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alphynix · 1 year

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Strange Symmetries #13: The Hermit Crab Cycle

Hermit crabs are crustaceans that first appeared at the start of the Jurassic, about 201 million years ago. Despite their common name they aren't actually true crabs, instead being a classic example of convergently evolving a crab-like body plan via carcinization.

They also have noticeably asymmetric bodies, with abdomens that coil to one side and differently-sized front claws.

Pagurus bernhardus by Arnstein Rønning | CC BY 3.0

And while modern hermit crabs are famous for inhabiting scavenged snail shells, their fossil record suggests this wasn't always the case.

Originally, they seem to have lived in ammonite shells.

Palaeopagurus vandenengeli lived in what is now northern England during the Early Cretaceous, about 130 million years ago. Around 4-5cm long (~1.6-2"), it was found preserved inside the shell of the ammonite species Simbirskites gottschei.

Its left claw was much larger than its right, and together they would have been used to block the shell opening when it was hiding away inside. And while the exact shape of its abdomen isn't known, it probably asymmetrically coiled to the side to accomodate the spiralling shape of the host shell.

Hermit crabs seem to have switched over to using gastropod shells by the Late Cretaceous, around 90-80 million years ago, possibly due to marine snails developing much stronger sturdier shells during this period in response to the increasing prevalence of specialized shell-crushing predators. The more upright snail shells would also have been much easier to drag around the seafloor than ammonite shells – and meant that they were ultimately less affected by the total disappearance of ammonites during end-Cretaceous mass extinction.

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#science illustration #strange symmetries #paleontology #paleoart #palaeoblr #palaeopagurus #paguridae #paguroidea #hermit crab #anomura #decapod #crustacea #arthropod #invertebrate #art #carcinization

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fbwzoo · 3 years

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So whats the difference between the hermit crabs you raise, and the full aquatic kind?

If I remember right, the hermit crabs you have are predominantly terrestrial, where as the others are predominantly aquatic, as well as significantly smaller than their terrestrial counter part.

But are they like off shoots of the same species?

I’m not sure of evolutionary development, unfortunately, and that’s more complicated than I can look up right now. I would assume they have a shared ancestor, but possibly several? Wikipedia says “The fossil record of in situ hermit crabs using gastropod shells stretches back to the Late Cretaceous. Before that time, at least some hermit crabs used ammonites' shells instead, as shown by a specimen of Palaeopagurus vandenengeli from the Speeton Clay, Yorkshire, UK from the Lower Cretaceous.“

But these days, there are over 800 different hermit crab species, and they’re all in the same superfamily, called Paguroidea. There’s 7 families in that group, and 6 of those families are marine/aquatic hermit crabs. The seventh family, Coenobitidae, is all terrestrial hermit crabs, like mine!

The terrestrial hermit crabs include 17 species, including the largest one, the coconut crab! It’s the largest land-dwelling arthropod. We get questions about if anyone’s ever had one on my crab groups sometimes and the consensus is “Fuck no, where would you put it”.

(Photo source)

Coconut crabs are in their own genus, and the rest of the terrestrial crabs are in another. Not all of them are kept as pets, and they live in different parts of the world - there’s only two that live on the west hemisphere. One is Coenobita clypeatus, aka the purple pincher or Caribbean hermit crab. This is also the one that’s mostly found in the pet trade in North America, and most commonly kept as a pet. This is the species all of my crabs are! Also the ones that are available for adoption from Mary at the moment. The other species is Coenobita compressus, the Ecuadorian hermit crab, and they are the other most commonly found in North America’s pet trade.

All of the other species, to my knowledge, are found in the East hemisphere, largely in the tropics. Australia has one species, Coenobita variabilis. A lot of the others are found in similar areas, on the Indo-Pacific Ocean islands. We have many members in a couple of my hermit crab FB groups that live in the Philippines and other islands that have many of these species - we’re jealous of their crabs & they’re jealous of ours! There’s differences in the color ranges, exoskeleton features, sizes, behavior, types of shells they like, and development between the different species. This site talks about each species & a lot of these characteristics for each!

Also, marine hermit crabs are way more varied than I thought! There’s a family that have matched claws instead of one being bigger, and they hide in wood, rocks, sponges, and other things instead of shells.

There’s two species that have toothed mandibles!!

I’m glad my hermit crabs don’t have teeth.

Also you mentioned size, and while there are a lot of marine hermit crabs that are much smaller than land hermit crabs can get, there’s a lot that are quite big too! I think it might actually be more normal for marine ones to be bigger than terrestrial ones, with water helping to offset the weight of a big enough shell.

Look at this guy!

(Photo source)

I probably went overboard with this answer, but I hope it’s informative & fun to read! Thank you for the excuse to infodump about hermit crabs! ♥

#the-fantabulous-satan #hermit crabs #crustaceans #invertebrates #land hermit crabs #hermit crab species #asks #long post

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