Posts Tagged molluscs

Sunday Spinelessness – How snails conquered the land (again and again) David Winter Aug 05

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Christie Willcox wrote a nice article this week on how one small group of organisms called “vertebrates” first evolved to live on land. Since you are a vertebrate who lives on land, you should probably go and read Christie’s piece. I wouldn’t want you, however, to go around thinking those first fish to leave the ocean behind were pioneers making a uniquely difficult transition. By my figuring, onycophorans (velvet worms like peripatus), tardigrades, annelids, nematodes, nemerteans (ribbon worms) and quite a few arthropod lineages have also taken up a terrestrial lifestyle. Many of those lineages were already breathing air before Tiktaalik, Ichthyostega and your other long-lost relatives came along to join them on land. But if you want to talk about transitions from marine to terrestrial lifestyles then you really want to talk about snails. You can find snails living in  almost every habitat between the deep ocean and the desert, and snails have adapted to life on land many different times. In fact, a litre of leaf litter taken from a New Zealand forest can contain snails representing three separate transitions from water to land.

Almost all the land snails I’ve talked about here at The Atavism are descendants from just one invasion of the land. We call these species the stylommatophorans and you can tell them from other landlubber-snails because they have eyes on stalks (as modeled here by  Thalassohelix igniflua):

These snails are part of a larger group of air-breathing slugs and snails (including species living in fresh water,  estuaries and even the ocean) called pulmonates or “lung snails”. As both the common and the scientific names suggest, pulmonates breathe with lungs. Specifically, the mantle cavity, which contains gills in sea snails, is perfused with fine veins that allow oxygen to permeate the snails’s blood. In relatively thin-shelled species you can often see this “vasculated” tissue in living animals:

Blacklight photo of Cepaea nemoralis showing ‘vascularised’ lung. Photo is CC BY-SA via Wikipedian Every1Blowz
The pulmonates can also regulate the amount of air entering their lungs with the help of an organ called the pneumatostome or breathing pore –  an opening to the mantle cavity that the snail can open or close at will:

A leaf-veined slug from my garden – the small opening near the “centre line” of the slug is the pneumatostome. Interestingly, leaf-veined slugs don’t have lungs, the pneumatostome opens to a series of blind tubes not unlike an insect’s respiratory system

So that, along with a whole load of adaptations that prevent a fundamentally wet animal from drying out, is your basic land snail. But those little leaf-litter snails I’ve been talking about for the last couple of weeks provide a good reminder that other snail lineages have left the life aquatic. Here’s a species you find almost everywhere there is native forest in Otago, Cytora tuarua:

Holotype of Cytora tuarua B. Marshall and Barker, 2007. Photo is from Te Papa Collectons onlne, and provided under a CC BY-NC-ND license
Cytora is from the superfamily Cyclophoroidea, a group of snaisl that have indepedantly adapted to life on (relatively) dry land. (The weirdly un-twisted Opisthostoma is in this post is another cyclophoroid).  Cyclophoroids share some stylommatophoran adaptations to life on land, they’ve lost their gills and replaced them with a heavily vesculalised mantle cavity. Slightly oddly, cyclophoroids also breathe with their kidneys. Or, at least, the nephridium, an organ which does the same job as a vertebrate kidney, includes “vascular spaces” that the snail can use to collect oxygen from the air. Cyclophoroids don’t have an organ equivalent to the breathing pore to control the flow of air into the mantle cavity. Instead the mantle cavity is open and air enters by diffusion, or in larger species, as the result of movements of the animals head. 
For the most part, the respiratory and excretory systems in cyclophoroids are not as well adapted to life on land as those in their stylommatophoran cousins. For this reason, most cyclophoroids are only active in very humid conditions. In my limited experience, Cytora species are usually found deep in moist leaf litter and soil samples, and I’ve never seen one crawling about. Nevertheless, some species can survive in drier situations, and these are certainly terrestrial snails.
Local leaf litter samples reveal a third move from the water to land. I don’t have nice photo of Georissa purchasi, and I can’t find anything else on the web either, so you’re stuck with a crumby drawing from my notebook:

