It's about squid and it's about sperm, and apparently the combination is sexy enough to get this research onto the BBC, MSNBC, Discover, and more. (Not to malign io9--that's actually where I first read about it.)
The particular squid in question is Loligo bleekeri--whoops, I mean Doryteuthis bleekeri--er, I mean Heterololigo bleekeri. Taxonomists*, can we please get this species complex sorted out??
ANYWAY, as is common in loliginid squids, male bleekeri come in two flavors: consorts and sneakers. Consorts are larger and engage in all the classic male mating behaviors: competing with each other for female attention, inserting spermatophores inside the female's body, and guarding her after mating.
Sneaker males are smaller and don't bother with any of that stuff. They're drive-by maters. They jet in, drop spermatophores on the female's skin, and jet off again.
You'll notice I'm using a long fancy word instead of just saying sperm. That's because squids (and octopuses and all other cephalopods) package their sperm up before passing it from male to female. These packages (each of which contains billions of sperm) are called spermatophores, and they break open at the right time, allowing the sperm inside to do their fertilizing duty.
In the case of consort males, who left spermatophores inside the female's body, their sperm springs into action as soon as the female's eggs start oozing out of her oviducts. That's internal fertilization. But in the case of sneaker males, their sperm doesn't get to touch the eggs until they're already outside her body, having passed by the consort's sperm. That's external fertilization. Seems crazy, but apparently they still have a fighting chance, because eggs do get fertilized by both consorts and sneakers.
This curious system raises a lot of questions. Why do both strategies work? Do the sperm from consorts and sneakers compete with each other? And what makes a male a consort or a sneaker, anyway?
You might guess that younger, smaller males act as sneakers until they grow big enough to duke it out, then they switch to being consorts. But the authors of the study that's been causing all the media hullaballo call such a transition "unlikely". (They don't cite a source, however, which seems odd.) Supposing they're right, one has to wonder: Is it genetic--are some squid born consorts and others born sneakers? Or is it environmental? Or simply a personal squid preference?
Whatever it is, it has to be pretty fundamental. As it turns out, consorts and sneakers differ in more than just body size. They also make differently sized spermatophores AND sperm. Wacky!
The difference in spermatophores was discovered back in 2007 by a couple of Japanese squid researchers named Iwata and Sakurai. Consort males, they found, produced larger spermatophores than sneaker males. But although consorts' bodies and spermatophores are bigger, when it comes to sperm--where the rubber meets the road, if you'll pardon the expression--
Sneaker males make bigger sperm.
That was the discovery made by Iwata, and published in BMC Evolutionary Biology four years after his spermatophore paper, which has all the science news sites abuzz. I'm delighted to see something this weird make big headlines, and I think it is legitimately a very cool study, but don't take my word for it. The BMC in the journal title stands for BioMed Central, which is an Open Access Publisher, so you can read the whole article for yourself. Yay open access!
This is the first time that physically different kinds of sperm, associated with different mating tactics (consort and sneaker), have been described within a single species. Plenty of other animals have "alternative mating tactics" in which males employ one of two or several strategies to pass their genes on. But none of these other species has such a difference between sperm.
So--why bleekeri? What makes it special? The authors think that the key lies in where the sperm meets the egg. Although males of other species employ various mating tactics, when it comes to actual fertilization, it always happens in the same place, either internal (for some species) or external (for other species). But in bleekeri, the two different strategies actually result in fertilization in two different places: internal (for the consorts) and external (for the sneakers).
Since the inside of the female's body is a radically different environment than outside in the open ocean, they suspect that it must be better to have small sperm for internal fertilization and large sperm for external fertilization.
But why? (Being a good scientist is basically the same as being a whiny child, repeating "but whyyyyyy?" over and over again.)
The authors have this to say: "At the moment the mechanisms by which sperm polymorphism has evolved remain elusive."
That means: we don't know. Yet. I'm certainly looking forward to the next Iwata paper!
Iwata, Y., Shaw, P., Fujiwara, E., Shiba, K., Kakiuchi, Y.,&Hirohashi, N. (2011). Why small males have big sperm: dimorphic squid sperm linked to alternative mating behaviours BMC Evolutionary Biology, 11 (1) DOI: 10.1186/1471-2148-11-236
* Taxonomists? Hello? Anyone? . . . Oh right, we don't make taxonomists anymore. Sigh.
Flexible Sex: Squid Tailor Sperm To The Environment