Saturday, 10 November 2012

The Banana Slug – Nature’s Giant Recycler


Perhaps it is the mucus, perhaps the snake-like appearance or the habit of many species of slug to regard your garden and the carefully cultivated plants within as dinner – but the slug generally has a pretty bad press.

So, if you just groaned in horror at the picture above, you are in good company. A lot of people don’t like slugs. The sight of them in a garden has been known to turn even the most mild mannered in to mad mollusk murderers. Yet the giant Banana Slug, the second largest in the world (after the European Limax), has more than just its size and resemblance to a certain yellow fruit as a claim to fame. This is one of the unsung champions of the forest, for the banana slug only eats dead organic material which they then turn in to soil.

It is the color which first attracts. Even those averse in general to slugs can find the banana variety compelling.  As a member of the mollusc phylum they have soft bodies and no obvious shell.  A single foot, the muscularity of which would shame a young Schwarzenegger, transports the slug, found on the western seaboard of the North America, from California all the way up to Alaska, via a system of rhythmic waves. To ensure this foot does not get damaged it also secretes a layer of mucus and it is on this layer the slug travels rather than the ground itself.

The mucus has much more than just a single role, however.  Another purpose for this sticky secretion is to enable the slug to breathe.  They are pulmonates and this means that they only have a tiny pair of internal lungs. These are exposed to the outside via a pneumostone – have a look at some of these pictures and you can see it when it is open. The pneumostone collects moisture out of the air and extracts oxygen and is used when the slug is doing something particularly laborious and an extra supply is needed. Otherwise, the slime keeps the skin wet so oxygen can be breathed through it.

The hump on the back of a banana slug (Ariolimax columbianus) is called the mantle and that is also there for a purpose.  Underneath it are some rather important organs, the genitals and also the anus. It is the mantle which protects them. In some species, including the banana slug, this is where the mollusk will have a very small and flat shell.

The two sets of feelers on the slug’s head help out in a sensorial manner. The top ones are used to detect light and the lower ones provide the slug with its sense of smell.  The slug can retract these feelers, but in case they are destroyed they can be replaced – they will simply grow back!

The Banana Slug can grow up to 25cm in length and although they are often yellow the color depends very much on their recent diet, the amount of moisture in their locale and the light. They can look like an overripe banana, with black mottling appearing on their skin and they can also come in shades of green and white. Although rare, almost white examples have been found too.  Their various colors also help as a form of camouflage – on the floor of the forest they can blend in completely.

Yet when they are spotted by predators, the Banana Slug has yet another use for its mucus. It really doesn’t taste very nice and a slug will often be spat out by an animal before they can be chewed.  The mucus also contains an anesthetic which can make the predator’s mouth tissue numb, making chewing all but impossible. Not only that but the mucus will become stickier, making the slug the worst kind of chewing gum ever.

Most animals learn the lesson once and move on.  However, raccoons have been seen rolling slugs in the dirt, to cover up the mucus. They can then be eaten. Some snakes and birds will also eat the slugs without complaint, not to mention the salamander above! Another interesting fact about slime is that it can be used, if the slug has climbed a tree, as a string which can safely and quickly transport the slug to the ground.

As the banana slug travels (albeit very slowly) along the forest floor it sweeps up dead leaves and animal droppings (it also has a great fondness for mushrooms) in to its mouth, grinds them up. It deposits the digested remains, recycled soil humus, via its anus. As such it is called a detritivore, and the banana slug contributes greatly to the decomposition and the nutrient cycles of the forests in which it lives.

The Banana Slug is a hermaphrodite which means that they contain female and male organs. The mucus is used in the mating process as well – they leave another chemical in their slime which other slugs will find irresistible.  When they meet for mating purposes, banana slugs form a heart shape and exchange sperm. After they have laid their 70 or so eggs they will leave them to their own devices – banana slugs don’t do parenting.

The Banana Slug can live to be a few years old and so will see a number of winters and summers. When the temperature gets to just below zero they will hibernate. They will also estivate during hot spells – burrowing in to leaves or soils and insulating themselves from the heat in the cool, surrounded by a bubbly layer of, you’ve guessed it, mucus.

Perhaps slugs will never make it on to your own personal list of charismatic creatures. The Banana Slug, however, cares not if we find it charming, captivating or compelling. It does, however, contribute to its own ecosystem in ways which belie the reputation of its phylum and should be treasured as such.

First Image Credit Flickr User Going Slo

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