Looking round me again, I saw that, quite near, what I had taken to be a reddish mass of rock was moving slowly towards me. Then I saw the thing was really a monstrous crab-like creature. Can you imagine a crab as large as yonder table, with its many legs moving slowly and uncertainly, its big claws swaying, its long antennæ, like carters’ whips, waving and feeling, and its stalked eyes gleaming at you on either side of its metallic front? Its back was corrugated and ornamented with ungainly bosses, and a greenish incrustation blotched it here and there. I could see the many palps of its complicated mouth flickering and feeling as it moved.
“As I stared at this sinister apparition crawling towards me, I felt a tickling on my cheek as though a fly had lighted there. I tried to brush it away with my hand, but in a moment it returned, and almost immediately came another by my ear. I struck at this, and caught something threadlike. It was drawn swiftly out of my hand. With a frightful qualm, I turned, and I saw that I had grasped the antenna of another monster crab that stood just behind me. Its evil eyes were wriggling on their stalks, its mouth was all alive with appetite, and its vast ungainly claws, smeared with an algal slime, were descending upon me. In a moment my hand was on the lever, and I had placed a month between myself and these monsters. But I was still on the same beach, and I saw them distinctly now as soon as I stopped. Dozens of them seemed to be crawling here and there, in the sombre light, among the foliated sheets of intense green.
- H G Wells, ‘The Time Machine’
TRIGGER WARNING: THE SECOND IMAGE SHOWS A SPIDER, FIVE LINES BELOW THE FIRST.
As Wells’s Time Traveller approaches the end of the world, he stops his time machine and witnesses a cold, almost dead world populated by only a small number of remaining species. There’s the large butterfly-like animal in the sky. There are lichens on the rocks. There is the later tentacled thing in the water. Also, famously, there is the gigantic predatory crab. This is what I want to talk about now. For the purposes of ‘The Time Machine’ at least, H G Wells clearly expected one of the last terrestrial animals living on the surface of this planet to be a giant crab.
In Arthur C Clarke’s 1973 novel ‘Rendezvous With Rama’, which I’m about to spoil, one of the characters crashes his sky bike in an alien habitat to find it being “eaten” by a giant biological robot resembling a crab. So there: crabs again.
Thirdly, on a personal note I once tried to design a terrestrial animal without making any assumptions who could manipulate their/its environment, and found myself coming up with a crab. That is, a squat box of a body with incorporated mouth and sense organs at the front, bilaterally symmetrical with jointed legs and a pair of pincers. I can’t remember the exact reasons why this seemed a good idea at the time, and it would be good to do so because it might explain a certain tendency which has been observed and also exaggerated.
Okay, so there’s this thing called “carcinisation”. It’s been inaccurately described as “every living thing is evolving into a crab”. This is obviously not true. For a start, many living things are evolving into trees instead, and scorpions, pseudoscorpions and lobsters are all suspiciously similar, but there is indeed something about animals called crabs. It’s actually two different things. One is that we tend to use the word “crab” to describe a lot of different animals who are definitely not crabs and the other is that there are a lot of crustaceans who are very crab-like indeed, and their ancestors were often much less crab-like. I’ll start with the first bit.
So, there are horseshoe crabs, crab spiders and crab lice. None of these are crabs and with the possible exception of horseshoe crabs, nobody really thinks they are. This is a horseshoe crab:
I don’t think anyone thinks these are really crabs do they? The main part of their body is not longer than it is wide, is rather flattened and is covered by a chitinous exoskeleton, and they live in salt water and on beaches, so in that sense they’re like crabs, but they’re more closely related to crab spiders than actual crabs, whatever actual crabs are. They’re remarkable in all sorts of ways, but also remarkably, I’m not going to digress about them here.
Speaking of crab spiders:
. . .that’s what this is. These live in flowers awaiting insects landing on them such as bees, and they’re unusual for British land animals because they can change colour to camouflage themselves against their backgrounds. They look a lot more like crabs than their relatives. By “crab”, I mean the classic flat and wide bodied decapods with claws.
Then there are these delightful individuals:
It’s unfair to judge parasites for their life styles. Nobody ever asks to be an obligate parasite as far as I know. Then again, we presumably have our own instinctive revulsion of organisms, which we can’t really help either. Actually we can, but it can be difficult. I used to be a koumpounophobe, so I do realise people can’t do anything about this stuff unless a miracle occurs (which it did for me). I mean, you can have therapy I suppose. These are of course insects and there are only two species of them, whose other lives on gorillas. This makes me wonder why it’s just them and us. They can also live on eyelashes, particularly children’s, and that’s a whole rabbit hole I’m not going to talk about. Presumably they’re that shape because it makes them harder to catch, and like head and body lice they’re dorsolaterally compressed, and I think they cling on with their front legs. And they’re not crabs.
