Not many people give slugs enough time in my view. Sure, there are a handful of pest species that bring the reputation of this group of marvellously muculent molluscs into disrepute, by nibbling at your pansies or destroying your dahlias. But if you’re prejudiced against slugs, you risk missing out on a fabulous array of species and behaviour.
This month, however, I’m not writing about slugs per se, but about some other creatures they host. Next time you find a nice big slug, maybe the great black slug, Arion ater, I implore you to spend a few moments watching it. As the beast grooves around on its rippling lubricious foot, a large hole opens periodically on the right-hand side of its body. This is the pneumostome, or breathing pore, and it is the portal to the slug’s single lung. Now peer closer, and you may also notice something else.
Tiny white animated specks appear from within. Look closer still (you may want a x10 hand lens for this). If one of the specks pauses long enough, you might just be able to count its legs – eight of them. This pallid lifeform is in fact an arachnid that goes by the name of Riccardoella limacum, or the white slug mite. Slug mites have fascinated me since I first noticed them when snail-racing as a child (snails carry a similar species, the white snail mite.) It’s the way these mites suddenly appear. A heavily infected slug may harbour dozens. When the cavernous lung opens and the mollusc takes a breath, a frenetic flurry of these animals scampers over its body. In most cases, they somehow get back inside the lung moments before it slams shut, like a movie hero running for a closing door.
It was once supposed that the mites were simply nibbling their host’s mucous. However, we now know they’re blood-feeders. These are true parasites, and most of their sanguisugent lifecycle presumably occurs deep within the lung cavity itself. This is where, by definition, there has to be blood-rich vascular tissues over which gas transfer occurs. So if you’re a small mite, even one with the secret weapon of a stylosome (a hardened funnel caused by the host’s body re eacting to the secretions of the m mite’s mouthparts), this would makem sense.
Far from harmless passengers, th he mites cause much lower su urvival rates in their hosts. A slug’s feeding, breeding an nd winter hibernation are all af ffected – and the more mites, th he bigger the problem. Studies on n the related snail mites have fo ound that up to 40 per cent of a hosth population can be affected – a mighty impact for mini mites.
The mites lay their eggs in nside the natural life-providing ch hamber of their host’s lung, and th heir three nymphal stages all ta ake place there, too. So, given th his utopia within every slug (or sn nail), why bother to leave and g alavant around on the surface? Well,W as with much about these an nimals, all we can do is watch an nd make educated guesses.
It is possible that the adult mites’m propensity to seek out fr resh slime may be a behaviour th hat enables them to leave their hosth and transfer and infect new sl lugs or snails. Perhaps mites wew see on the surface are those withw eight itchy feet, individuals doingd a quick run-around to ch heck the slime conditions for sl lug-to-slug transfer.
Slug mitesm can be seen neear a slug’s pneum mostome or breathi ing pore.