These Little Bugs have Comically Giant Sperm
Why do some animals evolve to have really big sperm? You may be thinking “Oh, by ‘giant sperm’ they probably just mean it’s like two or three times the size of human sperm, but still totally microscopic”.
And now you may be thinking “Well, if they’re doing that thing where they go ‘you may be thinking’, that means some animal sperm are so big that they aren’t even microscopic!”.
And now you may be thinking “Wow, they sure are overusing the ‘you may be thinking’ gag and should really just get on with the article.”
Yes, some animals have sperm cells so ridiculously large they can be seen with the naked eye. Imagine that if we humans, instead of making a whole bunch of little sperms, just squirted out one big one and it just kinda flopped around the place. Yeah, some animals basically do that, and it’s really weird! And they’re probably the last animals you’d expect, too.
The Most Giant Sperms
For reference, human sperms range from 55µm to 65µm. That’s in micrometers, which is to a millimeter what a millimeter is to a full meter.
The largest known spermatozoa by length is over 58mm. That’s not micrometers, millimeters. For the metrically challenged, that’s more than two and a quarter inches long. And get this; it’s produced by Drosophila bifurca, a relative of our widely beloved lab fruit fly Drosophila melanogaster. So that sperm is coming out of a single fruit fly, which is fruit fly sized. Here’s a picture.
D. melanogaster’s sperm is less big at just 1.8mm (Fabian & Brill, 2012). Which is still pretty impressive when you consider that whole sperm gets absorbed into the egg. Only the head of D. bifurca‘s does.
The Most Fat Sperm
The largest sperm by volume at 65,000 μm3, though *only* 11.7 mm long. It’s produced by Australocypris robusta, which is an ostracod that is just over 3mm. A. robusta are ostracods, which are these cute little shrimp guys inside of little clam shells. Very common marine and freshwater plankton. Also, ostracod sperm cells tend to have a literal drill head on them, so there’s that. (Smith et al 2016)
The giant sperms (“giant sperm” is the actual scientific term) of ostracods are so much longer than the male itself that they require a special muscular and chitinous sperm pump called the Zenker organ just to ejaculate. (Yamada et al 2012). It can take up like a fifth of the ostracod’s body size but might only ejaculate one sperm at a time. Ostracods be zenking on their Zenker’s til they zenk.
Also, I just want you to look at this diagram (https://royalsocietypublishing.org/cms/asset/e67bd042-0750-4338-9327-c37049f6d4a2/rspb20140394f01.jpg) and just appreciate this ostracod’s completely bonkers anatomy. Their reproductive system takes up HALF of their body, their vas deferens CIRCUMNAVIGATES their WHOLE FUCKING BODY for no apparent reason, then gets in a tangle before entering the actual seminal vesicle. Then the zenker organ doesn’t even have a straight shot to the hemipenis, and also is larger than the hemipenis. Wtf?
The Giant Sperm Receptacle
One thing you have to understand about bug sex is that it’s pretty weird compared to how it works in us vertebrates. In crustaceans, such as ostracods, females have sperm receptacles the males zenk out their one sperm into. Insects have a very similar organ. But theirs is called the spermatheca because it sounds fancier.
In both cases, sperm is stored in this receptacle for later use in fertilization. In a lot of species of bee, for instance, the queen only needs to mate once in her life and can keep making baby bees from the sperm stored in her spermatheca. Imagine if that was the case with humans and married couples only ever have sex once on their honeymoon and never again. So imagine if it was like with humans.
This fancy organ allows the female to choose when and if to let the sperm into their eggs, possibly based on their skill in bed judged on a scale from 1 to 10. So scientists thought the sperm is kinda like a peacock’s tail. And indeed experiments have shown that. When a female fruit fly or ostracod has multiple male’s sperms in their spermatheca they usually choose the one with the larger sperm (provided it actually fits in the spermatheca). (Miller & Pitnick, 2002).
It’s a way that the female can know which fly has the best genes. Because if they didn’t, presumably they would be dead after spending so much time and energy trying to produce one comically large sperm and shrivel up into these little mummified husks with big bug balls. Never having even gotten the opportunity to zenk all over the place.
Also, perhaps so much cell contents being absorbed into the egg reduces the amount of nutrients and energy the female has invested into it. Probably not a lot though, giant sperm still tend to be mostly mitochondria and protein. (Click here for my other article on sperm mitochondria and why they are the reason why gender exists!)
Scientists Baffled by Giant Sperm
The general scientific consensus is that we don’t entirely understand giant sperm yet. This stuff is pretty cutting edge science, right up there with cold fusion and AI-generated medicine. You could tell as much because nearly every article I’ve cited was published within the last two decades, which in biology means it’s cutting edge science. I’m not even joking, that’s generally the rule of thumb on how long it takes biologists to get stuff mostly figured out.
For instance, we still don’t know why there’s a pretty significant amount of variation of sperm length in Drosophila. Both between species, as we saw with bifurca and melanogaster, as well as within species. (D.J. Hosken, 2003). Also, in the aforementioned study, sometimes the smaller sperm would be chosen over bigger ones for no apparent reason (Miller & Pitnick, 2002).
Studies have also found that in some species, such as D. hydei, smaller males have bigger testes and sperm relative to their body size than larger males while also reproducing less (Pitnick et al 1994).
And it just seems kinda weird that it’s the females driving the evolution of giant sperms. Because the males have to spend so much energy making each sperm they also make fewer, meaning there will be fewer successful inseminations. The females have to spend even more time and energy zenking off more dudes just to get pregnant. (D.J. Hosken, 2003).
Also, these males take a lot longer to reach sexual maturity as they need to grow these ridiculously long cells (D.J. Hosken, 2003), which kinda defeats the whole point of being a fruit fly. Like, their whole deal is that they can reproduce really, really quickly. Also eating fruit and being hard to swat. Why select for something that makes you worse at one of those three things? Ostracods are basically microscopic turtle bugs so higher parental investment makes sense for them. But fruit flies?
Citations
Hosken, D. J. “Sperm biology: size indeed matters.” Current biology 13.9 (2003): R355-R356.
Pitnick, S.; Spicer, G. S.; Markow, T. A. (1995). “How long is a giant sperm”. Nature. 375 (6527): 109. Bibcode:1995Natur.375Q.109P. doi:10.1038/375109a0. PMID 7753164. S2CID 4368953.
Miller, Gary T., and Scott Pitnick. “Sperm-female coevolution in Drosophila.” Science 298.5596 (2002): 1230-1233.
Yamada, Shinnosuke, and Renate Matzke-Karasz. “How is a giant sperm ejaculated? Anatomy and function of the sperm pump, or “Zenker organ,” in Pseudocandona marchica (Crustacea, Ostracoda, Candonidae).” Naturwissenschaften 99 (2012): 523-535.
Smith, Robin J., Renate Matzke-Karasz, and Takahiro Kamiya. “Sperm length variations in five species of cypridoidean non-marine ostracods (Crustacea).” Cell and tissue research 366 (2016): 483-497.
Fabian L, Brill JA. Drosophila spermiogenesis: Big things come from little packages. Spermatogenesis. 2012 Jul 1;2(3):197-212. doi: 10.4161/spmg.21798. PMID: 23087837; PMCID: PMC3469442.
Pitnick, Scott, and Therese A. Markow. “Large-male advantages associated with costs of sperm production in Drosophila hydei, a species with giant sperm.” Proceedings of the National Academy of Sciences 91.20 (1994): 9277-9281.