Seeing as I’ve mentioned crab spiders, I may as well mention spider crabs. I won’t be showing a picture of them because of arachnophobes who might be reading this, but they’re not spiders but crabs. Or are they? This brings up the whole issue of what a crab is. This is yer bog standard crab:
Now this is definitely a crab. As the caption says, they’re Cancer pagurus, the so-called “edible crab”. I don’t know about you, but I’d be a little disturbed if some Dannelian decided to call me an example of an “edible human”, and it seems a bit unfair to define their identity in this way. In a way of course, they do crawl around being edible, but then many other animals do, not always by humans but by some other species. Also, we don’t call sheep “edible sheep”, and other species of crab can be eaten, so why?
Anyway. . .
This is a crab, that much is for sure, and there are other species of crab related to them who are presumably also crabs, such as the swimming crab:
This is also definitely a crab, whose back legs are specialised for swimming, and I’d be gobsmacked if this species turned out not to be closely related to the previous one. This makes me wonder how much crabs who are not crabs resemble crabs who are. Nonetheless, there is this thing called carcinisation.
The issue was first raised formally in 1916 by the zoölogist Lancelot Alexander Borradaile, who referred to it as:
… the phenomenon which may be called “carcinization” … consists essentially in a reduction of the abdomen of a macrurous crustacean, together with a depression and broadening of its cephalothorax, so that the animal assumes the general habit of body of a crab
- British Antarctic (“Terra Nova”) Expedition, 1910–1913. Natural History Report. Zoology. 3 (3). British Museum: 111–126.
What he meant by this was that many crustaceans evolve in the direction of having a carapace wider than its length, the belly side is fused into a kind of breastplate similar to that of a tortoise, turtle or terrapin, and the rear portion is flattened and bent over, completely invisible from above. The crucial feature distinguishing true crabs from merely carcinised crustaceans is that the latter only have six walking legs. Hence the porcelain crabs, for instance, are not true crabs:
These have three pairs of legs plus claws at the front, so they presumably started off with four pairs and the front pair became claws. In fact, one particular porcelain crab is said to be “hypercarcinised” in that they are not only crab-like but the males have a smaller pleon (the bent-forward underside bit) than the females, which is also the case for true crabs. This is the crab in question:
Crabs have apparently evolved five times, or rather the crabs who are still around today are of five only distantly related lines. There are the hermit crabs, who are distinctive in being asymmetrical and gave rise to the coconut crabs:
These are the largest land-living arthropods of all, weighing four kilogrammes and are almost a metre across with their legs spread out, and they don’t spend much time in the water at all. Adults actually drown in water, so their arrangement is like amphibians, with larval forms in the water crawling onto the land to become adults. Their affinity to hermit crabs can be discerned through their relatives the king crabs, who are still asymmetrical, which hermit crabs need to be so they can fit themselves into whelk shells. And of course hermit crabs are not crabs. Along with porcelain crabs, they belong to the order anomura rather than the true crab order brachyura. They’re related to squat lobsters:
Hermit crabs might be described as needing to live in shells (they also live in rubbish like bottle caps) because their exoskeletons are too soft, but it also makes sense to say they save themselves the bother of growing hard carapaces by using shells instead. Some other animal builds the armour, dies and they then recycle it.
A further, related example is the king crab:
Hairy stone crabs are yet another example:
Although all the other examples given so far are crustaceans in the infraorder anomura, whereas true crabs are brachyura, that’s not an order exclusively of crabs and their ancestors weren’t crab-like. There’s a further, long-extinct, example, consisting of an entire order, the cyclida, living from the Carboniferous into probably late on in the age of (non-avian) dinosaurs, who looked like this:
Palaeontologists disagree about what cyclids were. Although they’re usually considered crustaceans, a minority believe they were chelicerates, i.e. related to spiders and horseshoe crabs. If so, it’s all the more remarkable that they’re like crabs.
To a limited extent, turtles are somewhat crab-like, although since they lack the general body plan of arthropods they aren’t that similar. All of the crab-like animals mentioned here are arthropods. Crab lice and crab spiders are apparently merely coincidentally crab-like, as they live out of the water.
