Saturday, 27 August 2011

An impressive slug

ResearchBlogging.orgA showery day, I come across this enormous Arion slug crossing the garden path. It must be close to 15 cm. Identification to species level is difficult to impossible as they are distinguishable by examination of internal genitalia characters - or molecular genetic analysis, and hybrids are common. British large Arion slugs are made of several species complexes, Arion ater/rufus A. lusitanicus and A. flagellus. Both groups are very polymorphic slugs, from almost white to black, including all shades of brown yellow red and orange. I have never found the the black morph in the garden, but a good representation of the others abound. The polymorphism of these slugs in the U.K., not only in colour, but also in genital characters, might stem from common hybridisation between several subspecies that colonised the British Isles from Europe, postglacially but also in more recent human introductions. Distribution changes might also come about due to expansions as a result of climate change. In addition, the slugs - except for A. flagellus - are able to self-fertilise and form relatively homogeneous populations, which might give the impression of being a separate species.
 Arion slugs had deep tubercles on their backs, and no keel. At the end tip is the mucus gland, producing an extremely sticky mucus. Unlike the yellow slug, which is strictly nocturnal, Arion slugs are active all around the day, provided that it is damp. When disturbed they show a characteristic behaviour: they contract their bodies into an almost round shape and retract their tentacles inside their mantle. Inspired by a post in MyrmecosI set the photo vertically. I had to use a white plate - instead of bowl - and allow the slug to relax and start to crawl out. 
A very yellow specimen
A disturbed Arion
A selection of colours found in the garden
The black morph of Arion sp.
Noble, L. R. Jones, C. S. (1996). A molecular and ecological investigation of the large arionid slugs of North-West Europe: the potential for new pests The ecology of agricultural pests: biochemical approaches Ed. William O. C. Symondson, Systematics Association, special volume. Clarendon Press., 53, 93-132
Evans, N. (1986). An investigation of the status of the terrestrial slugs Arion ater ater (L.) and Arion ater rufus (L.) (Mollusca, Gastropoda, Pulmonata) in Britain Zoologica Scripta, 15 (4), 313-322 DOI: 10.1111/j.1463-6409.1986.tb00232.x
UPDATE 30/08/11: reference added and Arion ater in a photo caption changed to Arion sp.


Nyctalus said...

Hi Blackbird,
I was delighted to read this as I have been cogitating on the various colours of Arions myself recently ( and failing to find a satisfactory answer. I assumed that they were all Arion ater but obviously not. Thanks for bringing some expertise to bear on this! I have only found the black versions once you get into Northumberland and your observations seem to confirm this more northern distribution. That in itself is a curiosity. Any thoughts on why it should be so?
That is one impressive beast and a lovely photo, as ever,
best wishes

Blackbird said...

Thank you Allan. I have found the black ones in Top Hill Low, a bit north of Hull. I read in the same reference I give (freely accessible at Google Books) that the black form might be more common up north or higher up in the mountains. Also, some of the native forms, being self-fertilising are more inbred and are easily displaced by the more healthy introduced forms. I am no expert, though. After my little excursion into that book chapter I concluded that from now on the safest thing is to talk about Arion sp.

Ray said...

You have inspired me to look closer at, and photograph these little and not so little guys. Ray

Ruth said...

Hi there, Think that we have a few black ones in cambridgeshire. I have spotted some large black ones on the path next to fields recently. Very shiny in the trodden down grass. :)