Species of the week- 14

A little while ago, I claimed that in order to keep my species of the week blogs variable through the winter, I’d be choosing some summery species, otherwise it’d become slug of the week! It’s been brought to my attention that I haven’t actually had a ‘slug of the week’ yet, so here goes…

The species this week is Arion owenii, currently given the vernacular name Tawny Soil Slug, though I’ve always known it nicknamed the Irish Garden Slug. Here is the marvellous little creature:

S3750079.JPG
Arion owenii at Kennall Vale nature reserve in Cornwall

I’ve particularly like slugs, as they were one of the very first invertebrate groups that I learnt about more seriously. This species however, I found for the first time only last year, since living in Cornwall at university. Arion owenii is fairly common in Devon and Cornwall, and isn’t especially choosy when it comes to habitat, though from my experience it favours woodland. There are also smaller hotspots for the species in South Wales and Western Scotland. Outside of these areas, it is rare, but strangely widespread. Here is its distribution map:

https://data.nbn.org.uk/Taxa/NHMSYS0001701908/Grid_Map

Arion owenii, like many slugs, spends a great deal of its life in the soil where the moisture content it high. It therefore feeds on both the roots and shoots of living plants. That said, it is rarely abundant enough to be considered a pest species. Copulation in A.owenii, as in all slugs, is quite odd. They are hermaphrodites, impregnating each other simultaneously using penises of similar length to their bodies!

Identification of this species can be challenging at times, but with experience becomes more straightforward. There are two species which resemble A.owenii, they are: A.distinctus and A.hortensis. Both of these species are much commoner the A.owenii, and are more likely to be found in synanthropic habitats (near humans: in gardens, allotments etc.). However, it is not infrequent that all three species occur together. Coming up is a table I’ve made to help with identifying these three species. Unless you’re a serious slug identification fan (like me), I’d scroll past it.

Species Arion owenii Arion hortensis Arion distinctus
Base Colouration Typically a warm orangey-brown with dark pigment along the centre of the back. A dark blue-grey-black colour, rarely with any hint of brown. A mid-dark brown to grey-brown.
Lateral Bands Dark brown, extending almost to, or to the foot fringe, though diffuse at base. Blackish, with paler pigment beneath. Clearly not extending to the foot fringe. Dark brown to blackish. Broad, usually reaching foot fringe.
Mantle Bands Dark brown, +- parallel when viewed from above. Running over/through breathing pore. Blackish, +- divergent when viewed from above. Usually running over breathing pore. Dark brown to blackish, +- convergent when viewed from above. Running through-under breathing pore.
Tentacles Black to purple-brown. Black to red-brown. Black to blue-black.
Tubercles

 

Coarse, giving a slightly prickly appearance, particularly when dry and/or at rest. Small and relatively fine. Small and relatively fine, containing very small dots of yellow pigment.
Adult size 25-40mm when extended. 20-35mm when extended. 25-40mm when extended.
Adult season Probably all year. Usually autumn-winter, occasionally all year. Usually spring-summer, occasionally all year.

I hope you’ve enjoyed the blog! It’s ended up being rather more detailed than usual. The identification paragraph was meant to be quite short, but one thing lead to another, I got carried away, and now there’s a whacking great table sitting in the middle of my blog. Hopefully it helps somebody out…. happy slugging!

 

 

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