two pairs of tentacles and a fleshy foot. And both slugs and snails have similar behaviors. They are both gastropod mollusk members, and they both have shells. But here are a few ways to tell them apart. 

Classification

Scientifically, snails and slugs have different classifications. Yes, they’re both in the mollusk phylum and the gastropod class. But these mollusks are classified under different suborders. For instance, the snail falls under superorders of Neritimorpha, Caenogstropoda, and Heterobranchia. The slug, however, falls under Orthurethra and Sigmurethra. Each superorder has different subfamilies and superfamilies within them. 

You might wonder how classification has any bearing on the differences between snails and slugs. Know that taxonomy is based on key divergent traits and behavior. Let’s talk about those.

Behavior

Slug, snail – it’s all the same, right? When it comes to behaviors, not so. Snail species spend their lives in search of food. So, they don’t have regular sleep cycles. In fact, they sleep on and off for periods of 13 to 15 hours. Wild snails then awake from their slumber. They then come out of their hiding places and use all their energy for the next 30 hours in search of nutrients. When they sleep, they relax their muscular foot and retract their tentacles slightly. If it’s not moist enough, snails have a survival advantage in their ability to sleep for 3 years! In damp areas, they take on a more regular sleeping/waking schedule. 

The slug on the other hand is nocturnal. That’s why they’re so hard to control in gardens and that’s why you won’t find slugs on the ground during the day. They wait until dusk to make their way out of hiding places to feed on plants. And while they do share some of the same diet as their snail cousins, many slugs feed solely on plants. There are a few key species that have evolved to eat insects, but most slugs focus on plant matter as a main source of nutrients, in a general sense. 

Environment

Both slugs and snails live in similar conditions. But snails tend to be much more versatile in their habitats. You probably already know about the land snail, but did you know there are sea snails, city snails, desert snails, and wetland snails? They can live under the earth, in trees, and under leaves like a slug. Snail species are very specialized according to the environments they’ve evolved within. Freshwater snails live in water (like sea snails) but also on river banks. You might even find them in your aquarium eating plants. Land snails live in moist forest floors, but also deep under the desert sand. Because they’re tender, they hibernate in winter and proliferate in spring. 

On the other hand, slugs belong chiefly in cool, dark, moist areas. There just aren’t slugs found in deserts or within living trees, as snails would be. Slugs prefer a more specific niche in terms of their overall distribution. They are also more selective about seasons. A slug loves to feed on plants when it’s warm, especially in spring and early summer. That might make you think slugs and snails share winter behaviors. However, that’s not the case. A slug doesn’t hibernate as snails do. They overwinter. To protect their internal organs in the harsh cold, they burrow and live below ground where it’s warm in the adult stage. Sometimes they die before winter comes, and their eggs remain below the soil surface where baby slugs hatch and emerge in spring. 

Movement

There’s a huge (albeit hard to see) difference between the way snails and slugs move. The snail’s pace is based on the foot that must completely retract in order for any movement to occur. Snails have to make a lot of movement to get where they want to go. With waves of contraction and relaxation, land snails navigate the expanse burning a lot of calories. 

A slug moves at a slow and exacting pace. They maneuver much more than snails to get where they need to go. Instead of compressing the foot, they use their feet to choose which direction to go. A slug is faster than snails, though. Snail mail is called just that for a reason. Snails move at one millimeter per second, on average. Slugs move at different speeds depending on the species. 

Shells

Snail crossing crack
The foot of a snail can easily bridge cracks or gaps. Source: eliduke

The shell is the focus of divergence between these two animals. When we think of mollusks, we think of the shell they carry through life. Most gastropods have a shell, too. The snail is identifiable by the large exterior shell it carries on its back. This protective shell allows a snail to completely retract within if it comes into contact with sharp objects or predators. A snail is born without a shell. As baby snails feed and grow, their mantles (or the developmental sacs on their dorsal region) store calcium. Then the center of the shell spiral forms, and growth occurs outward in a clockwise spiral. The external shell is fully formed in adulthood, and calcium carbonate storage only serves to thicken the shell at this point. When the outer shell of a snail is complete, snails can mate. 

Did you know that technically both slugs and snails have a shell? This is one thing that solidifies their position in the gastropod mollusk classification. The difference between a slug shell and a snail shell is that slugs have a small internal shell. Their coiled shells are made of the same material and are also meant to store calcium, just like that of the snail. But the slug has what’s called a reduced shell or an internal vestigial shell. It’s still a coiled shell like the snail’s, but it’s an internal shell rather than external. Instead of protecting the slug (snail shells are primarily for defense), the coiled shell helps slugs retain nutrients. 

This key difference is one reason you’ll find snails in the human diet, but not slugs. Internal shells make consuming slugs difficult. 

Lifespan

Slugs and snails have different lifespans too. A wild slug tends to live anywhere from 1 to 6 years, while a wild snail lives from 2 to 3 years. In captivity, a snail can live to be 10 to 15 years old. That’s as long as the family dog! A slug in captivity won’t have as much divergence in its lifespan as a snail would. Slugs and snails have these different lifespans based on habitat and behavior.  

Size

Slugs and snails have divergent characteristics when it comes to size too. On average a snail can reach 10 inches long. Slugs reach a mean length of 15 inches. However, there’s more variation in size between slugs and snails than the average would suggest. The large black slug, for instance, only reaches 2.5 centimeters in length as an adult. Australia is home to the largest snail on earth, called the giant whelk. This marine snail has a shell that is 28 inches long. That’s just the shell! Banana slugs are the biggest slug in North America and top out at 8 inches long. If you’ve ever seen them come out at dusk, you know they’re a sight to see. 

A Word on the Slug and the Snail as Pests

Banana slug
The banana slug can be absolutely massive. Source: megankhines

Slugs and snails have a lot in common. But as far as animals go, the slug and the snail aren’t the same. Knowing the difference helps you determine the best control for pests in your garden, and pests like slugs can easily take out a plant in no time. When you control these pests, measure the traits we discussed here to see if you can tell the difference. Use specialized controls either for slug or snail. That way you don’t do ecological damage to one or the other.


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