Many people wonder why snails have a large shell on their back and slugs don’t. Well, the short answer is that snails haven’t changed much over the past 500 million years but slugs are the snails that made some changes…
Snails have shells to keep their body moist and protect them from predators. Slugs evolved from snails and lost their big shell over the years as they found ways to survive without one. Most slugs still have a small shell under their skin that you can’t see, but the rest have no shell at all.
Knowing how slugs survive without a shell and what snails do with theirs will help you find slugs and snails in your yard and understand their behaviors and the damage they cause. You can then take the best steps to get rid of slugs and snails or keep them off your property as best you can.
How slugs became snails without shells
Slugs are snails that found ways to live without an outer shell and lost theirs over time. Slugs and snails fall in the same class called gastropods, but they’re like cousins – related to each other with similar traits, but each part of its own family. Snail families have an outer shell, slug families don’t.
Here’s how some snails turned into slugs…
Snails came first. For a snail to be called a snail, it must have a shell that’s big enough to fit its entire body inside. Over time, some snails’ shells started getting smaller, but their body stayed the same size.
When their body couldn’t fit into their shell, they stopped being snails.
Snails with a visible outer shell that’s too small for their body to fit inside are called semi slugs. These are snails that have started turning into slugs, but they still have a shell on their back where their internal organs sit for protection. Snails also keep their internal organs inside their shell.
There are still many semi slugs today. They are mostly found on tropical islands where there’s a lot of humidity in the air to keep them moist and few predators to eat them.
Slugs evolved from semi slugs and their shells are even smaller: Most slugs have made their shell so small that it stays under their skin, but some got rid of their shells completely. Slugs keep their internal organs in their foot body as they don’t have a shell for organ storage.
The shell is one of the main differences between modern snails and slugs. Click here to find out all the differences between slugs and snails.
The following video from the Carnegie Museum explains how snails became slugs without a shell, with examples of snails, semi slugs, and slugs…
Why do snails have shells?
Snails have kept their shells for the past 500 million years because their shell helps them to survive:
- A snail’s body is soft and spongy, but a snail’s shell is its hard skeleton on the outside of the body. This exoskeleton protects the snail from predators that want to eat it. When a snail is threatened, it pulls its soft body into the hard shell for protection.
- A snail’s internal organs, including its heart, lung, anus, and reproductive organs, are found inside the shell. The shell protects these organs throughout the snail’s life.
- A snail’s body must stay moist, otherwise the snail dries out and dies. When the weather gets too hot and dry, the snail crawls to somewhere safe (like up the walls of your house) and pulls its body into the shell before sealing the shell with a mucus plug. The snail’s body can stay moist and healthy inside a sealed shell for a long time, even years.
- In areas with long, cold winters, some snails retreat into their shells to hibernate until the days get longer and the weather gets warmer.
This means that you find snails in areas where there is a lot of calcium to eat to grow strong shells, where there is water or humidity in the air to keep a snail’s body moist, and where there are places for snails to hide or climb high enough to be out of reach of predators.
Why don’t slugs have shells?
Slugs don’t have outer shells because they evolved from snails and lost their external shells over time. They no longer needed these shells because they found ways to survive without having a shell to hide in for protection or using a shell to store their internal organs.
Slugs stopped growing outer shells because:
- They wanted to be able to live in areas where there isn’t much calcium to eat. Because a slug doesn’t have to eat a lot of calcium to keep its shell strong and healthy, it can live in many more places across the world and in areas where only small snails with small shells can survive. Slugs are happy to live in acidic soil, where there is very little calcium.
- They didn’t want to carry the weight of a shell on their back. Some slugs can move a bit faster than snails because of this, but it also depends on the size of the slug. However, slugs save a lot of energy by not carrying a shell wherever they go.
- They wanted to fit into spaces that are too small for a snail shell. Without a shell, slugs can crawl into spaces where a snail with the same size body (but a shell attached) will never fit.
To survive without a shell, slugs stay in areas with a lot of rain, water or humidity, so they don’t dry out. When the weather gets too hot or too cold, slugs bury themselves deep in the soil to stay alive.
How to get rid of slugs and snails in your yard
If you want to get rid of slugs and snails in your yard, below are the best tips and recommended products from Amazon to get the job done:
- Sprinkle these organic snail and slug killer granules to draw slugs and snails out of hiding and kill them. The good things about these granules are they’re biodegradable, safe to use around pets, children, and wildlife, and are effective in all types of weather (even rain).
- If you have a problem with slugs or snails on certain plants or in certain areas, spray the plant or area with this non-toxic slug and snail repellent made with essential oils. This spray can be used indoors or outdoors, and even around the perimeter of your yard to keep slugs and snails out.
- In the evening, water your garden well and set a beer trap by placing a small plastic bowl in the ground near the plants the slugs or snails are eating. Put the bowl deep enough to leave 1 inch above the ground or cover the trap with a loose lid to stop insects from falling in. Fill the bowl halfway with fresh beer. Empty it out and put fresh beer in every night until you no longer find slugs or snails in the morning. If you don’t want to make your own beer trap, you can buy beer traps from Amazon.
- The DIY option is to go out and pick up slugs and snails with your hands (I suggest wearing gloves for this). The best time to look for slugs and snails is after sunset and rainfall. Click here to find out what to do with snails collected from your yard.
- If you have rose slugs, try making this dish soap spray and follow the instructions to kill the slugs with a direct spray of the soapy solution.
Click here for my full article on how to stop slugs from eating your plants.