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!
Eitel, M., and B. Schierwater. 2010. ’The phylogeography of the Placozoa suggests a taxonâ€rich phylum in tropical and subtropical waters.’ Molecular Ecology 19: 2315-2327. doi:10.1111/j.1365-294X.2010.04617.x.
Schierwater, Bernd, Michael Eitel, Wolfgang Jakob, Hans-JÃ¼rgen Osigus, Heike Hadrys, Stephen L Dellaporta, Sergios-Orestis Kolokotronis, and Rob Desalle. 2009. ’Concatenated analysis sheds light on early metazoan evolution and fuels a modern ‘urmetazoon’ hypothesis.’ PLoS Biology 7e20. doi:10.1371/journal.pbio.1000020.