Sorry for anyone looking forward to the next part of a series on the first animals - that posts needs more editing than I have time to do this evening.

Instead, I'm going to jump on an internet bandwagon and show you a surprising video that's been doing the rounds:

As I've said before, we have some seriously big invertebrates in New Zealand, but none of them are more impressive than our giant carnivorous snails. We tend to think of snails and slugs as pests that destroy our lettuce plants, but snails are the most diverse group of molluscs and they have adapted to eat a whole range of food. Most snails scrape algae and fungi off surfaces, others are plant eaters, a few are parasites with no mouth at all and a surprisingly large number of them are carnivores. The scrapers and the herbivores eat by extending a rasp-like organ called the radula out from their mouths to chip away add the food at hand and rake into their mouths:

Typical snail feeding anatomy from wikimedia user Debivort - image is CC 3.0

Great as this method is for eating immobile plants and algae, it doesn't really work for carnivorous snails whose prey has the ability to run away. Indeed, most carnivorous snails have seriously re-arranged their feeding anatomy to accommodate their lifestyle. The video above gives us a rare chance to see it in action. Once the snail has worked out where the worm is (using its two sets of tentacles - the smaller ones below are for smelling while the longer ones have eyes on their tips) its pharnyx fulls with blood and is rapidly thrust outside of its body, surrounding the worm. Once the worm is enveloped, the snail's sharp radular teeth will hold on, and start to break its body up as its dragged deeper into the digestive system. Although the actually moment of capture happens with a swiftness that belies snails' reputation as slow moving animals, the rest of the eating process takes a bit longer. The radular teeth are not particularly efficient and it will take several passes for the (still living) worm to be sloughed off into edible pieces.

All of New Zealand's carnivourous snails are from the Southern Hemisphere family Rhytididae. All told we have around 60 species in 6 genera. The video doesn't tell us what species were looking at, but it's probably from the one of the two related genera Powelliphanta and Paryphanta (if you forced me to pick, I'd says this was Po. augusta since the video comes from DoC and, as we'll see, they have a population of that species in captivity). Both these genera contain large worm-eating species with extraordinarily beautiful shells:

Image is CC 2.0from Flickr user SidPix

Most of the Powelliphanta species aren't yet formally described, and seems like there is some interesting evolutionary biology going on in this group. A number of species appear to be linked to each other in what is called a 'ring species' - a long chain of populations in which those adjacent to each other can interbreed but populations distant from each become quite distinct (and probably couldn't interbreed given the chance). I'd really love to get the chance to apply some genetic tools to understanding what's going on there, and, in fact, sorting out the number of species in this genus has important conservation implications. Unfortunately, for all their fearsome eating habits, most of our rhytidids are at risk of extinction. Like the rest of our fauna, they have no natural defence against introduced mammalian predators like possums. Habitat destruction also threatens their future, since some species appear to be adapted to very fine-scale differences in habitat, which makes the risk we took in translocating an entire species that had the temerity to live on mountain with a coal seam seem utterly crazy to me.

Labels: powelliphanta, rhytididae, sci-blogs, snails, sunday spinelessness

I see that Prime is playing a documentary by David Attenborough on the origin of the animals. One of my favourite people talking about one of my favourite topics is motivation enough to dust off one of the many posts in The Atavisms massive "drafts" folder....

You and I are connected. Trace our lineages back long enough and we are guaranteed to find a shared ancestor. In fact, we are each connected to all of life on earth. Every cell in your body can trace its existence back through an unbroken chain of cell division and DNA replication to the origin of life. The same is true of every creature on earth, so, in a very real sense you are connected to all life on this planet.

Animals are a bit weird though. Every piece of DNA in your body is the latest link in chain that goes back to the origin of life, but most of your cells can't continue that chain. Very early on in our development, there is a fundamental distinction between the cells that make our germline (and will go on to make sperms and eggs) and those that make our bodies (the so called somatic cells). It seems natural to us for some of our cells to have no chance of leaving descendants that outlive us. But how the transition from singled celled organisms, each with a chance of reproducing, to multi-celled creatures in which most cells on reproduce 'by proxy' happened is one of the most fascinating questions in biology. So, how did animals evolve?