I did warn you that it was a crumby drawing. In life G. purchasi have an orange-red sort of a hue, and you can often see patches of pigment from the animal through the shell.  Georissa species are from the family Hydrocenidae and are quite closely related to a group of predominantly freshwater snails called nerites. Just like the other lineages discussed, the Hydrocenidae have given up their gills and breathe through a vasculated mantle cavity. Very little is known about the biology of these snails. G. purchasi is sometimes said to be limited to very wet conditions, but I’ve collected (inactive) specimens form the back of fern fronds well above ground so it can’t be completely allergic to dry . 
So, in a handful of leaf litter collected from a Dunedin park you might have cyclophoroids, hydrocenids and  stylommatophorans – descendants from three different moves from sea to land. If we look a little more broadly,  there are are many more examples of this transition.  I’ve written about the the helicinids before, then there are terrestrial littorines (perwinkle relatives) some of which have both gills and lungs. Plenty of other pulmonate lineages that have also taken up an entirely terrestrial lifestyle. Because some of these groups have adapted to life on land multiple times, there have probably been more than 10 invasions of the land by snails.

Most of the description of Cyclophoroids here is taken from:

Barker, GM (2001) Gastropods on land: phylogeny, diversity and adaptive morphology In Barker (Ed.),  The biology of terrestrial molluscs (pp 1146) CABI Publishing.

Sunday Spinelessness – Hairy snails David Winter Jul 29

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Here’s another of these tiny native snails I talked about last week. Aeschrodomus stipulatus:

Not the best photo I’ll admit, but it records enough detail to see the two things that set Aeschrodomus apart from most of its relatives in New Zealand. It’s tall and hairy. I’m not sure if there is an accepted definition of “hair” when it comes to snail shells, but plenty of different land snails groups have developed processes that extend form the shell. In New Zealand we have the fine bristles of Suteria ide, the filaments of Aeschrodomus and the spoon-shaped processes of Kokopapa (literally ”spoon-shell”):

K. unispathulata Photo is from David Roscoe / DoC and is under Crown Copyright

I try very hard to avoid the sloppy thinking that presumes there is an adaptive explanation for every biological observaton, but it’s hard to see how these hair-like processes would evolve if they didn’t serve a purpose. The larger hairs are presumably made from the same calcium carbonate minerals as the rest of shell, and calcium is a precious resource for snails (so much so that empty shells collected from the field often show signs of having been partially eaten by living snails). In those species with finer projections, the hairs are an extension of the “periostracum”, a protein layer that covers snail shells.  If we presume that snail hairs come at a cost, in either protein or calcium, what reward are they hairy snails reaping from their investment?

Markus Pfenninger and his colleagues asked just that question by looking at snails from the Northern Hemisphere genus Trochulus (doi: 10.1186/1471-2148-5-59). This genus contains many species that sport very fine and soft hairs. Pfenninger et al.collected ecological data for each species, and used DNA sequences to estimate a the evolutionary relationships between those species. From these data, they were able to infer the common ancestor of modern Trochulus species was probably hairy, and three separate losses of hairyness can explain all the among-species variation in this trait. Moreover, it appears the loss of hairs in Trochulus is associated with a switch for wet to dry habitats. Given this finding, Pfenninger’s team hypothesised that, in Trochulus at least, hirsute snails might stick to host plants more effectively than their bald brethren. Indeed, in experiments it took more force to dislodge a hairy shell from a wet leaf than non-hairy one.

Pfenninger’s study makes a neat case for the maintenance of hairy shells in Trochulus, but I don’t think adherence to leaves can explain all the hairy snails we know about. In New Zealand, most snails with shell processes are limited to leaf litter, a habitat that would seem to make adhering to leaves a positive hindrance to getting around. I don’t know if we’ll ever have a simple answer as to why some of our snails sport these attachments, but Menno Schilthuizen‘s work might give us a couple of clues as to why these sorts of shell sculpture arise and stick around. In 2003, Schilthuizen proposed many shell features may arise because those individuals that have them are more likely to procure a mate (or perhaps a desirable mate) (doi: 10.1186/1471-2148-3-13). Although there is quite a lot of evidence for sexual selection in land snails, I don’t know of a study testing Schilthuizen’s hypothesis on shell sculpture. On the other hand, Schilthuizen’s group has found evidence that elebaroate shell sculpture can arise as a response to predation (doi: 10.1111/j.0014-3820.2006.tb00528.x). Opisthostoma land snails from Borneo have extradonary shells, with unwound shapes, ribs and spines:

Opisthostoma mirabile
In Borneo, Opisthostoma species live alongside a predatory slug that attacks these snails by boring a hole into their shells. The unique shape and ornamentation of Opisthostoma shells appears to have evolved to hinder slug attacks. Even more interestingly, geographically distinct populations of slug appear to attack snails in different ways. This local variation in predator behavior could well be a response to local variation in the shell ornamentation – a so called Red Queen process in which each population evolves rapidly while maintaining more or less the same relative fitness

There are certainly plenty of snail-eating animals in New Zealand. Several species of Wainuia land snail appear to specialise in eating micro snails, which they scoop up and carry off using a “prehensile tail” (Efford, 1998 [pdf]). It’s entirely possible that the relatively small projections that some our snails sport are preforming the same job that those weirdly distorted Opisthostoma shells serve.

