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Crab Eating Frog

CRAB EATING FROG (Fejervarya cancrivora)

Image adapted from Wikipedia
Original photo by WA Djatmiko
"Crab eating frog" sounds like a sideshow act where you get all hyped up to watch a frog eat a crab, but it's actually a crab that eats frogs. I don't know why a crab eating a frog is more plausible to me than a crab-eating frog. Maybe because crabs are horrible monsters and frogs are, generally, extremely cute.

So, are crab eating frogs actually cool, or a rip-off? Let's find out together.

Appearance

Crab eating frogs look a lot like regular frogs, but their humble appearance hides a secret  while most amphibians cannot tolerate any kind of salt content, crab eating frogs (also called mangrove frogs after where they live) have a fairly high salt tolerance. Amphibians have very thin skin, so you would think that water would just leave the frog because osmosis. So how in the jesus jingling fuck do crab eating frogs not just shrivel up like salt mummies? One of the reasons is that they have these special mucous glands in their skin that aren't found in other frogs, which may have something to do with ion transport. In other words, they coat themselves in magic snot that stops salt from entering and/or water from leaving their bodies.

Points: 1/1

Behaviour

Unlike some animals with wildly exaggerated names (looking at you, Goliath bird-eating tarantula or, as I like to call it, the Spectacularly Disappointing Spider), the crab eating frog does actually eat crabs. Yay! Not big ones, because the frogs themsevles are tiny. And not all the time, because a lot of them live in freshwater and eat bugs like regular frogs. So let's not get too excited.

Still, frogs don't have any teeth, so I was kinda curious about how they subdue and swallow something as hard and violent as a crab. Unfortunately, about 200% of the research on crab eating frogs has to do with their salinity tolerance. I did find one paper that looked at gut contents and found whole animals in their stomachs, and some entire shells in their butts, so draw your own conclusions there I guess.

Points: 0.5/1 because they eat crabs whole which is badass, but not all the time which is disappointing

Distribution

Hands down the wildest thing about crab eating frogs is that they can live in brackish water, and sometimes venture into whole-ass marine habitats for food. The second wildest thing about them is that they can also live in freshwater. Most animals can deal with freshwater or saltwater, and being able to functional perfectly well in both is relatively rare. But the crab eating frog is a bad bitch and does whatever the fuck it wants.

I shouldn't say they can do whatever the fuck they want. You can't take a frog from a brackish swamp and plot it in a freshwater lake. I mean, I guess you could, but the frog would die and you would be an asshole. What you can do undickishly is gradually increase or decrease the salinity of the liquid that the frog is in over a period of several days, and the frog would deal with it.

So they're pretty smart for figuring that out. They're also smart for living all over the goddamn place in southern Asia. They are shockingly one of the few amphibian species the IUCN lists as having an increasing population (for context, the scientific consensus on the future of amphibians is that they're proper fucked).

Points: 1/1

Ecology

When I said they live all over the goddamn place, I meant it. Swamps and wetlands? Yep. Coasts? Sure. Forests? Yes. Rice fields? They're there. Irrigation ditches? People's gardens? Cities? Frogs. Everywhere, frogs. Even for a frog-heavy part of the world, that's a lot.

Because everybody is obsessed with how they handle salty water, there's been basically no research on their ecological impact, so I'm going to go ahead and talk out my ass for a bit here:

They probably control insects pretty well, right? I mean, they live everywhere, including cities and other man-made environments. They're abundant, as far as frogs go. And all frogs are really, really good at slurping up insects. So it would make sense if they contributed to pest control. I have nothing to back that up except my own massive ego, but I think it bears consideration.

Points: 1/1

Evolution

So they've evolved a sophisticated suite of mechanisms (the mucous thing I talked about, plus a bunch of other physiological and biochemical witchcraft) allowing them to live in both fresh and brackish water. But whyyyyy? Why go to the trouble?

The most closely related species is the Asian grass frog which also has a reasonable tolerance to salt water as a tadpole, but not really as an adult. The next closest relatives after that are not tolerant of salt even a little bit. So then, what? Salt tolerance is just this weird-ass thing that cropped up in this one species? No!

Other species of salt-tolerant amphibians exists, and they aren't even uncommon. Earlier, I said "most amphibians cannot tolerate salt of any kind," which was true, from a certain point of view. The vast majority of amphibian species cannot even with salt, but there are a lot of species of amphibians. There are about a hundred or so species (~1% of known amhibians) that can do salt, and these species are spread over the three different types of amphibians: frogs, salamanders, and caecilians. Crab eating frogs are by far the most famous salt-tolerant frogs, and they have the widest range of salt tolerance, but it's not just some crazy thing from out of nowhere. It's a sensible strategy.