What is an animal?

Before we can ask how animals came about, we have to know just what makes animals different from other types of life. If you studied biology at school you may remember that animals are "heterotrophs" - that is they eat food rather than make their own. Lots of singled-celled animals are heterotrophs, but almost all animals are multicellular (some very strange single-celled parasites called Myxozoa appear to have evolved from multicellular ancestors). All animals are capable of moving, at some stage in their lives. There are plenty of animals that spend their whole adult life on one spot, but they all have larval forms that can get about under their own steam. Finally, animals are the only group of organisms to go through a "blastula" stage in development, and the only creatures to use collagen to hold themselves together. Since these characters are all unique to animals, and found in all branches of the animal family tree, we'd expect the common ancestor that unites all animals to share them.

So how to we reconstruct this ur-animal? This is one of those exciting fields of science where a whole suite of different tools and methods need be used to try and arrive at a clear picture The events that we are talking about happened around 600 million years ago, there are lots of different ways to try an peer back to that time, but as we'll see, each of them has their own strengths and weaknesses. Over the next couple of weeks I'm going to look at evidence for different methods and see if we can't pull together a consensus view of what the first animals might have been like.

Fossils

If you want to know what animals looked like 600 million years ago, you'd think the obvious place to look was 600 million year old rocks. There used to be a real problem here. Until the 1950s the fossil record seemed to have a very abrupt start in rocks from the Cambrian period (around 540 million years ago). Cambrian rocks had plenty of fully developed animals, while earlier formations seemed to have no fossils at all. The problem of life seeming to arrive fully-formed in the Cambrian period has come to be known as Darwin's Dilemma. In Chapter 10 of The Origin (p308) Darwin talks about it (in a characteristic style that has been abused by creationists who seemed to think Darwin didn't believe his one theories). First he acknowledges the problem, and grants that it could be used to argue against the evolutionary origin of life

To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer... The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.

So Darwin was happy to admit, at least at the time that he was writing, that there was no clear reason why pre-Cambrian rocks had no fossils and this lack of fossils was a mark against his theory. But, he goes on to offer a particular explanation for why pre-Cambrian rocks might be fossil free even if their were many pre-Cambrian creatures (which, as far as I can tell turned out to be wrong) and a more general reason to be skeptical about claims that rested on a lack of fossil evidence. Darwin argued that the evidence geologists had so far uncovered was only a thin slice of the full history of earth, in fact it was like:

A history of the world imperfectly kept and written in a changing dialect. Of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved, and of each page, only here and there a few lines. Each word of the slowly-changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced. On this view the difficulties above discussed are greatly diminished or even disappear.

When we view fossils not as a complete record of life on earth, but as a few snapshots of different periods, the apparently sudden appearance of animals in the fossil record becomes less of a problem.

But Darwin was right, simply saying the fossil record was incomplete might be perfectly reasonable, but it wasn't the satisfactory answer he would have wanted. Darwin's Dilemma lacked that answer until the 1950s and the discovery of Charnia A frond-like impression in a rock discovered by a school boy in England was the first fossil to confirmed to exist in pre-Cambrian rocks.

The first pre-Cambrian fossil, holotype of the genus Charnia. CC2.5 from wiki-commons user Smith609

People had found things that looked like fossils in old rocks before, but they were dismissed or ignored either because the impression were presumed to be of an non-organic origin or because the dates of the rocks were disputed. Charina was different, the impression is clearly biological, and the rocks from which it was collected had been dated by the British Geological Survey. Once Charnia has been established as something real and something pre-Cambrian, people got a bit more serious aobut older fossils. In time an entire community of large creatures were discovered, what we now called the Ediacaran bioata

Though the Ediacarans were morphologically diverse, but there were made up from repeats of some pretty simple patterns. Charnia had is fronds growing from a rib Ediacaria left disc-like fossils and Dickinsonia was a flat oval made of tubular sections (which probably inflated like an air mattress)

Above: A Dickinsonia fossil. Below A reconstruction of an Ediacaran ocean (with way too much light!)