Sunday Spinelessness – New Zealand microsnails David Winter Jul 22


When I tell people I study snails for a living I get one of two replies. There’s either some version of the “joke” that goes “that must be slow-going” or “sounds action packed”, or there’s “oh, you mean those giant killer ones we saw when we went tramping?”. I guess the joke is funny enough, but I want to make it clear that those giant killer snails from the family Rhytidae, cool as they might be, are not the most interesting land snails in New Zealand.

The local land snail fauna displays a pattern that is quite common for New Zealand animals – we have a very large number of species but those species are drawn from relatively few taxonomic families. Since taxonomic groups reflect the evolutionary history of the species they contain, that pattern most likely arises because New Zealand is (a) quite hard to get to, so few would-be colonists make it here and (b) full of ecological niches and geographic pockets that can drive the formation of new species. In total, there are are probably about 1200 native land snail species in New Zealand – about ten times the number found in Great Britain, which is approximately the same size. That diversity extends to the finest scales – individual sites in native forest might have as many as 60 species sharing the habitat. New Zealand forests probably have the most diverse land snails assemblages in the world (although tropical ecologists, who generally hold that diversity in terrestrial habitats almost invariably increases as you approach the equator, have argued against this conclusion).

You may now be asking why, if this land snail fauna is so diverse, have you never seen a native snail. Well, you’ve probably walked past thousands of them without noticing. Most of our native land snail species are from the families Punctidae and Charopidae, groups that are sometimes given the common name “dot snails”. Meembers of these families are usually smaller than 5 mm across the shell, and are restricted to native forest and in particular to leaf litter. But in native forests, where there’s leaf litter there’s snails. Grab a handful of leaves, or pull up a log and you’re likely to find a few tiny flat-spired snails going about their business. Hell, down here in Dunedin you can even find charopids living under tree-fuschia in a suburban garden.

Like so many native invertebrates, we know very little about our land snails. Lots of people have dedicated substantial parts of their lives to documenting and describing the diversity of these creatures, but even so we don’t have a clear understanding of how the native species relate to each other or to their relatives in the rest of the world, or even where one species starts and another ends. Without such a basic understanding, its very hard to ask evolutionary and ecological questions about these species, so for now we remain largely ignorant of the forces that have created the New Zealand land snail fauna.

For the time being I can tell you that a lot of them are really quite beautiful. Since most people don’t have handy access to a microscope to see these critters, I thought I would share a few photos from this largely neglected group over the next few weeks. The 2D photographs, with the relatively fine depth of field, don’t quite record the beauty of these 3D shells, but I hope it’s at least a window into the diversity of these snails.

 Let’s start with a snail that is very common in Dunedin parks and forests. This is a species from the genus Cavellia (the strong, sine-shaped ribs being the giveaway) but I won’t be able to place it to species until a new review of that genus is published. 

This particular shell is from an immature specimen, and is about 2mm across. When flipped, you can see an open umbilicus that lets you see straight through to the apex of the shell.

Sunday Spinelessness – Cuttlefish in drag deceive their rivals David Winter Jul 08


One awesome mollusc deserves another, so let’s follow up last weeks octopus post with one on that group’s close relatives the cuttlefish.

Cuttlefish are relatively small (the largest grow to 50cm) squid-like cephalopods that present a nice soft and digestible meal to predatory fish and marine mammals. Having lost the shell that most molluscs use to protect themselves cuttlefish have had to develop other defences. Most strikingly, cuttlefish are masters of camouflage


The deceptive patterns that cuttlefish put on come from their remarkable skin, and are controlled by a pretty impressive nervous system. The skin is covered in cells called chromatophores which contain granules of pigment. When a cuttlefish decides it’s time to disappear it looks around its surroundings and, with the aid of nerves that lead from the brain to the the skin, stretch and twist the chromoatophores  on the skin’s surface in such as way as to change the colour of their cells, and ultimately their whole bodies. 
That impressive trick is principally used for camouflage, but cuttlefish and also use their skin as a sort of billboard to signal to other members of their own species, and even put on a strobing light show (possibly used to startle their own prey):

Just this week, researchers have reported evidence for a other trick that cuttlefish can pull off. When males of the Austrian Mourning Cuttlefish (Sepia plangon) see a female they put on a show, producing striped patterns that evidently impress the female. But these animals form male-dominated groups, and rival males often interrupt would-be woo-ers  in mid-display. So, when they spy a receptive female, males want to put on their flamboyant show for her to judge, but also want to make sure they don’t attract the attention of rival males that might want to spoil the party. The male Mourning Cuttlefish’s answer to this problem? Using only half of his body to put on the female-impressing show, and throwing would-be spoilers off the scent by mimicking a female with the other half.

This gender-splitting  tactic seems to be pretty common. In aquarium experiments about 40% of males would  attempt the deceptive signal when they were displaying in the presence of a rival. Just as the cuttlefish camouflage response requires information from the physical environment, the gender-splitting trick is influenced by what the male can learn of the social environment. If more than one female is available the male will display to both  without bothering to hide his intentions for observers (probably because working out an angle from which he could excite two females while staying under the radar is just not possible). Likewise, if more than one rival male is about that don’t bother with the deception – since it wouldn’t be possible to maintain the illusion for two rivals viewing from different positions.

Brown, Garwood & Williamson (In press) It pays to cheat: tactical deception in a cephalopod social signalling system. Biology Letters.

Sunday Spinelessness – The other mollusc shell David Winter Jul 01

Here’s a really cool animal, a female argonaut (sometimes called a paper nautilus):

It may not be immediately obvious from the photo, but argonauts are octopuses. Strange octopuses, because the seven species that make up the family Argonautidea are among a handful of octopuses that are capable of swimming through the water column (rather than hanging out on the ocean floor) and they are the only octopuses that fashion themselves a shell. 
The argonaut shell has been a topic of consideration, confusion and conjecture for biologists for a long time. Only females produce the shell. Male argonauts are tiny (about a tenth of the size of the female) and only really serve as sperm donors (in fact, they donate an entire sperm-transferring organ, called the hectocotylus). Once mated, a female argonaut starts producing her shell and lays her eggs in its base. This behaviour has lead some biologists to conclude the shell’s primary function is to act as an egg case.  We now know that shell is also used to help the argonaut maintain its position in the water column. By propelling herself to the surface and rocking back and forth an argonaut can introduce an air bubble into her shell. While she’s near the surface that air bubble will make her buoyant, but by diving downwards she can reach a point where the increasing water pressure (which compresses the air bubble, decreasing its buoyancy) cancels out the buoyant effect, letter her float in the water colum. At that point she’s free to swim about in two dimensions without having to maintain her vertical position.

You can watch this remarkable behaviour  here:

I don’t want to talk too much more about the purpose of the argonaut shell, partly because it has already been well covered. Ed Yong wrote a predicably clear and interesting post on the research which uncovered it (which also produced an interesting comments thread) and the lead researcher, Julian Finn from Museum Victoria in Australia, also discussed his work in a really great video.

Instead, I want to talk about the origin of the argonaut shell. Octopuses are molluscs, part of a group of soft-bodied animals that includes clams and mussels and snails. Most molluscs have shells. In fact, despite being arugably the most morphologically diverse of the 35 animal phyla, only a few small groups of molluscs don’t contain at least some species that produce shells. The easiest way to explain the presence of shells in so many different molluscan groups is to hypothesize that the last common ancestor of all molluscs had a shell, and most of that ur-mollsuc’s descendants have retained this organ. 

