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.

Semi-slug

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.

References:

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.

Tenerife Days: of Choughs and Chaffinches

As the last of the golden wych elm leaves settled on the woodland floor in the Blyth valley, the dank grey chill of late November sent me scuttling for an early winter reprieve in Tenerife.  Such a fascinating island for the naturalist and so easily reached from Britain.  All the sunseekers off to the bars and beaches of Los Cristianos and Playa de las Americas keep the travel costs down for those of us who are seeking the wilder side of the island.  Sometimes in the airport departure lounge there are other travellers in boots and walking gear and we exchange little smiles and nods, oddities like mackerel caught up in a shoal of herring.

 There can surely be few places in the world that pack as much variety into such a small space as Tenerife manages to.  The largest of the Canary Islands, it is a bit smaller than the island of Lewis and Harris in the Hebrides (perhaps not a hugely helpful comparison as I am certain that many more Brits, and indeed many more Scots, will have been to Tenerife than have ever been to the Outer Hebrides, so let’s say it’s about the same size as County Durham) and it is staggeringly mountainous with the volcanic cone of Mount Teide soaring to almost 4000m at its heart, and a great ridge of mountains with stupendous cliffs dividing the island into two halves with completely different climates. 

Sunrise over Tenerife

The moist trade winds hit the north-west side of the mountains brewing up clouds that keep them green and verdant, while the south-east side bakes in the rain-shadow.  You can walk over the crest of the ridge from shadowy, evergreen, subtropical cloud forest

In the Cloud Forest

to dry stony semi-desert with Euphorbias and other weird and wonderful succulents in a matter of minutes;

Semi-desert in springtime

while just a few km away, via woodland of tree-heather and (largely overgrown) farmland,

Abandoned farmland

are fragrant pine forests;

The Zen garden simplicity of pine forest on volcanic ash

and a few more take you onto the stark lava fields of Teide itself studded with brooms and the giant 2m tall flower-spikes of Tajinaste Echium wildpreti (related to Vipers Bugloss). 

The lava fields

Truly Tenerife is a pocket continent.

 At their closest the Canaries are only 100km from the coast of Morocco, and like North Africa itself they belong, biogeographically, firmly to the Palaearctic rather than the Ethiopean realm.  The remarkable thing is just how many endemic species and subspecies they support given this proximity to the continental landmass.  But then that is one of the fascinations of islands.  The mix of species they end up with seems so random, and the extent to which the ones that do colonise then diverge in their characteristics from their congeners on the mainland seems utterly unpredictable.  Take the birds.  The avifauna of the Canaries is, basically, a rather limited subset of widespread Eurasian species.  But why is it, given how many Palaearctic migrants turn up there en route between Europe and Africa, that so few have established resident populations?  And why is the Blue Tit the only tit that has made it there, and why is it so gloriously, deeply blue in comparison with its anaemic European cousins?  

Tenerife Blue Tit – photo by Conry

The Red-Billed Chough Pyrrhocorax pyrrhocorax has made it across from the Atlas Mountains of Morocco but is to be found only on the island of La Palma (essentially a smaller version of Tenerife) where it swaggers about in true chough style in large flocks.

Choughs – photo by lapalmabirds.blogspot


  Tenerife is clearly visible from La Palma, 85km away, but there are no living choughs there now, although sub-fossil bones found in caves tell us that there used to be.  Choughs are more than capable of sea-crossings – witness their recolonisation of Cornwall from Wales in 2002 (just in time to pre-empt a reintroduction project!) and have also been shown to move between Brittany and Britain.

Chough flock over La Palma

The distances involved are much the same as those between the different Canary Islands so what is stopping them?

