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.
Posts Tagged 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.
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
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. http://dx.doi.org/10.1098/rsbl.2012.0435w
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:
Here’s something cool that I’ve meaning to write about for a long time. A native Powelliphanta land snail with an apparently pigment-less foot and head:
That snail (a close relative of speedy carnivore featured here) popped up in Kahurangi National Park at the end of last year. Apart from just being kind of cool, the un-pigmented individual is interesting for a geneticist that studies land snails. For the most part, dark pigmentation in snails results form melanin (which is perhaps the most common pigment in the animal world). That’s true for pigmentation of the shell as well as the animal that caries it around. As you can see, this snail has normal pigmentation on its shell, so clearly its still able to make melanin. The genetic mutation (or developmental defect) that has left this snail white hasn’t broken the genes for pigmentation, just the mechanism that moves that pigment around the body wall of the snail.
The ghost Powelliphanta is a pretty cool snail, but there’s actually an albino snail that’s even more interesting. Every now and again a truly albino individual of the freshwater snail Biomphalaria glabrata pops up. Looking at these mutants we can learn something about the evolutionary history of the these snails:
Photo is CC 2.5 and comes from Lewis FA, Liang Y-s, Raghavan N, Knight M et al in PLoS Tropical Diseases
Free from the pigments that would usually make shell opaque we can see the feature that sets Biomphalaria and other species form the family Planorbidae (ramshorn snails) apart from every other snail. The planorbids are the only red-blooded snails on earth. So why are these snails so different?
As we all know, in order to live animals need to get oxygen from their environment into their bodies. For small animals this doesn’t represent a huge problem. Oxygen will flow form areas of high partial pressure (a concept analgous to concentration, but accounting for some of the weird ways gasses behave) to areas in which Oxygen is being used up. So, for instance, most insects pull air directly into their bodies with a set of open tubes (called tracheae). Once the air makes it into those tubes oxygen will passively diffuse into the insect’s tissues.
Big animals have a much bigger problem*. Not only do larger animals need much more oxygen to fuel their bodies, they also have to actively transport that Oxygen because the distances it is required to travel can’t be achieved by passive diffusion. Lungs and gills are both organs dedicated to pumping more oxygen into animal bodies, and many animals use blood, and special proteins dissolved in blood, to move oxygen about.
In vertebrates the oxygen-carrying protein is called hemoglobin. Very simply, a hemoglobin molecule is a cage used to hold iron atoms in such a way that they will bind to an oxygen atom. The iron containing group in the hemoglobin protein (called heme) gives our blood its red colour and its hemoglobin circulating through that snail’s body that makes it red.
As with every problem life faces, invertebrates have come up with many more interesting ways to move oxygen around than their spined relatives. Annelids (earthworms and their kin), brachiapods and spoon worms have a whole set of iron-containing proteins to do the job. Even more interestingly, molluscs and some arthropods have a protein that uses Copper rather than Iron atoms to co-ordinate an oxygen molecule. This molecule, called hemocyanin, takes on a green-ish blue hue when oxygen binds to it and changes its conformation.
Most snails get through life fine with hemocyanin as the only oxygen-carrying molecule in their blood, so why have Biomphalaria and their cousins become red-blooded? Part of the reasons lies in their lifestyle. Planorbid snails breath with lungs (which only work in air) but live underwater. If you make your living by holding your breath while diving then you really want to have some way of holding on to as much of the oxygen you get form each breath for as long as possible. It seems that Biomphalaria hemoglobin is more efficient at using the oxygen stored in lungs while diving than any hemocyanin could be.
It’s all well talking about why an animal might have evolved a particular trait. But in evolutionary biology it’s generally much more intresting to try and work out how. How does an air-breathing snail make its own hemoglobin from scratch? A team lead by Bernhard Lieb asked just that question a few years ago, and found the answer: Biomphalaria hemoglobin was made by cobbling together parts of existing proteins. When Lieb et al (2006, doi: 10.1073/pnas.0601861103) isolated hemoglobin from red-blooded snails they found it was made up of two different components (called peptides), each of which has 13 different sub-components (called domains). When the team compared the sequence of those peptides and their domains to other molluscan proteins they found similarties between the hemoglobin sequences and another iron-containing protein called myoglobin.