In evolutionary biology we call traits that are shared between organisms as a result of their shared evolutionary history “homologies”. Homologous traits are often compared with “analgous” ones, parts of organisms that are similar as the result of independent innovations in different evolutionary lineages. We can illustrate the concept using a bat’s wing as an example. The forelimbs of bats and whale are made up of the same bones, despite the fact that whales swim and bats fly. That’s because bats and whales are both mammals, and they inherited their forelimb bones from a common ancestor before each group radically repurposed their limbs. On the other hand, despite the fact that both bats and stoneflies fly, the insect wing and the bat wing are separate evolutionary inventions and not something the two groups share as a result of shared evolutionary history:


The protective shells of snails and clams are homologous to each other, and to the internatilized shells that some squids use to stay afloat. But the argonaut shell is something entirely different. The argonaut shell is made of calcite, where most molluscan shells are argonite. Moreover, the minerals that make up the argonaut shell are extruded from the octopuses tentacles, where other molluscs have an organ called the mantle that they use to produce their shell. 
The fact the argonaut shell is made of different stuff than other molluscan shells, and with the aid of  a different organ, suggests it is a unique evolutionary innovation. So how did shells evolve twice within the molluscs? I can’t provide you with a definitive answer, but I do like one (only slightly crazy) speculation. Earth’s oceans used to be dominated by another group of shelled molluscs called ammonites. Adolf Naef pointed out that argonaut shells are very similar to some ammonite shells, and suggested the ancestors of ammonites might have laid their eggs in discarded ammonite shells (some modern octopuses certainly spend time hanging out in mollusc shells). Naef suggested ancestral argonauts might then have acquired the ability to repair broken shells (developing the mineral secreting organs on their tenticles) and finally to create their own. 
It’s a pretty out-there sort of an idea, and I don’t know how you could actually test it. But wouldn’t it be cool if the ammonite shell was still being dutifully copied every day, 65 million years after the last ammonite died?

Sunday Spinelessness – They’re alive! David Winter Feb 26


It would take the most dedicated reader of The Atavism to remember the empty snail shell I wrote about last year. I’ll admit even I’d mainly forgotten about myself, but this weekend I went on a little mini-field trip to collect a few samples for a colleague’s ongoing project. In planning that trip I did remember the slightly mysterious shells I found last winter, and so decided to head back and see if I could get a few more to send along an an expert who might be able to put a name to them. 

Sure enough, I found plenty more empty shells in different states of aging , but deep within the leaf litter I also uncovered one shell that was still playing house to an animal. I couldn’t quite be sure there was a healthy animal in the shell when I first picked it up, since the snail was already retracted inside. Thhe easiest way to encourage a sleeping snail out from its shell is to warm in up, so I clasped it in my palm for about a minute and, well, here’s the result:

Obviously, having taken the photographs I put this snail back under the nice moist leaf litter from which I’d taken it. Since then I’ve done a bit of research and I’m fairly confident that I’ve now identified this population down to genus level. But I’ve wrong about these things before (most recently by en entire superfamily…) so I’m still going to send the empty shells I collected from the same site to someone who has much more expertise than I do. I’ll keep you updated on just exactly what these creatures are.

Sunday Spinelessness – How chitons are tougher than stone David Winter Sep 25

No time to say anything meaningful today, so here’s a pretty picture of a green chiton (Chiton glaucus)

Of course, I wouldn’t be a self-respecting invertebrate evangelist if I didn’t try an convince you that chitons represent more than a pretty shell and a stern test for junior rock-pool hunters’ ability to prize creatures from rocks. Apart from all that armour, the most interesting think about chitons is their teeth. Like most molluscs they feed with a rasp like organ called the radula, unlike most other molluscs their radulae are coated in an iron-containing mineral called magnetite. As the name suggests, magnetite is a mineral that can become magnetised (in fact, of all naturally occurring minerals its the most  magnetisable) but that’s not why chitons make cover their teeth in it. In order to eat, chitons need incredibly abrasive teeth that can scour away at rocks and expose algae, and that means the teeth need to be coated in a tough material.

“Normal”, geologically produced magnetite is pretty tough, but, remarkably, the magnetitie that chitons produce to coat their teeth is much tougher despite being made from the same molecules. The chiton’s biochemical toolkit is able to produce magnetite in which the three dimensional structure is tweaked towards a tougher end result.  Professor Kate McGrath from the MacDiarmid Institute spoke about this ‘biomineralisation’ process as part of her contribution to the Royal Society’s Marie Curie Lecture Series. Her research doesn’t aim to mimic the specific ways in which organisms create chemicals, so much as learn the various tricks that evolution has discovered and see how they might be applied to either tweak or completely chain the way we make useful minerals on industrial scales.

Sunday Spinelessness – Incertae sedis David Winter Apr 24

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I was going to start this post by saying taxonomy has a language of its own, but that’s not really true. Taxonomy just has a whole lot of Latin. When I’ve given talks in schools, I’ve almost always been asked why scientists insist on giving creatures those strange Latinised names. The answer is, we need names that anyone anywhere can recognise and pin to particular species or group. A common name like “black bird” might make sense in conversation in New Zealand, but in other circumstances it could refer to a single species (Turdus merula), the five species that make up Turdus, or a whole bunch of species in the new world family Icteridae. When taxonomy really got started Latin was the language which scientists from different countries used to speak to each other, so modern taxonomy follows Linnaeus in using Latinised names.

But the Latin terms used by taxonomists don’t stop with names, if you thought they were difficult to understand you should open a taxonomic work. The first time I did I was lost in a sea of lecto-, neo- and allo-types; homonyms junior and senior and Nomina nuda, obltum and protectum. The title of this post, Incertae sedis is another of those strange taxonomic terms and translates as “uncertain placement”. Most big taxonomic reviews include a few species the author is certain are well defined, but hard to place into a higher group. Those species get placed under the heading Incertae sedis.

I used that title because today’s subject is… a land snail… of some sort

I found this guy (actually, all land snails are hermaphrodites, so I need a better familiar term for them) in our garden and it’s pretty tiny (this is a 50 cent coin, almost exactly the same size as a quarter for North American readers):

Allan Solem (who knew a thing or two about land snails) regarded the New Zealand land snail fauna as one of the most diverse the world. Head into a patch of native bush and pick through the leaf litter and you’re bound to find small snails like this one eating the decaying matter, look around a bit more an there will be slugs like the leaf veined ones I’ve written about and much bigger species like giant carnivorous Powelliphanta. The massive diversity of our land snails makes their classification and identification hard work. This is probably a punctoid (a member of the super-family Punctoidea) but that’s a pretty lame identification from someone who’s meant to be some sort of malacologist (equivalent to a scientist specialising the cute and the cuddly saying a cat it probably some kind of mammal, since land snail families tend be much more anciently diverged groups that vertebrate families) and doesn’t help us very much with getting the species name because there are probably 450 punctoids in New Zealand, only half of them formally described.
So, for now, I’m just considering it Incertae sedis and admiring its finely sculptured shell

Sunday Spinelessness – Return of the spineless David Winter Jan 23

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It seems all the cool bug bloggers have escaped to the tropics just at the time I’ve got back to more temperate climes. I spent a couple of weeks in Vanuatu over the Christmas and New Year break and have a couple of memory cards full of Melanesian wildlife to share here over the next few weeks. I have another post based on life in Vanuatu I really want to finish editing before I talk about those bugs, and no shortage of real work to do before I can get to that. So, let’s kick of a new year of spinelessness with a lame joke:


(That’s a very sick Achatina fulica, one of the villains in this story and pretty common snail in and around Efate, the most populous island in Vanuatu. You can bet this very picture is going to show up as a slide is all my talks about Pacfic Island snails.)

Sunday Spinelessness – The origin and extinction of species David Winter Dec 05


I don’t use these pages to write about my own work very much, partly because it’s not yet published and partly because I write about that all day as it is. The shortest answer I can provide to the question “what do you do” is “I use genetic tools to study evolution” and I guess that makes me an evolutionary geneticist. You can split the people that work in our field into two groups: there are biologists that are really interested in a group of organisms and have learned some genetics to help their study of them, and there are people who are interested in a particular question and have chosen their study organisms to suit. I’m very much of the second sort, and like most people in that group I’ve caught myself saying “I’m interested in the questions, not the animals”. Paraphrased, that becomes something like, “Oh sure, I study Pacific land snails, but for all I care they’re just little bags of genes that help me answer questions”. But that’s a lie. You can’t work on animals without having them effect you. When I started my PhD I had no particular love of snails, but now I’m a complete snail fan-boy and I frequently find myself preaching on the wonders of life as a terrestrial mollusc to people whose only mistake was to ask me what I do for a living. Did you know most slugs retain the remnants of their shells? Or that almost all snail shells coil to the right? Or that mating in many land snail species only proceeds after one snail has stabbed the other with a “love dart”? A couple of weeks ago I was recounting the the sad tale of The Society Islands partulids to someone I’d met three minutes earlier, and today I’m going to tell you that story (though, of course you have an advantage over the first recipient of the story, since you don’t have to read this crap)

