Slugs, in all their slimy glory, aren’t usually associated with food. But banana slugs (genus Ariolimax) are named after their colour, not their taste. As tempting as it sounds, don’t try to lick one, because the slime secreted by these gastropods is an anaesthetic!
As detritivores, banana slugs – which can also be green, brown, or white – recycle dead organic matter by eating and excreting. They play key roles in decomposition and nutrient cycles, with their waste acting as fertilizer. On the other hand, they have plenty of predators: mostly small mammals, snakes, and salamanders. They’re not very good at escaping these hunters – they travel at about 10 metres an hour by contracting muscles in their foot and slithering across surfaces with the help of mucous lubricant. That mucous helps slugs defend themselves by numbing the tongue or throat of predators (and any moist surface with which comes in contact).
Slug slime is quite similar to human mucous. All mucous is made up mostly of proteins called mucins. These macromolecules form chains and become glycosylated, meaning carbohydrates (multiple sugar molecules) are attached throughout the polymer in repeated patterns. When exposed to water from the environment, the dry mucin granules expand to hundreds of times their volume. That said, mucins provide the viscous texture, not that numbing factor in banana slugs. Another use of the slime is to help the fruit-like creatures breathe by keeping their skin moist for gas exchange.
The mucous made of glycoproteins is called a liquid crystal, neither a true liquid nor solid, but in-between. The molecules are too dispersed to be a liquid but not compact enough to be a solid; they’re organized in a crystal-like pattern, hence the name.
Another fun fact about this genus consisting of six species is that they are hermaphrodites! This means that they have both male and female sexual organs. They can mate as either sex – or both, fertilizing themselves! This multipurpose mucous also aids in mating. When ready to mate, banana slugs release pheromones into their sticky trails to attract others. The mating ritual that follows is also quite unique. The courtship phase is very rough, in some species terminating in apophallation – the penis of one slug gets bitten off! Yikes! Some suspect this is an adaptive behaviour, to encourage the partner to devote resources to the fertilized eggs instead of continuing to mate as a male. Perhaps the limb simply gets trapped. It can be as large as the slug itself and emerges from the genital pore near their heads. It won’t grow back, but the slug can of course continue mating as a female.
Some salamanders and garter snakes are thought to be immune to the toxic chemicals found in the slime. Others aren’t so lucky. This numbing trait may have been discovered due to observations of shrews cleaning their mouths for up to an hour after eating a banana slug, or raccoons rolling them in dirt before eating them. I don’t think it was discovered by someone mistaking a banana slug for a banana split.
Haleh Cohn is studying Anatomy and Cell Biology at McGill University.