I spent quite a bit of time last week looking for Blue Chaffinches.  Unlike its Blue Tits, which are a single species found wherever there are trees and bushes, Tenerife’s chaffinches have split into two – a normal (albeit darker than ours) generalist chaffinch Fringilla coelebs canariensis,

Tenerife Chaffinch – photo by Paul Floyd
Blue Chaffinch – photo by Steve Lane

and the dusky blue-grey Blue Chaffinch Fringilla teydea that has evolved a rather chunkier bill.  This is found only in the pine forests and is said to feed primarily on the seeds of the endemic Canary Pine Pinus canariensis.  I have no doubt they do, although the ones I managed to watch were hunting for the seeds of Codeso Adenocarpus foliolosus (a leguminous shrub endemic to the Canaries) on the forest floor or fly-catching in the canopy.  Anyway it has found a niche for itself that has led to its divergence from the other chaffinches of the island.  But I wonder how secure that niche is?  Greenfinches Carduelis chloris have colonised Tenerife within the last fifty years and I saw one in the pine forest looking quite at home.  And what of those pine cone specialists par excellence, the crossbills Loxia sp?  They seem to be capable of evolving rapidly to tackle different conifer species and new situations (the Scottish Crossbill Loxia scotica has developed as a separate species within the last ten thousand years), are notoriously irruptive and live just across the water from the Canaries in the Atlas Mountains.  How long can it be before they make it over?

So much to ponder!

On the Autumn Symphony of 2018 and the Revival of the Wych Elm

No two autumns are exactly alike, and maybe no two person’s either – much depends on where you live and where you went – but for me this one has been very distinctive – a lovely, long, slow-moving symphony of an autumn.  The first movement opened on the Lake District fells in early September when already the deer-grass and the cotton-grass were beginning to turn (every stem banded with subtle colours), patches of bracken were turning tawny, and yellows, scarlets and coppers were appearing among the leaves of the bilberries (which fruited so early this year).

Ennerdale – the First Movement begins

Mid-September on the fringes of the Highlands in Perthshire saw the gold of the turning birches joining the expanding palette of the bracken and the sedges.

Perthshire  (note dead birches drowned by beavers)

By late September the leaves of brambles and rosebay in lowland Northumberland were beginning to glow as crimson as the heavily-berried hawthorns, and the hills were all in russet.

The second and third movements in October were unusual – the arrangement of the instruments atypical, with some stalwarts making only a brief contribution this year and some lesser contributors thrusting themselves forward for attention.  For a glorious week early in the month the country lanes were incandescent with the greeny gold of ash, which disappeared almost overnight so the naked trees loomed there like a memento mori as the other players took their turns.

A lane of Ash – like driving through a crisp white wine

That was the second movement.  After a long dry summer like this, the Swedish whitebeams (much loved by local authorities in Northumberland as street trees and for pit-heap reclamation schemes) normally give a few days of golden glory, lighting up even the dourest of council estates.  This year they and the rowans were stripped of most of their leaves by tree-toppling and branch-felling gales that ripped through Northumberland in late September and so could make no more than a token effort.  Many of those other landscaping stalwarts, the Norway maples, suffered the same fate, although some did contribute their New England scarlet right through the month – one of the advantages of having a tree flora with cultivars and varieties from many places is the phenological variation they bring.  Thus, most of the limes (all hybrids in this part of the world) flared and faded quite early in October, while others were still making brave contributions well into November.  However beech and birch, reliable as ever, played their parts right through this mellow third movement while never giving us the dramatic heights they are capable of.

So I thought we were set up for a slow, fading fourth movement in November.  But no, this autumn saved its best for last.  Almost overnight it seemed that every remaining leaf on every broadleaved tree and shrub burst into an effulgence of golds and yellows and ambers.

The view from my study – how am I supposed to concentrate!

Species that are normally only minor players in the autumn symphony were given their chance to shine in a glorious finale and took it in style.  Wych elm, sallow, hazel and hornbeam lit up the woods, glowing on the foggiest of days as if lit from within.