Believe it or not, land snails are one of the characteristic animals of Pacific Islands. Anak krakatau is so young it’s still smoldering, and it has a native land snail species and Rapa nui (Easter Island), which is arguably the most isolated island in the Pacfic, had its own land snail fauna back when it had forests. It’s not entirely clear how these unlikely colonists get to islands. Darwin was so interested in the question* he, ever the experimentalist, stuck snails to ducks’ feet to see if they’d survive an inter-island journey. Birds have been shown to carry snails great distances, but wind blown leaves are probably a more common mode of conveyance. We might not know exactly how snails get to islands, but we know what happens once they establish themselves. The land snails of the Pacific include some of the most outrageous explosions of diversity in the biological world. Chief among these evolutionary radiations were the partulid snails of The Society Islands (the French Polynesian archipelago that includes Tahiti). Partulids are very elegant tree snails that form part of the land snail fauna across most of Polynesia, in the Societies they made up most of the land snail fauna. In total, the tiny islands had 58 species of these snails with each of the main islands have their own endemic forms.

A plate from Crampton’s monograph on the partulids of Moorea

The Society Islands’ land snails were a marvel all by themselves, but they were also an extraordinary resource for scientists. The first person to seriously take up their study was the American embryologist and evolutionary biologist Henry Crampton. Crampton was working at the turn of the 20th century, a time in which the mechanisms underlying genetics and evolution were very much up for debate, and he hoped Tahitian and Moorean partulids could help set the story straight. Crampton’s monogrpahs are famous (at least among people that spend thier lives thinking about snails) for their detail. He collected and measured over two hundred thousand shells, then calculated summary statistics for each species, each site and each measurement. By hand. To eight decimal places.

Table #95 from Crampton's monograph. Three are approximate 150 cells.

Those massive tables (there are more than 100 pages of them in the Moorean monograph) might seem like an old-fashioned, descriptive, way to do biology. But in many ways Crampton was ahead of his time. For one, he was a Darwinist when not every evolutionist was. By the end of the 19th century Darwin had convinced the world of the fact evolution had happened, but relatively few naturalist bought his theory of how evolutionary change happened. The anti-Darwinian theories that prospered during the so called “eclipse of Darwinism” placed very little importance on the variation within species. The orthogenesists and the lamarckians thought evolution had a driving force, pushing species towards perfection. In their scheme variation within a species was deviance from the mainstream of evolution and was quckly stamped out by natural selection (which they didn’t deny, they just said it couldn’t be a creative force). Similarly, saltationists thought large-scale evolutionary changes occurred in a single generation, and the small changes you see in populations were of no consequence in the grand scheme of evolution. Crampton realised that, in a Darwinian world, variation within populations was the raw material of evolution. He was obsessive about measuring his shells because he knew could use the data he was recording to understand where species came from. In particular, we was able to show that isolated populations of the same species varied from each other. That finding that makes sense in light of Darwin’s theory, since species arise from populations evolving away from each other; but is harder to fit into progressive theories of evolution, in which you’d expect different populations of the same species to follow the same trajectory.

Crampton’s results influenced people like Dobzhanky, Mayr and Huxley who helped to re-establish Darwinism as the principal theory of evolution in the Modern Evolutionary Synthesis. But Crampton also predicted arguably the most important development in evolutionary theory since the modern synthesis. In the middle of the 20th century evolutionary genetics was defined by a single debate. The “classical” school held that populations in the wild would have almost no genetic variation, because for every gene there would be one ‘best’ version and every member of the population would have two copies of that gene. Arguing against the classical school, the “balance” school argued that, quite often, there would be no single best gene and organisms would do better having two different versions of the same gene**. The ballancers thought natural selection would keep lots of different versions of maybe 10% of a species’ genes. Both schools assumed natural selection was such a pervasive force that selection would dictate the way populations were made up, they just disagreed on what would result from it. Here’s the funny thing, they were both spectacularly wrong. When scientists started being able to measure he genetic diversity of populations in the 1960s it became clear almost every single gene had multiple different versions. Now, in the post-genomic age there is a database with 30 million examples of one sort of genetic variant amongst humans.

Faced with the overwhelming variation he recorded in partulid shells, Crampton had argued natural selection didn’t have a damn thing to do with it. Snails isolated from each other by a mountain weren’t adapting to their local habitat, they just varied with respect to traits that had no influence on their survival. The fact two populations were isolated meant each would follow its own path and two populations could drift apart from each other. Faced with the overwhelming genetic variation coming from studies in the 1960s Motoo Kimura proposed the neurtral theory of molecualr evolution. Kimura’s explanation was the same as Crampton’s, almost all of the variation we see at genetic level has no bearing on the success or failure or organisms so the frequency of different variants drifts around at random. The neutral theory is at the heart of a lot of modern evolutionary genetics, and Crampton had understood the underlying principle 50 years before we knew we needed it!