It's hard to say quite what the Ediacarans were. The rocks they are preserved in were formed at the bottom of deep oceans, so it's clear they were heterotrophs (because photosynthesis wouldn't work down there). The fronds like Charnia must have been filter feeders (or "absorbers" feeding by osmosis) while the surface spreaders like Dickinsonia also left trace fossils, suggesting they dragged themselves across the sea floor. It's not clear if there were any predators in these systems (leading some to call the time the "Garden of Ediacara"). So, the Ediacarans were heterotrophs, they could move and they were obviously multicellular - it seems like they must be animals. But placing them into the tree of life we've created by looking at modern animals has proved extremely difficult. When it was discovered Charnia was thought to be like a modern sea pen (a cniderian) but recent evidence evidence suggests that interpretation was wrong. Some researchers, most notably Adolf Seilacher, have argued that the Edicarans don't fit easily within modern animals because most of them aren't very closely related to them. In this view, most of the Ediacaran biota form a distinct group, which seperatley arrived at the idea of being large, multicellular heterotrophs. Others have argued that particular fossils fit into existing groups; Kimberella as a mollusc, or, more sketchily Vernanimalcula as an early Bilaterian.

It's very hard to come down on one side or another here. If the Ediacarans really are early animals, then when we look at them we are looking at the first twigs of what would go on to form the mighty branching tree of animal life. We shouldn't expect those first twigs, all those years ago, to contain the traits that would go on to define animal groups. On the other hand, the fossil record is so fragmentary, and gap between out time an theirs so great, placing any particular early fossil into a modern takes a bit of leap. It seems likely some of the creatures preserved in Ediacaran rocks are closely related to animals, and some others are experiments in multicellular life that burnt out. (For what it's worth, this guy thinks the are all lichens). Whatever the Ediacarans were, they left the scene, replaced rapidly in the fossil record by the Cambrian fauna. The amazing diversity that arose in the so called Cambrian explosion is worth more than blog post by itself, but it's not really relevant to the question at hand, reconstructing the first animals.

I should include a little discussion on some of the claims to "first animal fossil" you might have read in headlines. "Firsts" are important in paleontology, and they're a sure fire way to get your work into a good journal, but the further back we go the more circumstantial the evidence becomes. Some very old rocks have biological chemicals that are as far as we know only produced by modern animals. That is circumstational evidence for the presence of animals in those periods, but when the rocks are hundreds of millions of years older than the oldest known animals, it seems like good evidence that some animal-ancestors could make those chemicals too. Similarly, there are very old trace fossils (tracks left behind by crawling creatures), but we know modern single-celled eukaryotes are capable of producing similar tracks, so they don't provide water tight evidence for the presence of animals in the sediments that have been preserved.

So, fossils seem to leave us with as many questions as answers. The patchiness of the record means we can't be sure the Ediacarans were early animals, the earlier "animal" fossils are sketchy at best and the Cambrian fauna (wonderful as it is) is just too late to tell us about the origins of animals. Next week, I'll see if modern organisms can't help up understand how ancient animals might have got their start, but now I'm going to tune in to David Attenborough and see how much we agree with each other... (8:30, Prime TV)

Labels: animals, eumetazoa, evolution, first animals, fossil, sci-blogs, sunday spinelessness

The photos I share here are really not representative sample of the bugs I come across in my travels. For me to get photos of a bug I need to get pretty close to it, and that means i can't get nice pictures of creatures that are skittish or particularly fast moving. For instance, there's a longhorn beetle (Cerambycidae) species I find quite often in Dunedin but have never taken a nice photo of, because every time I line one up the subject runs out of frame or out of focus.

Even though I've never taken a good photo of this beetle, I do have one that displays it's most unusual feature. It has two ridges growing out of it's back:

I can't imagine what, if any, purpose those protuberances serve, but i quite like them. Their spikiness, and their shape makes me think of tiny little circular saw blades sticking out of a saw bench.

