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):

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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 1—146) CABI Publishing.

Labels: molluscs, native snails, photos, sci-blogs, science, sunday spinelessness

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.

Labels: molluscs, native snails, new zealand, sci-blogs, science, sunday spinelessness

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.

Labels: molluscs, native snails, photos, sci-blogs, snails, sunday spinelessness

Next week's post is going to be really good, but it's also going to be next week. Here's a little taster of a post I didn't leave myself a enough time to write today - a close-up of of the shell of native (Allodicsus sp.) land snail:

Labels: native snails, sci-blogs, snails, sunday spinelessness

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:

Labels: Dunedin, environment and ecology, molluscs, native snails, photo, sci-blogs, snails, sunday spinelessness