At the end of his monograph on the partulids of Moorea, Crampton said he’d got as far as his measurements could take him, and it was time for someone to study their genetics. In took a bit longer than Crampton might have hoped, but in the 1960s two leading geneticists took up the study of his snails. James Murray from Virginia and Bryan Clarke from Nottingham spent almost 20 years working in what they called, in more than one paper, the perfect “museum and laboratory” in which to study the origin of species. Their work helped scientists understand, among other things, how ecology can contribute the formation of new species and what happens to species when they hybridise with others from time to time. Then, in 1984, Murray and Clarke had to write the most heart-breaking scientific paper I’ve ever read. It’s written in the careful prose scientists use to talk to each other, but the message it delivered was devestating:

In an attempt to control the numbers of the giant African snail, Achatina fulica, which is an agricultural pest, a carnivorous snail, Euglandina rosea; has been introduced into Moorea. It is spreading across the island at the rate of about 1.2 km per year, eliminating the endemic Partula. One species is aiready extinct in the wild ; and extrapolating the rate of spread of Euglandina , it is expected that all the remaining taxa (possibly excepting P. exigua) will be eliminated by 1986-1987.

Euglandina rosea Cga33333

The bad guys: Euglandina on the left, Achatina on the right.

Euglandina rosea is better known as the Rosy Wolf Snail. It senses the mucous trails of other snails, tracks them down and eats them. It’s not clear if the wolf snail had any effect on the pest species it was introduced to control, but it had huge impact on the partulids. By the time Murray and Clarke wrote their paper, E. rosea had already done for one species and it was too well established to control. All they could do was watch as human stupidity and molluscan hunger slowly (1.2 km per year) destroyed the species they’d been studying for 20 years and Crampton had dedicated 50 years of his life to. The same slow torture played itself out in Tahiti and then the rest of the Society Islands. Where there were 58 named species, there are now 5 alive in the wild. Crampton’s hundreds of pages of tables should have been the starting point from which the evolution of the partulids could have been tracked. Murray and Clarke’s natural laboratory should still be open and be taking advantage of a new generation of technologies that might be able to reveal the genetic and genomic changes that occur when a new species arises. Extinction is a natural part of life, and the fate of all species eventually, but when it’s driven by human short-sightedness and robs us of not just a wonderful product of nature but a window through which we might have understood nature’s working it’s very hard to write about.

I should end by saying there is just a tiny scrap of good news in this story. The partulids are no longer an iconic species in the study of evolution, but they have become the pandas of invertebrate conservation. Murray and Clarke were able to get 15 of the species of the islands and into zoos and labs across the Northern Hemisphere. Breeding programs have been succesful, and new lab-based studies come out form time to time. The relict populations back in the Societies don’t have nearly the range they used to, but it appears they’ve held on to most of their original genetic variation. Perhaps, one day, Eulglandina can be taken care of and some of the partulids can have their islands back.

*Darwin had to be interested in dispersal. Before evolution was widely accepted naturalists thought creatures were created for their habitat (the modern creationist notion of a post-flood diaspora explaining the distribution of animals is almost entirely an invention of Seventh Day Adventists, no, really, it is), Darwin’s theory did away with special creation but still needed to explain how life came to live everywhere

** A concept similar to “hybrid vigour”, in which crosses between relatively unrelated strains/cultivar bring together different genes and do well as a result. You’ve seen evidence of this phenomenon any time you’ve eaten yellow and white “honey and pearl” corn. That corn is a hybrid between a white and yellow cultivar and if you count up the kernals you should get close to the 3:1 ratio Mendel preficts for a dihybrid cross.

Some further reading:

Stephen Jay Gould, who was a snail man himself, wrote and essay on Crampton and the Society Island partulids in which made a humanistic argument for the importance conservation. I resisted the urge to re-read it in researching this piece so anything I stole from him I stole sub-consciously!

  • Gould, S.J., 1994. Unenchanted evening in Eight Little Piggies: Reflections in Natural History

Crampton’s monograph on the Mooeran partulids (from which the figures above are taken) is available online

Finally, the paper in which Clarke and Murray told the world about the demise of their snails:

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