Labels: beetle, environment and ecology, photos, sci-blogs, sunday spinelessness

Sometimes it's tempting to look back on the history of science and feel just a little envious of the people that went before us. Imagine being alive at a time when you could jump on a boat, sail to some tropical country and discover thousands of species not yet known to science. Even better, you could use your discoveries to understand some of the most important ideas in biology. Darwin is the obvious example, but imagine being Wallace and finding flying frogs and searching for birds that were said to spend their whole lives on the wing and, so, have no feet. Or being with Banks and Solander as they arrived in New Zealand and discovered an entirely new flora to describe, catalogue and learn from.

It would be pretty great to live in those times, but I wouldn't swap places for the world - biology's age of discovery is still going strong. There are probably ten times as many species on earth as we know about. In the last couple of months we've heard news of new fish species discovered in the kermadecs, a nematode that survives a mile below earth's surface, bioluminescent fungi and an antelope species discovered in a meat market. Then there's biodiversity's dark matter; whole groups of creatures for which the fac they exist is all we know about them. Recently, DNA sequences have revealed a new group of fungi discovered in a pond in Devon, and maybe, just maybe, a whole new branch of life in a set of DNA sequences that are similar to each other but like nothing else we know. In the 21st century there is no lack of species for us to discover, and, in fact, modern tools mean we don't have to get on boat to discover them. Your average shovel load of soil almost certainly has bacteria and fungi that aren't known to science.

Sunday spinelessness has, rather arbitrarily, limited itself to animals (I am from a zoology department, these things rub off) - and one of my favourite examples of how little we really know about biology comes from some very strange animals. Meet a placozoan (literally 'flat animal'):



That's really an animal. It has no tissues, no organs, no front or back (let alone a mouth) and it has only a handful of specialised cell-types. But it's an animal none the less. Placozoans have been on earth for 600 million years, but it took us until the 1880s to notice them. F.E. Schulze was a German zoologist who specialised in simple animals like sponges, and when he stumbled across these guys living in marine aquariums he recognized them as animals and created the species Trichoplax adhaerens for them. Schulze also recognized his flat animals were quite unrelated to anything else in the animal kingdom. Unfortunately, other people had different ideas. Until the 1970s, text books said Schulze's Trichoplax wasn't a species in its own right, rather, they said, it was the larval form a some cnidarian (the group containing jellyfish, corals and their kin) or other. It wasn't until the 1970s that it became clear placazoans really were animals, and the ones people were finding in aquariums were adults. So, what have we leaned since then?

What do we know about placozoans?

They're small, up to 3 mm across. They don't have organs, or tissues (sponges are often cited as the only other animals to lack these, but I think some of the recently discovered small phyla don't have them either) but they do have specialised cells. In particular they have a layer of stretched out 'contractile' cells they use to move (FC in the diagram below) , these cells sandwiched between an upper- and a lower-level of epithelial cells (UE, and LE respectively).

When you zoom back from the cells and look at the animal you get... a blob (Mike Dickison tells me he thinks "they're tiny Blobs from Mars, and are slowly striving to meet up and form a giant Blob and destroy New York.")


They live in the 'littoral zone' - the part of the sea closest to the shore. Because they're tiny, and basically see-through, they've never been studied in their natural habitat. Instead, they're collected from rocks or microscope slides suspended in the water column. As the video above shows, they can be studied in the lab.

The adherens bit of their species name refers the fact they stick to surfaces (presumably rocks and shells in their natural habitat) where they eat algae and cynanobacteria. But how do they eat? I said before, they don't have mouths. Instead, they slide their entire body over a food particle and and fold-up to form a 'digestive cavity' into which enzymes that break down the food items can be secreted.

If I was writing this post way back when I first had the idea to, I would have said that placazoans definitely reproduce asexually and that they can probably also reproduce sexually. Last month Micheal Eitel and colleagues upgraded that 'probably' to an 'almost definitely'. Using genetics and some advanced microscopy they showed evidence for active growth of sperm and eggs in adults, and also detailed the stages of embryonic development.