"But whyyyyy????" you say. "You still haven't said whyyyyy." Read on, dear friend. There are two very (very) broad, generalized ecological strategies that tend to determine the kinds of places where organisms live.
Option 1: they can be really good at outcompeting other species in an easy environment
Option 2: they can be really good at living in a difficult environment where they have little competition
Since salt water is a very harsh environment for most amphibians, being adapted to living there is a good idea to succeed while avoiding competition. Imagine if you had the wonderful ability to live inside an active volcano. It might not be super comfortable, but nobody is going to come kick you out of your house, and you can eat all the lava you want.

The caveat is that this is neither a rule nor a guideline, it's just a trend we see sometimes and there are a lot of exceptions. Crab eating frogs can live in both hostile brackish water, and friendly freshwater, because life is chaos and god is dead.

Points: 1/1

Life History

Even the most mighty frog starts its life as a humble tadpole, and the crab eating frog is no exception. Both tadpoles and adults can survive in saline waters. According to one study from the 1960s, tadpoles grown in straight-up seawater don't metamorphose. Another study published forty years later said that was wrong and that they can metamorphose if given enough time to acclimate. So, you know, if they continue doing what they're doing there could, potentially, be fully marine frogs in the future.

There was also an off hand comment in one of the other papers I read about how 2 out of the 21 adults captured in a freshwater area were hermaphrodites. I can't find any information on whether that's the norm for them, or whether there's chemicals in the water making the frogs gay. Anyway, we demand more frog sex studies 2021.

Points: 1/1

Interactions with Humans

Frog eating humans hunt and farm crab eating frogs, and I would guess they probably taste... okay? Like, on the one hand, I never imagined that frogs tasted very good. They don't smell good, but who am I to argue with French cuisine. On the other hand, they live where it's salty so maybe their meat is salty too? I don't know. I'm on a pretty strict diet of kraft dinner and cheetos so that's well outside my culinary purview. Either way, it doesn't seem to be putting a dent in their numbers, so humans can have a little frog sometimes as a treat.

Points: 0.5/1 because I can't decide whether they would taste good or not

Final Score: 6/7
Crab-eating frogs are cooler than I expected. This is saying something, because I thought they monched down on, like, green crabs and shit and I was kinda disappointed to find out that they do not do that. Still, they come out on top. You go, crab eating frog.

Further Reading

  • Elliott AB, Karunakaran L. 1974. Diet of Rana cancrivora in fresh water and brackish environments. Journal of Zoology. 174: 203-215
  • Gordon MS, Tucker VA. 1965. Osmotic regulation in the tadpoles of the crab-eating frog (Rana cancrivora). Journal of Experimental Biology. 42: 437-445. Link
  • Hopkins GR, Brodie Jr. ED. 2015. Occurrence of amphibians in saline habitats: a review and evolutionary perspective. Herpetological Monographs. 29(1): 1-27
  • Lu C. 2010. Fejervarya cancrivora. AmphibiaWeb. UC Berkeley. Link
  • Ren Z, Zhu B, Ma E, Wen J, Tu T, Cao Y, Hasegawa M, Zhong Y. 2009. Complete nucleotide sequence and gene arrangement of the mitochondrial genome of the crab-eating frog Fejervarya cancrivora and evolutionary implications. Gene. 440(1-2): 148-55
  • Seki T, Kikuyama S, Yanaihara N. 1995. Morphology of the skin glands of the crab-eating frog (Rana cancrivora). Zoological Science. 12(5): 623-626. Link
  • Shao Y, Wang L, Zhong L, Hong M, Chen H, Murphy RW, Wu D, Zhang Y, Che J. 2015. Transcriptomes reveal the genetic mechanisms underlying ionic regulatory adaptations to salt in the crab-eating frog. Scientific Reports. 5: 17551. Link
  • Wu C, Kam Y. 2009. Effects of salinity on the survival, growth, development, and metamorphosis of Fejervarya limnocharis tadpoles living in brackish water. Zoological Science. 26(7): 476-482
  • Zhigang Y, Ermi Z, Haitao S, Diesmos A, Alcala A, Brown R, Afuang L, Gee G, Sukumaran J, Yaakob N, Ming LT, Chuaynkern Y, Thirakhupt K, Das I, Iskander D, Mumpuni, Inger R. 2009. Fejervarya cancrivora. IUCN Redlist of Threatened Species. Link

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