5d ago

Carcinisation

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Fun fact: horseshoe crabs are not only not crabs (infraorder Brachyura), not only are they not decapods (order Decapoda), and not only are they not class Malacostraca, they're not even crustaceans (subphylum Crustacea).
Instead, they're in the subphylum Chelicerata, making them more closely related to a tarantula, a daddy longlegs, and a tick than they are to any crab.
- They are part horse though, right?
  
  Actually, part shoe.
  
  And like many a shoe, has 10 eye-let
- My favorite horseshoe fact is that they have nine eyes, of all different types. And that's not even counting the tail which is itself covered with photoreceptive cells - so it's essentially a tenth eye.
  
  be not afraid
- I was under the impression that chelicerates were categorized as crustaceans, along with hexapoda. I'm not an expert in this, I just watch the phylogeny videos of Clint's Reptiles and AronRa.
  
  Close. Arthropoda is the phylum under Animalia. From there you have the four subphyla Crustacea (crustaceans), Hexapoda (insects), Myriapoda (centipedes and millipedes), and Chelicerata (chelicerates). So they are closely related relatively speaking, but they're separate subphyla.
  If anything, funnily enough, the clade Pancrustacea is comprised of just crustaceans and hexapods to the exclusion of myriapods and chelicerates. Even more messy – I learned just now that – is that Arthropoda can be divided into two clades Chelicerata and Mandibulata (where Mandibulata contains myriapods and Pancrustacea), so in reality, not only are horseshoe crabs more closely related to a black widow spider than to crabs, but crabs are substantially more closely related to a mosquito than to horseshoe crabs.
  
  !Subscribe to arthropod facts
  
  Welcome to Arthropod Facts! Did you know that king crabs aren't actually crabs? King crabs (family Lithodidae) sit in the hermit crab superfamily Paguroidea in the decapod infraorder Anomura, whereas true crabs comprise the decapod infraorder Brachyura. The current strong and widespread scientific consensus is that king crabs are hermit crabs which originated in the North Pacific about 15 million years ago and evolved a crab-like, calcified exoskeleton through carcinisation.
  Evidence for this includes:
  King crabs exhibit the same asymmetries that hermit crabs do, such as the asymmetrical abdomen (underside) found in females but not males.
  Lithodidae can be divided into the two subfamilies Lithodinae and Hapalogastrinae. Respectively, these have a calcified abdomen and an uncalcified/partially calcified abdomen. This suggests that Lithodinae fully calcified whereas Hapalogastrinae only partially calcified.
  Many king crabs (mainly Hapalogastrinae) are exclusively distributed in the North Pacific and are adapted to only living in cold waters.
  Molecular phylogenetics (the study of things like how species differ in their DNA) strongly suggests that Lithodidae is most closely related to the hermit crab family Paguridae.
  An easy way to distinguish a king crab from a true crab is to look at the number of visible legs it has. While all decapods have ten legs, the last (hind) pair of legs in king crabs are dramatically reduced in size and usually sit inside the branchial chamber – used for cleaning rather than walking. By contrast, crabs always have their last pair of walking legs, although in the crab superfamily Portunoidea these are flattened and used as paddles for swimming. Thus, when counting the chelipeds (claw-bearing arms) as legs, you'll see eight legs on a king crab (four rows of two) and ten legs on a true crab (five rows of two).
  Thank you for subscribing to Arthropod Facts!
  
  Thank you so much!
  
  Thanks for the info, I love learning this kinda stuff.
- A fellow Clint’s reptiles enjoyer?
  
  Is that a YouTube channel?
  
  It is! he’s a biologist that has gone through phylogenies of many living creatures with the goal of hitting every clade I believe. I’ve found him recently, so I can’t say I really know. But, I’ve been watching a lot of it. It’s fascinating.
  
  Omg that's so cool. New thing to binge at work on slow days. 😎
  Thanks!
- Well that checks out, considering they are more terrifyingly stranger then any other crab.

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