Wych Elm

Goat Willow

Hornbeam

Decked out in gold as they were, it made me appreciate just how significant a part wych elms still play in the ancient valley woodlands of Northumberland.  In the 1960s and ‘70s we had fears that Dutch Elm Disease was going to erase all the elms from the British landscape.  The disease did have a big impact of course, although not as severe a one up here as it did in southern Britain.  This was because the southern elms were clonal species, and in many areas suckering elm clones were the main hedgerow trees, and once one died the rest were sure to follow.  Wych elm, on the other hand, relies on sexual reproduction.  Also, it is more of a woodland than a hedgerow tree, so although it was far and away the commonest elm in the north, it wasn’t as prominent in our landscapes. Nevertheless, the big open-grown wych elms we did have all disappeared. There used to be two monsters close by where I grew up, and one of the signs of spring was when the flowers turned their great spreading crowns bruise-purple in March.  Each had a mass of epicormics twigs like a gigantic bird’s nests at its base, tempting places to hide in when playing hide-and-seek, but not wise ones if you were wearing shorts – books hardly ever tell you this, but elm leaves sting, not as much as nettles but it’s not pleasant when they get you on the tender skin behind the knees.

Wych Elm leaves – not quite as innocent as they look

In the Blyth valley woodlands wych elm was one of the commonest trees, many of them coppiced long before, and after a slow start Dutch elm disease ripped through these in the ‘80s leaving the valley sides dominated by stark crowns of dead stems.  In the part of the valley that is included within the Plessey Woods Country Park there were, of course, safety fears, so many of these wych elms were felled.  Then, panicking at the spectre of their woodland turning into a perpetual bramble thicket, the County Council decided to replace the wych elms with new trees, and so out came the tree tubes.  Many of their ash, oak and beech occupants, now ten or more metres tall, are still sporting their split and tattered tuley tube jackets, like Incredible Hulks on a rampage.

Grrr – let me out!

The wych elms did not, as it happens, die.  Generally Dutch elm disease prunes its victims back to the base rather than killing them, and in a little while new stems thrust up towards the sunlight.  Many of these new stems were cut back in their turn once their bark was thick enough to host the beetles that spread the fungus that causes the disease.  However as the years have gone by more and more of these stems seem to be getting away with it – just how many was brought home to me by this year’s late autumn colours.  Once more the multiple stems of the wych elms are a major feature of the Blyth valley woodlands, albeit as yet much skinnier than before.   Together with the ash, oak and beech (both planted and self-sown) they have suppressed the brambles in the country park, and elsewhere in the valley they fight it out with the hazel for the honour of being the principal component of the understorey shrub layer.  Some are even in the process of becoming decent sized trees once more.  It is good to see such resilience.

In the Wych Wood

WordPress Resources at SiteGround

WordPress is an award-winning web software, used by millions of webmasters worldwide for building their website or blog. SiteGround is proud to host this particular WordPress installation and provide users with multiple resources to facilitate the management of their WP websites:

Expert WordPress Hosting

SiteGround provides superior WordPress hosting focused on speed, security and customer service. We take care of WordPress sites security with unique server-level customizations, WP auto-updates, and daily backups. We make them faster by regularly upgrading our hardware, offering free CDN with Railgun and developing our SuperCacher that speeds sites up to 100 times! And last but not least, we provide real WordPress help 24/7! Learn more about SiteGround WordPress hosting

WordPress tutorial and knowledgebase articles

WordPress is considered an easy to work with software. Yet, if you are a beginner you might need some help, or you might be looking for tweaks that do not come naturally even to more advanced users. SiteGround WordPress tutorial includes installation and theme change instructions, management of WordPress plugins, manual upgrade and backup creation, and more. If you are looking for a more rare setup or modification, you may visit SiteGround Knowledgebase.

Free WordPress themes

SiteGround experts not only develop various solutions for WordPress sites, but also create unique designs that you could download for free. SiteGround WordPress themes are easy to customize for the particular use of the webmaster.