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Chapter 1: What unusual behavior did coconut crabs exhibit in 2017?
In 2017, behavioral ecologist Mark Ladry documented some pretty odd behavior from coconut crabs on a small island in the Indian Ocean. He had set up cameras to better understand their foraging patterns, and in the middle of the night, one of the cameras showed a coconut crab climbing a tree. And yes, climbing.
Eventually walking out into a low branch where a booby, a type of bird, mind you, was sleeping. At that point, it then proceeded to stretch out its claw, lock them around the bird's wing, and boom, snap, breaking its wing and causing it to fall to the ground.
Now, the bird tried to fight back, but he might as well have been fighting a living tank, as the crab did not give any quarter, and proceeded to smash the heck out of it, ultimately causing the bird to become paralyzed. And within 20 minutes of this starting, five more of its bloodthirsty homies had shown up to join the, uh, fun. And by morning, the bird was completely gone.
And when I say gone, I mean eaten. Now, Mark also surveyed the surrounding islands and found something rather grim, which is that on islands with high coconut crab densities, ground nesting seabirds basically do not exist.
On the flip side, the one nearby crab-free island in the survey found ground nesting birds were happily nesting on the ground like, well, any other self-respecting ground nester. So in other words, an entire seabird community had been bullied, or rather made locally extinct, by an arthropod. Oh, and by the way, coconut crabs aren't technically crabs. Yeah, the name's deceiving.
And as it so turns out, coconut crabs are not alone in this. A surprising number of the animals we casually call crabs aren't actually crabs in the taxonomic sense. They just look like them. King crabs, not crabs. They're hermit crabs that gave up on the shell. Porcelain crabs, not crabs either. They're more closely related to squat lobsters.
the hairy stone crab clinging to rocks in southern Australia, while also looking like a crab and being hairy, also not a crab. And coconut crabs, the nightmare fuel that they are, are, as we just established, also not crabs. And the really weird thing isn't just that they aren't crabs, it's that they aren't even closely related to them either.
These are completely separate lineages of crustaceans that somewhere along their evolutionary history independently arrived at roughly the same body plan, despite starting from totally different taxonomical places.
And this happens so frequently that there is even a name for it, carcinization, a term coined back in 1916 by an English zoologist named Lancelot Alexander Bordale, who described it as nature's many attempts at evolving, well, crabs. And in a 2021 phylogenetic synthesis, researchers estimated that the fully crab-like body plan has evolved at least five times independently in different lineages.
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Chapter 2: How do coconut crabs impact seabird populations on islands?
Now, the easiest way to tell them apart in person is to count the walking legs. True crabs have one pair of claws up front and four pairs of walking legs behind them.
However, on the flip side, many crab-shaped onomorins have one pair of claws and only three obvious pairs of walking legs because the back pair is reduced and tucked up under the carapace, where it mostly gets used for cleaning the gills.
Small anatomical difference, yes, but it's important, as it means a king crab and a Dungeness crab, which is, yes, its actual name, who might be on your plate, are in reality, despite appearing to be cousins at the very least, are separated by hundreds of millions of years of independent evolution. So yeah, they are more distant related than we are to literally any other living mammal on Earth.
Anyways, going back to the star players here, the crabs, well, not crab crabs, but crabs in name and crabs in shape and, uh, well, you get the point. So, king crabs are one of the more dramatic transformations in this story, at least evolutionarily speaking. You see, they aren't crabs, which, yes, has been established, but get used to hearing this.
They're rather hermit crabs that decided to be a bit more social. So, hermit crab lineages have spent over 100 million years renting out discarded snail shells, and their entire anatomy has been shaped by that lifestyle. Asymmetric claws, because the larger one has to plug the entrance to the shell like a cork,
a soft, uncalcified abdomen that corals perfectly into the inside of a gastropod shell, and a growth ceiling that's basically capped by how big of an empty snail shell they can find on the seafloor, which means a hermit crab's entire life is essentially one long, stressful search for a home.
In a landmark study from 1992, place king crabs phylogenetically inside the hermit crab genus Pigurus, and pretty much every molecular study since is confirmed a hermit crab ancestry. So the idea is that somewhere in the past, selection started favoring hermit crabs that could calcify and harden their own abdomens, basically growing their own shells instead of renting one out.
And once that shell size constraint relaxed, well, level up as the lineage produced some absolute giants. For example, a big red king crab can reach roughly 28 centimeters or 11 inches across the carapace, and then a leg span of around six feet or 1.8 meters and weigh up to about 28 pounds or 12 kilograms. So in other words, basically a small dog with very, very long claws.
And if you flipped one over, the abdomen is still subtly asymmetric, a leftover from an ancestor that once coiled into a spiral shell. Porcelain crabs, the family Porcelanidae, are another case of carcinization, but they went in a totally different direction.
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Chapter 3: What is the significance of carcinization in crustaceans?
Which for their environment, it actually was. Then speaking of environments and lifestyles, the crab body plant is pretty much like a Swiss army knife, because it seems to do it all. Without the tiny end of the spectrum, you've got the peak crabs in the family Pinotharidae, which grow to just less than a centimeter and spend most of their lives living inside oysters, clams, and mussels.
Well, then at the giant end, you've got the Japanese spider crab, Macrochira caimferi, whose leg span can reach roughly 3.7 meters, or about 12 feet from claw tip to claw tip, making it the longest leg span of any living arthropod.
then you have ghost crabs on the other hand who are quite small but can then sprint across beaches at speeds of about 2.1 meters per second or roughly 6.9 feet per second all the while having muscle contractile properties and locomotor characteristics that look weirdly similar to those of a mammal not a crab which is not a sentence i would ever expect to say about well a crab and then there's the coconut crab which as we covered found the word pairings of ocean and no thanks with chicken and yes thanks quite tantalizing
So clearly, the crab shape isn't just specialized for one thing, which thus begs the question of why this shape is such a versatile, well, shape in the first place. And the answer is mostly mechanical.
First off, a flat, wide body gives a lower center of gravity that makes the animal more stable on uneven surfaces like intertidal zones, while then also giving it its key ability, the sideways shuffle, which allows the crab to dart left or right while most of its predators are forward-moving animals.
then of course the tucked tail removes what would otherwise be a perfect grabbing zone for would-be predators, as you literally cannot grab a crab by the tail, because duh, there is no tail to grab. Now, interestingly, true crabs likely originated around the Jurassic period based off of fossil evidence.
But a lot of their modern lineages diversified during an interval of roughly 145 to 66 million years ago, and paleontologists call this stretch the Cretaceous Crab Revolution.
And it overlaps with a broader event called the Mesozoic Marine Revolution, where marine predators evolved stronger jaws, more specialized drilling and crushing structures, and more efficient ways to crack, peel, and grind hard prey. And of course, if that happens, the pressure rounds the other way too, for prey.
And so the whole marine ecosystem was basically a slow motion arms race for millions of years. Now, was it this event that drove the massive diversification event in crabs? Hard to say, but funny enough, it's often crabs that are included in the list of those shell cracking predators, not as the prey themselves. So there's definitely something going on here.
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