Tenerife Days: Beware of the Semi-Slugs


The sub-tropical cloud forest of Tenerife is known as the ‘laurisilva’ – the laurel woodland, and much of it feels like the grounds of a long-neglected British country house.  For the most part the ground is covered in dead leaves, but on the exposed earth of banks and tracks and on the windward side of tree trunks there is a rich growth of mosses.  Occasional crashings in the canopy are generally one of the two species of endemic laurel pigeons, bickering over laurel fruit while frequent rustlings in the undergrowth usually turn out to be blackbirds which are very common.  The dawn chorus in these mountain forests is truly wondrous in its richness and simplicity – the whole island seems to be ringing with the songs of countless blackbirds punctuated by the occasional grunts and squeaks of a roding woodcock.

As ever in the Canaries, there is a sense of strangeness that can be difficult to pin down, and when you do it is often what is not there that is at the root of it rather than what is.  On the forest floor, it finally dawned on me in December, there is a remarkable paucity of ants.  Not a total absence, but they do seem to be very scarce indeed. Grubbing about in the leaf litter and under dead wood produces plenty of non-ant life however – small crickets, cockroaches, earwigs, bristletails, millipedes, snails etc – all, no doubt, good food for blackbirds.  But the largest and most striking of the forest floor invertebrates are the semi-slugs.  These are 5-6cm long and, to my eye, have great charm as they cruise about in stately fashion and plain view.

What is a semi-slug?  Well, the snail body form, while clearly very successful, has several disadvantages.  You can’t go very fast while carrying a shell, and it also limits your ability to squeeze into confined spaces.  Hence a recurring theme in snail evolution has been trying to do without a shell i.e. becoming a slug.  Converting yourself from a snail to a slug requires radical internal reorganisation.  This can’t be done in a single step and evolution has left various partway stages scattered about the planet – from snails that are a little bit too big to withdraw entirely into their shells, through to slugs with a flake of shell hidden under the skin.  More or less halfway along the continuum are slugs that still carry a recognisable but absurdly small shell on their backs (or you could say snails with absurdly large bodies for their shells).  These are the semi-slugs.  Tenerife has several species, although all the semi-slugs I have seen there have looked very similar – with slender chocolate-brown bodies and a small, somewhat flattened amber-coloured shell halfway along the body and largely covered by flaps of skin. The shell sits at a stylish angle so that it looks as though the semi-slug is wearing a beret.


The area of laurisilva I was exploring in December (the Monte del Agua) has a vehicle track running through it, unsurfaced but excellent for spotting semi-slugs and I soon came across two of them engrossed in eating something.  What they were eating appeared to be another semi-slug, but I have always assumed that slugs and snails were essentially vegetarian and, bearing in mind the bizarreness of their sex lives and sex organs, I thought I might be mistaken and crouched down for a closer look.  It still looked like two semi-slugs eating a third, but to be quite sure I picked one up and moved it to one side.  Yes, it was definitely a case of cannibalism.  This was interesting enough, but what followed was even more so.  The one I had moved slid back to resume its meal and as it did so it touched tentacles with the other diner. This one, which happened to be considerably smaller, then appeared to shoot out its mouthparts at the other one which recoiled as if stung, turned away and slid off at considerable (by mollusc standards) speed.  I had been thinking that what I was seeing was cannibalistic scavenging of a semi-slug that had got crushed by one of the occasional landrovers using the track, but this made me wonder if they might actually be cannibalistic predators.  Leaving the forest that evening I came across two more semi-slugs sharing a meal – a large chicken dropping.  So although my Tenerife ‘bible’ (the ‘Natural History of Tenerife’ by the inspirational Philip and Myrtle Ashmole) said that they were leaf-eaters, it seemed that they must like a bit of animal protein as well.

Anyway, on my next visit to the laurisilva I engineered an encounter between two semi-slugs that had been innocently minding their own business, putting a fragment of cheese down beside one of them.  It soon got stuck in, so I then moved the other one close to the cheese.  This too appreciated my gift, and as you can see in the attached video clip they contentedly mumbled on their cheese reminding me of a pair of Asian small-clawed otters sharing a large fish. 

However I then removed the cheese and the second video clip shows what happened once they had touched tentacles.

Given the slowness with which they go about their everyday affairs I find the speed of the strike by the attacker quite startling as, indeed, is the speed and flexibility of the response of the victim, appearing as it does almost to turn around within its own body and speed away.  It made me wonder if they had some kind of stiletto in the mouthparts, or even if they might actually be venomous.  Anyway, I decided to be more cautious in my handling of them in future!

Subsequent on-line research has revealed more about these strange creatures, but also indicates that there is still a lot to learn about them.  Tenerife has several species of semi-slug, all in the family Vitrinidae and all now put in the genus Insulivitrina, but previously included in the genus Plutonia.  I have not found a key to enable me to identify the ones I was watching to species.  Nor have I found any mention of venomousness in any terrestrial slug or snail, but Insulivitrina and Plutonia do have particularly sharp and prominent spines on their ‘radulae’ (the gastropod tongue) and some authors think that they are primarily carnivorous, their main prey probably being earthworms.  I presume that like many true slugs they must be covered with a distasteful slime or the blackbirds would have long since driven them to extinction.  

I found a particularly interesting paper by German zoologist Bernhard Hausdorf exploring the biogeography of the Vitrinid semi-slugs.  It seems that the greatest variety of species is found in Central Europe, particularly in the high mountains, and that those that do live in the lowlands are mostly active in winter.  They are all rather small.  He thinks this is because they have been driven to such specialised ecological niches by the more highly evolved true slugs.  There are no indigenous true slugs on the Canaries, so when in the dim and distant past they were colonised by semi-slugs these had free rein to grow large and to occupy (or reoccupy) much warmer habitats than they could on the European mainland.  Inevitably, with the colonisation of the islands by humans and the introduction of agriculture, true slugs have now arrived.  As yet no one seems to be focussing on the threat this may pose to the semi-slugs – the main conservation concern has been to ensure that the laurisilva itself was not entirely destroyed taking the entire ecosystem and all its endemic species with it – but it is one to watch.


Ashmole, P. and Ashmole, M. (2016) Natural History of Tenerife. Whittles.

Barker, G.M. (2004)  Natural enemies of terrestrial molluscs.  CABI.

Hausdorf, B. (2001)  Macroevolution in progress: competition between semislugs and slugs resulting in ecological displacement and release. Biol. J. Linn. Soc. 74: 387-395.