Green Scene

The unlovely slug — What exactly are these little creatures?


Years ago when I was young and foolish, I thought slugs were interesting.

I had my first camera and was looking for subjects everywhere. And so I found these long creatures with retractable antennae (whose movements are governed strictly by blood pressure, since they do not have a skeleton) that left a shimmering trail behind.

They looked just like snails, minus the shell.

Well, I lived to learn that no one has a good word for them, least of all gardeners. They can chew their way through an enormous amount of your favorite plants. They are horribly squishy should you accidentally step on one, And that shimmering trail is disgustingly slimy and hard to wash off — try vinegar instead of water, should you get it on your hands.

Fossilized ancestors have been found from the Cambrian period (between 541 and 485 million years ago), so they are certainly keepers. What exactly are they? They are clearly not insects and certainly not reptiles. And they are definitely not higher animals.

Well, it turns out that they are actually pulmonate (having lungs instead of gills) mollusks — phylum Mollusca, from the Latin mollis, meaning “soft” — making them related to bivalves such as oysters, mussels and clams, as well as squids, cuttlefish and octopi.

However, snails and slugs are the only members of the class Gastropoda, which includes only land mollusks. Gastro derives from “stomach,” and pod means “foot” — giving us “stomach foot.” Most explain that gastropod then refers to the peristaltic motion of the slug as it moves along on its muscular foot.

Personally, I think it would be more accurate to say a “stomach on a foot,” which is an apt description of their voracious eating habits.

While there are about 100,000 different mollusk species today — of which approximately 80 percent are gastropods — the American gardener has been blessed with only 32 different species. However, there are basically three species that can be found in Riverdale, none of them native. They are the leopard slugs — Limax maximus — the Arion slugs, and the Drocerus slugs.

So is a slug just a shell-less snail? Essentially yes. Clearly snails have shells, requiring them to live in calcium-rich environments, which adds both to their protection from predators as well as their ability to survive droughts by walling themselves into their shells to wait for wetter times.

Despite great diversity within the phylum, they all have a mantle, two nerve cords. The anus and genital openings are located toward the head because of evolutionary reasons. Slugs are descended from snails and the forward position prevented snails from pooping inside their shell.

While slugs inhabit almost every type of terrain excluding arctic conditions and deserts, their entire life revolves around moisture — how to find it, and how to keep their bodies moist. Therefore, they are out and about primarily from dusk to dawn when relative humidity is highest.

Mucus is also part of the solution. There is locomotion mucus, which allows them to glide over abrasive surfaces.

They have a different mucus on their backs which prevents moisture evaporation.

Slugs are hermaphrodites, meaning that they have functioning male and female organs at the same time. However, like many plants, they do not self-fertilize. Generally, the fertile adults seem to mate with both partners using the male form. But afterward, each partner lays eggs.

There are many suggestions as to how to manage slugs in the garden. Many assume that sharp materials will damage the slug’s skin. These materials include diatomaceous earth — the fossilized remains of a hard-shelled alga called diatoms.

Personally, I tried using the spiky fruit of the sweet gum tree — Luiquidambar styraciflua — and crushed eggshells. They never worked particularly well since, as stated earlier, the slug secretes a mucus designed to protect it as it moves over different terrain.

There are several commercial products based on iron phosphate or copper compounds that have been used successfully. Since slugs like dark, moist locations, I am going to try laying out upside-down flowerpots or wooden slats in strategic locations, checking underneath them for the culprits in the morning and removing them (I find that New York Times newspaper wrappers are perfect for the job).

There are several other anti-slug recommendations. Keep garden litter down. Concentrate on slug resistant plants, which means primarily those with leathery, stiff and aromatic leaves. Do not overcrowd your plants in order to allow proper air circulation.

Despite my aversion to slugs, it seems that they have many predators, including beetles and fireflies, toads, snakes, turtles, and various birds. Perhaps a kind word is, therefore, in order.

In any case, despite this year’s abundance of rain, I have seen remarkably few of them.

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Cambrian, Drocerus, Arion, New York Times, Sura Jeselsohn