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Uploaded:2021-02-11
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This episode is brought to you by the Music for Scientists album! Stream the album on major music services here: https://biglink.to/music-for-scientists. Check out the “For Your Love" music video here: https://youtu.be/YGjjvd34Cvc.

When a species evolves from living in water to living on land it’s called terrestrialization, and it’s not an easy task. Yet crabs keep making the jump from seas to shore. What things have they had to change to adjust to life on land?

Hosted by: Michael Aranda

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Sources:
https://research.nhm.org/pdfs/31132/31132.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3926130/
https://www.journals.uchicago.edu/doi/abs/10.1086/318099
https://jeb.biologists.org/content/221/21/jeb185421
https://pubmed.ncbi.nlm.nih.gov/17279390/
http://www.sfzoo.org/animals/sculptures/coconut-crab.html
https://www.nature.com/articles/30724
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7575764/
https://www.nature.com/articles/4401005a
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https://jeb.biologists.org/content/221/21/jeb185421
https://research.nhm.org/pdfs/31132/31132.pdf

Image Sources:
https://commons.wikimedia.org/wiki/File:Caribbean_hermit_crab.JPG
https://www.flickr.com/photos/davidstanleytravel/24952104493
https://www.scirp.org/pdf/OJE_2013051316263259.pdf
https://www.flickr.com/photos/moondance38/10161169963/in/photolist-4CChPi-gtUDPz-7FE2Wu-dg6JhE-QVuEtv-cLVTtd
this episode is brought to you by the music for scientists album now available on all streaming services when most of us think of crabs we probably picture marine animals living in the ocean eating fish and things the life aquatic but there are also quite a few crabs that have basically abandoned the water we're talking land crabs not just semi-aquatic beach crawlers but crabs that spend most of their lives up here and there are even some that live in trees this process of a species evolving from living in water to living on land is called terrestrialization and depending on how you count it's happened at least five times in crustaceans as a whole several groups even seem to be undergoing it right now but exactly what's pushed crabs to make the jump is a bit of a puzzle and it could involve anything from food to predators to oxygen availability but one way or another it wasn't an easy switch fresh realization requires nearly everything to change from how an animal eats to how it has babies so here's how crabs manage it when you're moving from water to land probably the most obvious thing that has to change is the whole breathing part like many other animals aquatic crabs get oxygen they need from a series of gills feathery structures that are chock full of blood vessels and can extract oxygen from the water but if you've ever seen a fish flopping around on land you'll know that gills often don't work so well in the air the main problem is those feathery structures that make up the gills can collapse in on themselves leaving the animal unable to breathe and therefore dead but land-going crabs have found ways to adapt around this some herbert crabs use the water in their shells for gas exchange for instance a more impressive adaptation though is lungs which show up in many land-bound crabs like the robber crab which actually only breathes air they'll enter the water to drink or release eggs but they can't stay too long or they'll drown those crabish lungs aren't exactly like ours though they're modified branchial chambers which is also where the gills are so these crabs breathe in through openings near the base of their walking legs then out through their mouths instead of the in and out mouth breathing we're used to in land crabs the tissue on the inside of the chamber has become convoluted and spongy increasing surface area and allowing for gas exchange so their lungs aren't entirely new organs they're more like heavy modifications to the structures that were already there meanwhile the gills also tend to change a bit although how depends on the type of crab in some cases gills may get smaller stiffer and have spacers on them all of which help prevent that collapsing problem we mentioned a lot of crabs can also kind of switch between gills and lungs as needed to optimize oxygen delivery based on their level of activity and environment another species the gills mostly stick around to regulate the crab's internal ph levels and to get rid of waste products they also do that as a secondary function in marine crabs but in these land crabs it becomes their main job of course just being able to breathe air isn't enough to get around on land i mean it's important but you also need to be able to support yourself since buoyancy is much less of a thing when you leave the water that's why if you put a whale on land the force of gravity eventually crushes its tissues thankfully though crabs are not whales as you may have noticed most importantly here crabs don't use the same internal bony skeletons whales and other vertebrates use they have hard kitenous exoskeletons instead in the ocean this is great for protection against predators but it also really comes in handy for walking out of the water in general the crab shell is pretty well built and more than enough to deal with gravity and mechanical strain but that's not to say there's no challenge the biggest problem is what happens when land crabs outgrow their exoskeleton and molt when this happens the crab sheds its exoskeleton unveiling a new soft version that will harden and expand over the next few days the problem is that during that time they lack the strong support system that's been holding them