Myoglobin is a small molecule that is usually restricted to muscles where is acts as a store of Oxygen (in snails, myoglobin is most commonly found in the muscles that drive the radula, the rasp like organ used to break down food). The Biomphalaria hemoglobin sequences are more closely related to each other than they are to myoglobins from any other species. This pattern suggests the sequences that make up the snail hemoglobin descend from a single common ancestor. Subsequent changes to each of these descendants have allowed the descendants proteins to group together and become ”super myoglobins” capable of transporting oxygen through the body.
*The huge number of ways size matters in biology were wonderfully explained by JBS Haldane. I’d reproduce the most famous passage here, but it’s probably even better if you discover it by yourself.
I though I’d do something a little bit different today. Instead of coming up with anything new to say or show you I’m going to
steal from give a shout-out to a few New Zealand organisations that highlight some of the amazing ways that spineless creatures get on with business of living.
Let’s start with Landcare Research (Manaaki Whenua), the Crown Research Institute that focuses on bioiversity and environmental issues. As you’d expect, Landcare do lots of work on invertebrates an that’s refelected in their public face. Their “What is this bug?” site is a great starting point for anyone trying to put a name to some weird critter that’s crawled out from the garden, and topic pages on some of our most interesting creatures (Onychophora, stick insects and our amazingly diverse moth fauna) make for a nice introduction to these groups.
The Landcare site I really want to pull out for special focus is their recently developed guide to freshwater invertebrates. Freshwater invertebrates are often use as “indicator species”. Because certain groups of stream invertebrates are very susceptible to pollution or changes to a stream’s natural flow, the presence or absence of these groups in particular stretch of water can give us an idea of the health of that water. In order to help community groups or landowner monitor their streams, Landcare has produced some beautiful photographs of stream invertebrates (along with information on how to sample them, and how well each species acts as an indicator). You really should check out the whole site, because some of them are quite beautiful, I’ll just give you a taster here:
Left: Kempynus lacewing sporting some impressive ‘tusks’. Right: Head shot of the larvae of an Onychohydrus diving beetle. Both images © Landcare Research
Phronima having recently evacuate its salp (© Owen Anderson). Tiny octopus! (photo from Ocean Survey 20/20)
In Dr Zhivago Boris Pasternak describes an epiphany that sneaks up on one of his characters thus:
For a moment she rediscovered the purpose of her life. She was here on earth to grasp the meaning of its wild enchantment and to call each thing by its right name…
I’ve recently learned the name of two species that turned up on these pages unnamed. So, let me introdue you to Thalassohelix igniflua (last seen in “they’re alive!”):
This behaviour seems to be particular common in snails that live in rocky outcrops, and those that live in the tropics, especially the Pacific. I’m not sure about what species the snail depicted above fall into – but they are from the sub-family Microcystinae, which is one of the dominant groups of land snails in the Pacific and is made up entirely of live-bearing species. The large evolutionary radiations that used to live in Hawai’i and the Society Islands were also all live-bearers.
So why give birth to live young? It is easy to see why live-bearing is an advantage to snails living in rocky habitats with few places to deposit eggs. It’s less clear why the Pacific is full of live-bearers. It has been suggested that tropical weather can lead to unpredictable patterns of boom and bust – with snails that can hold on to and grow their offspring in the bad times and release them “ready to go” when conditions are better having an advantage over egg-layers. As far as I know no one has ever come up with a way of testing that idea, so the reasons for the prevalence of live-bearers in the Pacific remains an open question.
Yup, I’m continuing with last week’s theme of terrible jokes. Of course you know the punchline for this one, what’s brown and sticky? A stick:
Both the objects in that photograph are brown and fairly sticky, but the one in the background is a bit more interesting.
That’s not a stick – it’s an insect doing a very convincing impersonation of a stick. Stick insects ( ‘walking sticks’ in North American, Phasmatodea everywhere in the world) are among the most impressive mimics in the biological world. As you can see, their bodies mirror the tiniest details of the plants they live on – right down to having stems and buds. The stickyness of stick insects goes deeper than their remarkable appearence – they also act like sticks. The rigid pose you see above is the result of my disturbing this one while trying to take a photo. The insect was so dedicated to its role I could easily pick it up and place it on its leaf while it maintained its spread-eagle pose.
A few minutes later it was on the move: