by Joseph DeSisto
Although we like to imagine that slugs are, by and large, peaceful herbivores, the truth is they are highly diverse and occupy a wide range of niches. It’s true, there are many slugs that persist on living plants, and can become the bane of flower or vegetable gardeners. Others are detritivores, and eat decaying organic material, including leaf litter, animal waste, and carrion. A few are fungivores and eat mushrooms — some are especially fond of plasmodial slime molds (Keller and Snell 2002). And some, such as Testacella and Selenoclamys, are carnivorous.
But first, a short taxonomic digression. The term “slug” doesn’t refer to any one lineage of organisms, but instead describes a number of lineages within the Gastropoda (snails), all of which independently lost their shells. Shell-lessness evolved at least seven times within the terrestrial gastropods (Wade et al. 2001). As to why so many groups lost their shells, a number of hypotheses have been proposed. The one I like best is that slugs are better adapted to living in small spaces, such as in rotten logs or underground, where large shells would be a hindrance. Alternatively, shells may simply have disappeared in taxa where they were not needed to serve their primary purpose, preventing water loss.
Although it might seem like losing shells would make slugs more vulnerable to predators, and so be a poor adaptation, it is important to remember that shells are energetically expensive to make, and not making shells might have allowed slugs to invest more energy in reproduction. Also, shell-lessness freed slugs from depending on a diet rich in calcium, which can limit the ability of snails to colonize certain habitats.
Testacella, a genus of slugs from Europe and North Africa, is unusual for a number of reasons. First, all six species of Testacella have the remnant of a shell on their posterior. This shell may be vestigial, in which case it has no function and, like a whale’s pelvis, is merely a reminder of its ancestry. Or, the shell may be a form of protection specific to Testacella‘s burrowing lifestyle. Unlike the slugs gardeners are more familiar with, Testacella spend their time underground, tunneling through the soil. Here a large, spiral shell would be a handicap, but a small, flat shell protecting the slug’s posterior might provide some defense against predators that approach from the tunnel behind. Either way, the structure is what gave the genus its name: Testacella is the diminutive form of the Latin word testaceus which means “shelled” (Scarborough 1992).
The reason for talking about Testacella here, however, is because it is one of the slugs best-adapted to a carnivorous lifestyle. Like shell-lessness, predatory behavior has evolved numerous times in both terrestrial and marine gastropods, but Testacella provides one of the best examples. Burrowing by day and emerging on rainy nights, they specialize in preying on earthworms. The best description I can find of the feeding strategy of Testacella, and in particular T. scutulum, comes from Liberto et al. (2011). If you are interested in this sort of thing, I strongly recommend checking out the original paper here, not just for the description but also for the photography showing each step of the feeding process. The whole affair is quite complex, but I’ve tried to summarize it here.
Like all mollusks, Testacella has in its mouth a radula, which is a hard, chitinous structure bearing rows of teeth. In most slugs, these teeth are small and used to scrape off bits of material from a leaf or other vegetable structure, but in Testacella, the teeth are large and hooked, and used in prey capture. The radula is supported by an underlying structure called the odontophore, which is made of cartilage and extensible — this is how the mollusk ejects its radula to feed.
When Testacella detects an earthworm, it ejects its odontophore and hooks the hapless annelid on its long radular teeth. With the worm secure, but still alive, the slug retracts the odontophore back into its mouth, and begins to swallow. At this point the radula “collapses” so that it surrounds the earthworm, eliminating any possibility of escape, and the worm is swallowed alive through a combination of suction and movement of the odontophore. The whole process can take an hour or more.
In 2006 a new species of carnivorous slug was discovered in a garden in Wales. It was named Selenochlamys ysbryda (Rowson and Symondson 2008), and received quite a lot of media attention. Selenochlamys isn’t related to Testacella; their carnivorous habit evolved separately, and ysbryda lacks the distinctive miniature shell of Testacella. However, both species are burrowers, prey on earthworms, and share a number of adaptations. Most conspicuously, both have a long, extensible odontophore and a radula lined with many rows of long, recurved teeth for use in prey capture. The specific name ysbryda translates to “ghost” in Welsh, a reflection of its striking pearly-white complexion, and the slug is popularly known as the “ghost slug.”
Ghost slugs began turning up elsewhere in Britain, always in gardens with a healthy population of earthworms. At first it was surprising that a new slug should turn up in a back garden in Cardiff, but it turns out that S. ysbryda is probably an introduced species from the Crimean Mountains, where an apparently native population was discovered by Balashov (2012) alongside its closest known relative, Selenochlamys pallida. Probably ysbryda was first introduced to British gardens in plant pots. Clearly there are many marvels left to be explored. The discovery of the ghost slug teaches us that our own backyards can be as good a place to start as any.
Balashov, I. 2012. Selenochlamys ysbryda in the Crimean Mountains, Ukraine: first record from its native range? Journal of Conchology 41(2): 141-4.
Keller, H.W. and K.L. Snell. 2002. Feeding activities of slugs on Myxomycetes and macrofungi. Mycologia 94(5): 757-60.
Liberto, F., W. Renda, M.S. Colomba, S. Giglio, and I. Sparacio. 2011. New records of Testacella scutulum Sowerby, 1821 (Gastropoda, Pulmonata, Testacellidae) from Southern Italy and Sicily. Biodiversity Journal 2(1): 27-34.
Rowson, B. and W.O.C. Symondson. 2008. Selenochlamys ysbryda sp. nov. from Wales, UK: a Testacella-like slug new to Western Europe (Stylommatophora: Trigonochlamydidae). Journal of Conchology 39(5): 537-52.
Scarborough, J. Medical and Biological Terminologies: Classical Origins. Norman, Oklahoma: University of Oklahoma Press, 1992.
Wade, C.M., P.B. Mordan, and B. Clarke. 2001. A phylogeny of the land snails (Gastropoda: Pulmonata). Proceedings of the Royal Society of London B 268: 413-22.