up and they become at risk of turning into a crab pancake or a crab cake if you will if this happens their new shell will harden in the wrong shape impeding further molting and likely killing the crab the way they've adapted to this is through what's sort of their other skeleton the hydrostatic skeleton it's another way crabs usually support themselves involving pressurized internal fluids that help keep the crab inflated think less knight and armor more balloon animal all crabs water bound or land bound have this but to stand up to the force of gravity on land the fluid pressure has to be much higher in land crabs and that comes with some trade-offs for one thing the greater internal pressure puts them at higher risk of the newly grown exoskeleton rupturing it also becomes more expensive energy-wise to move around compared to marine crabs how strong this hydrostatic skeleton can get might actually be the major limit on the size of crabs like coconut crabs which is lucky or unlucky i suppose depends on how much you like giant crabs anyway that's skeleton and gravity once an animal's gotten onto land walked around a bit and taken in the air it's eventually going to get hungry and it turns out that open-air dining is a bit different than eating underwater marine crabs mostly eat algae and small animals but for land crabs the food source that seems to come easiest to them is vascular plants fruits leaves that kind of thing these kinds of foods are a lot tougher than what's in the sea compare a nice soft clam to something crunchy and tough like celery they can also be full of tough hard to digest fibers like cellulose and even compounds that are potentially toxic to the crab like tannins and they can be pretty nutrient poor compared to seafood too especially when it comes to calories and nitrogen to deal with this herbivorous land crabs have developed a number of adaptations for one land crabs tend to grow more slowly and live longer which lets them deal with the low nitrogen they'd otherwise need to grow like the coconut crab can live to be 60 years old they also have enzymes in their gut that can help digest the cellulose and other fibers some of these enzymes may be made by the crabs themselves but one of the keys to exploiting this new food source may actually be microbes we're still in the very early stages of research on land crab microbiomes the communities of microbes that live in their guts but we know that other land arthropods depend on their microbiomes for instance in isopods which are crab relatives like roly-polies it's been shown that their microbiomes are critical to digesting plant material microbes in their guts produce enzymes called carbohydrate active enzymes which help digest and get energy from woody plant fibers they may also help in other ways like reducing water loss and digesting nitrogen so breathing walking eating these adaptations all help individual crabs survive on land when it comes to continuing entire crab lineages though there's a whole other set of adaptations involved i mean those 60 year old coconut crabs are impressively old but the crab lineage has been on land for millions of years to achieve that they need to reproduce and that leads to some new challenges in this case land crabs have mostly got themselves a pass that's because they're essentially still at what you might call the amphibian stage of conquering the land in other words they still depend on water for their larvae that said many have had to change up how they do things because you can't just release your eggs whenever you want if you live kilometers from the ocean for example one species of hermit crab can live near high cliffs and throws clumps of hatchlings over the cliff edges to reach the ocean some other species have solved this through brooding where they use pouches to carry their young until the time is right kind of like marsupials the abdomen actually folds under to create a brood chamber a kind of sealed water-filled chamber for the embryos or at least a super-humid one other crabs like the mandarin vampire crab let their babies ride around on top of them but some of the most remarkable species actively care for their young and raise them away from streams or oceans notably the bromeliad crabs raise their young in bromeliad plants in the spaces where water collects between the leaves and the stem the mother keeps the water quality high stirring it for oxygen adding snail shells to tweak the ph and killing predator larvae they feed the offspring for several months and they can even have later broods in the same plant making a family group meanwhile jamaica's snail crab keeps its young and water-filled snail shells the mother refills the water turning the snail shells upside down to catch rainwater or carrying water to the shells she also feeds the offspring for several months these couple of species are special because they're the only known cases of active brood care and probably the result of one evolutionary jump by an ancestor so yeah crabs have had to change a lot in order to make it onto land and we've only been able to touch on some of the challenges because moving to a whole new world is complicated but move they have and they've done it time and time again understanding crab evolution requires pushing boundaries and expanding our ideas about the world and that's the heart behind the album music for scientists patrick olsen wrote and recorded these songs for scientists and science communicators and they were all inspired by the beauty of science they celebrate exploration understanding and also the people who push those fields forward if you want to check out the album you can stream it on all major services or click the link in the description to catch the music video for for your love.