Hid­den Bri­tain

BBC Wildlife Magazine - - Wild October - NICK N BAKER is s a nat­u­ral­ist, au­thor and TV pre­sen­ter.

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.

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