What, then, are the pressures that tend to make decapod crustaceans into crabs? Well, the living examples are all decapods, which incidentally seems to specialise in convergent evolution as it also includes the scorpion-like lobsters and crayfish, so they have somewhat similar genes which may mutate in the same direction. The intermediate forms are usually like squat lobsters. Although none of their ancestors were similar to crabs, they were in fact similar to each other. The ancestor of the true crabs was also, unsurprisingly, not particularly crab-like:
This, Eocarcinus, was early Jurassic and once again somewhat squat lobster-like. I don’t know how much cyclids and early brachyura overlapped, but I suppose it’s possible that the latter were “new and improved” and edged the others out, presumably sideways.
There are other factors. Their relatives flex their hind ends to escape rapidly but because crabs’ are bent under, this is no longer possible and they have to run instead. This is also true of coconut crabs because they live on land, so they don’t have the advantage of pushing themselves around in water.
All this, of course, is an example of convergent evolution, although in fairly closely related animals who already have the genetic precursors and appropriate environment to help them do so. To clear up the big myths, this is not an unusual process and not everyone is turning into a crab. Humans have lost their tails, are dorsoventrally flattened and have grabbing hands, but we’re not becoming any more crab-like than that and apart from the opposable thumbs, these are not to do with the pressures on crabs to go that way. Even so, it’s conceivable that because this process also leads to front claws that can manipulate objects and is encouraged by living out of the water, other intelligent life forms in the Universe might actually look like crabs, which explains the Macra on Doctor Who.
To me, there is a much more spectacular example of convergent evolution in the form of pitcher plants. We have for a few years now had Nepenthes pitcher plants growing in our utility room:
A while back, I also tried to grow Sarracenia but it died:
These are trumpet pitchers and are in the same order as heather. They mainly grow in Canada, which is why I thought they’d be fine here in England, but actually they died, possibly because I accidentally chose a species native to subtropical North America instead. The Nepenthes, though, have thriven. These are Old World tropical plants in a huge order including cacti, chickweeds and soapwort, although there are so many of them that this is a pretty arbitrary choice of examples. A third lot of pitcher plants is in the bromeliad family along with pineapples and papaya, including tank bromeliads, which are quite amazing. They have a whorl of leaves, really blades as they’re related to grasses, in whose centre rainwater collects and into which, like the others, insects fall and are digested. Unlike the other pitcher plants, though, tank bromeliads have entire ecosystems living in the water too, including frogs and salamanders. One example of a tank bromeliad is Brocchinia reducta, one of three carnivorous species of the plants:
It’s fairly easy to understand how a bromeliad could become carnivorous. Pineapples and papaya both contain enzymes which break down protein (proteases). Pineapples also have a whorl of leaves at the top which would collect rainwater. In a tropical environment, leaf litter and also dead insects are likely to fall into this water, and in the latter case drown. From this, it’s a small step to them eating insects, using them the way other plants use leaf litter and decaying animal life to fertilise their roots. Incidentally, in a side note, it’s also easy to see an affinity between pineapples and sweetcorn cobs.
A further example is Cephalotus follicularis, the Australian pitcher plant:
Once again, the same thing has happened and this time the plant is a lot more like Nepenthes, but again is not closely related to any of the others. Thisses family contains just this single species, but they’re related to wood sorrel and a number of other plants more familiar to Europeans.
Apparently this has happened eleven times. In the case of Nepenthes, two species have symbiotic relationships, one with treeshrews and another with bats. The bats roost inside the pitchers and poo into them, and the treeshrews use them as toilets, in both cases providing nitrogen. Pitcher plants sometimes appear to be related to flypaper trap plants such as sundew, which operate by having sticky leaves which trap insects, and may either be evolving into pitcher plants or have evolved from them. Again, this has happened several times in unrelated lineages.
There are many other examples. Koalas, for instance, have fingerprints indistinguishable from ours and also opposable thumbs, although in their case both the first and second digits are involved. That occurs in humans sometimes in a condition known as Robinow Syndrome, which I don’t actually consider to be a valid syndrome so much as an individual variant. There’s also the very obvious ichthyosaur/dolphin/sawfish business.
So carcinisation, though interesting, certainly doesn’t mean everyone will eventually turn into a crab. Some carcinised species have also become less crab-like over time. For some reason, people have latched onto it and turned it into a seemingly bigger thing than it really is. It’s interesting, and it’s a good example of convergent evolution, but I very much doubt that any of my descendants will ever be living on a beach with a carapace and claws.
A Box Of Chocolates 