The Crab with Two Backs: ‘Natural’ Resource Economics

ALERT: Don’t read this if you’re offended by sex, bad jokes, or specifically bad sex jokes.

We all like sex, right? (To my younger readers, you’ll get there. To my asexual readers, that is the majority experience and I apologize if you feel marginalized. Irrespective of the necessity of sex for the propagation of the species, the fact is many of us pursue it simply for pleasure. I refuse to cite evidence for that. Let’s move on.)

Wow, what an opening. Such insight, such nuance. I’m really hoping this one gets the clicks, guys (this despite a previous post mocking clickbait articles). But really, sex is something that occupies much of humanity’s collective brainpower at any given point, so it’s no surprise that scientists are concerned with how much baby-spray male Chilean rock crabs (Metacarcinus edwardsii) are squirtin’ out during each sesh. You see, these crabs, as well as other decapod crustaceans (mostly other crabs and lobster), have a degree of control over the volume of their sweet release – a trick the scientists felt sure has an evolutionary purpose. Except for humans and dolphins (the two biggest asshole species – go figure), sex for animals is still all about producing offspring to further your own genetic line. However, the polyandrous nature of crab mating – where each female may lie with more than one male – makes this a sticky situation. What’s a guy to do?

Introducing the sperm economy hypothesis or ‘the most boring combination of words that are still technically about doin’ it’. The SEH “predicts that males can adjust the amount of their ejaculate during copulation in response to (1) individual traits of females according to potential female fecundity, (2) future mating opportunities, and (3) risk of sperm competition”(Pardo et al. 2018). Basically, the SEH predicts that if a female looks like a blue-ribbon baby-maker, the male’s going to give her a high caliber round. If there are numerous females around ready to settle down and have a few (million) larvae with just this one lucky gentleman, he’s going to be sparing with that man-juice to make sure there’s enough to go around. And if there are other dudes hanging about vying for daddy duties? He’ll again set that hose to ‘soak’ in order to crowd out the field. You know, throw in all the euphemisms and the sperm economy hypothesis is actually pretty fun.

Wolf Of Wall Street Oh My God GIF - WolfOfWallStreet OhMyGod LeonardoDiCaprio GIFs
This is what Wolf of Wall Street was about, right? From

But back to (the) business. SEH is only a hypothesis, so of course the fine folks over at the Instituto de Ciencias Marinas y Limnológicas and the Centro de Investigación de Dinámica de Ecosistemas Marinos de Altas Latitudes (get all that?) set out to test it re: Chilean rock crabs. They set up a three-part experiment. In part one, a single male crab and a single female crab were placed together and allowed to ‘become familiar’. The scientists examined a number of factors they thought might contribute to differences in sperm allocation: size of the male, size of the female, the difference in their sizes, whether the female had any ejaculate already stored from previous matings, etc. You’ll notice that part one of the experiment is designed to test point (1) of the SEH up there. To test points (2) and (3), the scientists arranged ménage à trois scenarios with two females and one male or one female and two males, respectively. No word on whether any males exchanged high fives with their bros before or after returning from their M-F-F threesome.

I didn’t even make this myself, it was posted on a bodybuilding website. Nothing makes sense; reality is a farce. From

So how did the SEH hold up? To use a sexually charged word: adequately. Ejaculate volume was dependent on male size in the one-on-one pairings, which makes sense – the bigger guys can store more of the good stuff. Looking at the threesomes, only point (3) of the SEH found any evidence: in the presence of another male, pearly loads were weightier.

Triple Graduated Pearl Necklace
No, no – different shellfish. From

This suggests that Chilean rock crabs can pick up on situations where their little swimmers will be cannonballing into a crowded pool, but not the opposite scenario. Low sample size may be the culprit here, only 7 M-F-F matings were observed because it required two females to be ready and willing at the same time. If more of those scenarios had been available, it’s possible the researchers would have seen a change in ejaculate volume.

An interesting factoid from this study is that, although both ejaculate volume and sperm count increased with male size, sperm count reached a saturation point at a certain size, but ejaculate volume did not. In other words, the largest crabs aren’t firing off any more sperm than the smaller guys, but they are adding in a lot more of the other stuff, mainly seminal fluid. This seems counter-intuitive, especially considering seminal fluid is more costly for the hopeful dad to produce. What’s the deal?

The fact that this is one of the first 20 Google image results for ‘confused dad’ says a lot. From

Crab biology is the deal, specifically the biology of the female reproductive system (frightening to men, no matter the species). You see, after female crabs mate, they store that liquid love – sometimes for years – in organs called seminal receptacles. Fertilization doesn’t occur at the time of mating, it’s not until the female lays her eggs and they’re passing out of her vagina that they’re exposed to the male’s sperm. Under this scheme, it pays to take up as much space in that seminal receptacle as possible if you want to pass on your genes. There’s also a chance all that seminal fluid will form a sperm plug, which pretty much seals (pun intended) the paternity. Note, DO NOT visit the Wikipedia page for ‘sperm plug’ unless you want to see a stoppered squirrel vagina.

frustrated squirrel.
“Hey baby…” From

Alright, well, this is all great and good. You now know more about crab sex than you probably ever wanted or will need to know. I read this paper because it has ‘ejaculate’ in the title, I can admit that, but I wasn’t interested until I got to the very end. The SEH and changing ejaculate loads and crab vagina all have very important consequences for conservation and the continued survival of this species. Chilean rock crabs are commercially harvested. As with many other harvested crustaceans, the large males are the primary component of the catch. In areas subjected to years of heavy fishing, not only will there be fewer male crabs available to mate, but this can result in a reduction in male average size. As we already know, the smaller males produce less seminal fluid and sperm leading to the possibility of ejaculate depletion in males and sperm limitation in females. Meaning, there’s simply not enough sperm to go around for all the eggs and you face the serious potential for local population collapse.

I think, at this point, most people are aware of how overfishing works even if they don’t believe it’s real. You fish too many fish and you run out of fish. But in this scenario, you’re also potentially reducing the baby-making ability of whatever’s left, making these crabs even more susceptible to overfishing. This is an issue expertly tackled by the lobstering industry in Maine. Besides having a minimum harvest size to prevent the taking of young individuals who haven’t yet had a chance to reproduce, there’s also a maximum size of lobster you can take. The idea is that these large lobsters are the oldest, most capable of survival (the best genes), and, because reproductive output scales with size, they’ll make the most little lobbies! Unfortunately, I wasn’t able to find any concrete information on the regulations for Chilean rock crab. Fingers crossed the sperm economy is self-regulating.

Johnny Venger has stared into the abyss and it was a squirrel vagina.


Pardo LM, Riveros MP, Chaparro OR, Pretterebner K (2018) Ejaculate allocation in Brachyura: What do males of Metacarcinus edwardsii respond to? Aquat Biol 27:25–33

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