Amazingly, one of the few things I can say we know about the Placazoa is the complete sequence of their genome. In 2009 researchers put the 90 million DNA letters together ( that 's about 33 times smaller than our own genome, and about 150 smaller than an onion's before you get too carried away about what that number might mean). The placazoan genome is of particular interest to cancer geneticists, as it has versions of two genes that usually protect us from the mutations that kick off the uncontrolled cell growth that characterises that disease.

What don't we know about placozoans?

Placazoans are almost always described as either 'ancient', 'primitive' or, worst of all, 'a living fossil'. Ancient really doesn't make any sense, all animal phyla have been on earth for 600 or so million years, so it's hard to see how a rabbit or a chicken is anymore ancient than a particular placazoan species (even if there really is only placozoan species at the moment). Primitive is not much better. The term means "similar to the ancestral state", there is no doubt that the first animals were simple, but it doesn't follow that because placazoans are simple they are like the first animals. After all, placazoans have been evolving for 600 millions years too, and it's entirely possible that they have ancestors that were more complex than the modern examples (the idea that evolution always creates more complex organisms seems to be one of the more persistent misunderstandings of biology). I'm not even going to talk about "living fossil".

So, placazoans aren't necessarily like the first animals, but that is a hypothesis we can test. If all other modern animals are more closely related to each other than they are placozoans then it would be more likely (still not certain) that modern placazoans are similar to ancient animals. If, on the other hand, placazoans fit somewhere in the middle of the animal family tree, it's more likely that their simplicity is something they evolved (what biologists called a "derived character"). We don't know which of those scenarios is the right one. Phylogeny (the methods we use to reconstruct the relationships between organisms) is really really hard when look way back into deep time, so different studies into this question get conflicting results. The placozoan genome study included a phylogeny that suggested sponges were first group to diverge from the animal tree, with placazoans more closely related to you and me than spognes (increasing the odds that placazoans are derived forms of more complex animals) but that study didn't include many other animals, possibly reducing its power. Another study suggested placazoans form a group along with sponges and cnidarians, but that study treats a whole set of gene sequences as a single piece of data. I don't mean to be unkind, but I think that's the worst way to do phylogeny. Every gene is an independent witness to the evolution of a group, and munging a whole lot of them into a single piece of data it just a terrible idea. So, we remain uncertain as to where placazoans fit into the animal kingdom.

We also don't know how many species there are. At the moment only one is described, but there are clear morphological and genetic differences between different placazoans and it seems likely there are several more.

We really don't know where placazoans live. They've been recorded in most of the places that people have looked for them in the tropics, as well as decidedly more temperate places like the Pacific Coast of the USA and the Sea of Japan. They will almost certainly be present in Northern New Zealand, but, as far as I know, no one has looked. If anyone up there wants a more exciting animal to study for their Year 13 Biology course, the best places to look are sandy beaches with some cover and a weak current. The easiest way to collect them is to suspend a box of microscope slides in the water column and leave them for a week or two to let a nice little community of algae, and hopefully a few algae eaters, develop on their surfaces. Then you'd need a dissecting microscope to examine your slides. You might not find anything, in which case you could fall back on a sea monkey colony or whatever the usual study animal is, but wouldn't it be fun to try?

What don't we know about life?

Placazoans are just one example about how much we still have to learn about life on earth. If we limit ourselves to animals, there are two phyla (remember vertebrates and all the lions, tigers, bears, fish, eagles, frogs and lizards you can think of are only part of one phylum) that were only discovered in the late 20th century (Cycliophora and Loricifera). There is a huge amount that we still don't know about life of earth. But just as international travel gave Darwin and Wallace and Solander and Banks a new chance to understand biology; DNA technology, new imaging methods and the possibility of collaboration with experts through the Internet givens 21st century scientists a unique opportunity to fill in the gaps in our knowledge. There has never been a better time to be a biologist!

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Eitel, M., and B. Schierwater. 2010. “The phylogeography of the Placozoa suggests a taxon‐rich phylum in tropical and subtropical waters.” Molecular Ecology 19: 2315-2327. doi:10.1111/j.1365-294X.2010.04617.x.

Labels: animals, placazoa, sci-blogs, science, sunday spinelessness