Common Name: Black Slug – From the Middle English slugge which is of Scandinavian origin slugga subsequent to the Norse invasion of Britain in the 9th century, it was used to describe a heavy, slow person. The ponderous motion of the mollusk is sluggardly.
Scientific Name: Arion ater – For reasons that are obscure, the generic name Arion is taken from a semi-mythological ancient Greek poetic singer who is most well-known for having been saved from pirates by attracting a dolphin to the ship with his song who saved him when he was thrown overboard. The savior beached dolphin died ashore; the slug, like a slimy marine mammal, a possible resurrection. The species name ater is Latin for black.
Potpourri: As slug is a sobriquet for a particularly lazy and inept individual, it is not surprising that the opprobrium carries over to the unwitting mollusk. It is hard to find terms of endearment for a slimy blob that consists primarily of a rapacious mouth that opens into a stomach that is dragged along the ground by a single, mucilaginous muscle called a foot. But when one considers that a slug is essentially a snail that ecologically outgrew its shell, there may be room for mitigation. And in that they are one of the only members of the prolific and edible oceanic mollusks that have managed to subsist in the desiccated terrestrial environment, one might even consider that they are due a modicum of respect.
A slug is a terrestrial gastropod mollusk that has a reduced or non-existent shell; the term semi-slug is sometimes used for the species in transition from shell-bound snail to bareback slug. Mollusca is a phylum of the animal kingdom and comprises about one quarter of all known marine organisms; Gastropoda is the class of snails and slugs that are its most diverse sub-group. About half of the 70,000 species of gastropods are terrestrial and they are sometimes referred to as pulmonates in reference to having traded the aquatic gills for aerobic lungs (pulmo is Latin for lung). The most commonly seen slug in forest habitats is in the taxonomic group Arionidea, which are sometimes called round-back slugs. In spite of what might appear to be a remarkable degree of similarity, slugs are a diverse, polyphyletic lot, the result of several independent transitions from their shelled forebears which is called convergent evolution. While counterintuitive, the rejection of the palladium shell to “slugdom” conveys survival advantages that evidently are successful; slugs are found in multitudes in almost every conceivable environment on a global scale. It is estimated that one acre of farmland has about 250,000 individual slugs from over 50 different species. There is obviously more to slugs than meets the eye, especially since most of them are hypogeal; they spend 95% of their summer hours underground.
The transition from snail to slug is evident in nature as there are about 1,000 species of semi-slugs and only about 500 species that are true slugs. The ecological forcing functions that militate against having a protective shell are not obvious, as its advantages seem compelling; what is a turtle without a shell? Carrying your home on your back requires both energy and resources. The calcareous shell requires calcium, which is deficient in some terrestrial habitats. The energy required to build the shell is substantial; it must also be maintained and transported using the life energies that would otherwise be employed in corporeal growth and subsistence. Shells are bulky, restricting passage in narrow spaces that might otherwise be employed for concealment. It is quite evident that in the survival trade off between having a bulky, heavy, protective shell and doing without, the latter is frequently preferential. The only other problem is water retention. In that the vast majority of the mollusks are aquatic, the pulmonate slug transition to land required adaptations for the retention of water that are physiological and behavioral; not as serious for the snail whose shell serves as a sealed sump. The most significant of these adaptations was the synthesis and distribution hygroscopic mucus that is the essence of the slug.
That mucus is perceived as pejorative is unfortunate, as it is a vital and ubiquitous substance produced by animals for protection against environmental hazards; its etymological origins from the Sanskrit word for slippery – muncati – attest to its pedigree. It is important for humans and quintessential for slugs. There are at least two different kinds of mucus produced by the slug’s epithelium skin layer: a mostly aqueous (97% water) compound produced by the foot-skin cells for locomotion; and a more viscous variant produced by the upper body mantle. Research by limacologists (limax is Latin for slug) has revealed that slime precursors are elongated granules produced within the cell that break open when released to hygroscopically absorb water to over one hundred times their initial volume. The result is a complex fluid with non-Newtonian properties, acting like solid glue at rest and reverting to liquid when stress is applied, the reason slugs can ascend vertically inclined glass by using stick-slip for locomotion. This factor alone would warrant some consideration for the complexity of slug slime. However, there is much more to it than that.
While still in a nascent state, research on the chemical composition of limacine mucus suggests that it varies according to species and that many compounds are involved including glycoprotein enzymes, hyaluronic acid, antimicrobial peptides and zinc, copper, iron and manganese ions. The driving force of nature’s innovative chemistry that promoted slug survival in a world filled with profound challenges offers promising complex molecules for human applications. There is some historical, though largely anecdotal evidence to support this. The skin sealing and healing properties of slug slime were acclaimed by the renowned Hippocrates of Ancient Greece; more recently, Chileans harvesting snails for the French escargot market reported its remarkable skin lesion healing properties. On the far side, the swallowing of garden slugs to relieve the symptoms of gastrointestinal distress was ostensibly practiced in Italy, an application supported by formulations for syrup made from slugs in southern Canada to treat ulcers. There are even slug slime wart removal remedies such as one from an 1898 label on a jar containing a slug in a museum in Oxford, England to “Go out alone and find a large black slug. Secretly rub the underside of your warts and impale the slug on the thorn. As the slug dies the warts will go.” While all of this may seem a bit phantasmagorical, recent research in the use of slug mucus as a wound healing pharmaceutical has proven some efficacy; U. S. Patent 2009026349 was filed for its use in burn treatments. It has recently been demonstrated that there credible wart removal properties. Leeches were approved by the FDA several years ago as medical devices for skin graft therapy; adding a slug or two to heal the wound might be next.
The success of a species is dependent on three factors: finding enough to eat; not being eaten; and successfully propagating the next generation in quantities that equal or exceed the current population. Slugs excel in all three. Slugs are omnivorous eating machines that speciated to consume everything from algae to other slugs including fungi, carrion, feces, insects, and, to the dismay of gardeners, plants. The chopping-block maw affords access to replaceable rows of multiple tiny teeth called the radula (from rado which is Latin for scrape) that, as its name implies, scrape the substrate of nutrients to pass ingest to the stomach for processing. Some slugs can and do consume over twice their own body weight in a single day. While slow, they inexorably extract nutrition, the characteristic fenestrated leaves are testimonial evidence of their repast.
Not being eaten would seem to be a challenge for a plump, boneless meal of muscle proteins with many notably pernicious predators including most reptiles, many birds and a few mammals. While slugs can see from the tops of their extended tentacle-born eyes and can smell from the two lower and shorter tentacles near the mouth, they can hardly run or fight. One study found that a garter snake was able to assimilate 62 percent more of the energy from slugs as compared to fish due to the relative ease of digestion. Slugs require defenses to survive; sliminess, crypsis and behavioral adaptation are manifestations to this end. While the thin mucus lubricates propulsion for offense, the thicker mucus provides defense; it is secreted copiously when under attack. Not only is a slimy, adherent, rounded blob difficult to pick up and hold, particularly with the chop-stick beaks of birds, but it can actually clog the mouths of its smaller predators like lizards and shrews; there have been several reports of dogs gagging on slugs. There is also a matter of taste; the mucus of some slugs is repellant to some predators – this is likely due to association with alkaloids that indicate toxicity. Cryptic coloration and morphology are employed in the earth toned flesh of most slugs that may also be grooved to simulate the texture of plants. Those that are toxic are generally brightly colored, employing aposematism to alert predators in advance of an ill-advised attack. Perhaps most importantly, slugs hide. Feeding primarily in the darkened nocturnal hours, slugs spend only about five percent of the daylight hours where they can be seen. Their morphology is subterranean as both the two larger eye-stalk tentacles and the two smaller olfactory tentacles can be retracted for streamlined burrowing. Sometimes they don’t move at all, estivating summer analogous to bears hibernating winter.
While eating and not being eaten are admirable slug attributes, their ability to reproduce sexually has no equal in the animal kingdom. They are all hermaphroditic so that copulation, when successful, results in mutual insemination and both becoming gravid. In isolation or due to social rejection by another slug (certainly not for ugliness), a slug can even mate with itself, thereby fulfilling the scatological human epithet that is usually rejoined by the observation that it can’t really be done. Slugs are the Q in LGBTQ. As a matter of evolutionary interest, they all start out as males and develop female reproductive organs only when mature at an “age” of about 6 months. This will probably fail to inform the gender equality debate among humans, but it is an interesting case in point. Slug courtship is a surprisingly complicated affair. Colocation and species validation are achieved by one slug following the slime trail of another, using the olfactory tentacles to ensure that the mucilaginous pheromones are consistent with those sought. Following a tactile encounter to establish mutual acceptability, eversion of the genital openings precedes a four hour long copulation during which both partners remain motionless, a rather sluggish affair. Consummation entails the successful transfer of a spermatophore from one slug to its partner and vice versa to effect mutual fertilization. They then part and go their separate ways to lay eggs in clutches of up to about 50. As a measure of the sexual drive that is powered by evolutionary survival adaptations, each mother/father slug can engage in multiple tristes in a single year to produce up to ten broods; even considering low survival rates, 500 eggs per year per slug is prodigal.
Slugs are highly successful organisms by virtue of adaptations to eat almost anything that doesn’t move, to avoid almost anything that does move, and to replicate themselves in geometric proportions. They are accordingly agricultural pests on a scale commensurate with rodents, insects, and fungi. Slug control measures range from chemical limacine poisons to dishes of beer placed strategically around the garden depending on the scale and degree of infestation. The most prevalent slug poison is metaldehyde, a chemical used in solid fuel pellets for camping and military applications. Although possibly apocryphal, the use of metaldehyde for slug control was purportedly discovered accidentally in the 1930’s by farmers in southern France (possibly South Africa) who found dead slugs in campsites where the so-called meta-tablets had been used. The chemical apparently works by disabling the mucus production glands so that the slug can neither move nor protect itself. From this humble beginning, the efficacy of “camp stove fuel” spread through gardening groups which resulted in the commercialization of slug pellets comprised of metaldehyde and other related compounds. The burgeoning use of these products with the expansion of suburbia and its attendant “shruburbia” has had the now expected but unintended consequences. Cats and dogs apparently find them close enough in taste and texture to their own edible food pellets that they make the sometimes fatal mistake of verisimilitude; the National Animal Poison Control Center reports slug bait poisoning as one of its major claimants.
Attendant to mammalian similarity, it is likely, though unproven, that other wildlife is similarly affected; avian deaths have been reported. There are many alternatives, even a benign iron phosphate compound named, inevitably, Sluggo. For the environmentally orthodox, slugs can be averted to some extent by surrounding a garden with a narrow strip of material that slugs traverse only with difficulty, such as sand, ashes or soot. Britain, with its moist climate, is the self-proclaimed slug capital of the world. It is likely that the use of beer traps originated there; the local ales are seen as panacea for all of life’s labors. Field experiments have demonstrated that slugs are attracted to the beer but that they typically take a sip and move and that only a few fall in and drown. Apparently, the slugs are drawn to any mixture of sugar and baking yeast, so, one might well ask, why waste perfectly good beer?