I’m always struck by how remarkably boring human sperm are compared to many less interesting-seeming animals (like the giant sperm of the fruit fly, for example). But here we have a case of an awesomely interesting animal with equally wacky sperm. I give you: octopus sperm.
Octopus sperm is sneaky. It starts off all innocent and normal looking, while it’s sitting there in the testes waiting to go to bat. Then, once in the female, the acrosome reaction begins and the sperm shows its true, screwy self.
And I do mean screwy. Seriously. It looks like a screw.
The head of the sperm contains a part called the “acrosomal screw”. But the screw doesn’t actually reveal itself until the sperm is activated for fertilization, during a sequence of events known as the acrosome reaction. Normally, there are two membranes on the acrosome of a sperm (the part of the sperm head that functions in penetrating an egg). During the acrosome reaction, the outer membrane is broken down. The inner membrane remains, and plays an important role in fertilization.
Octopuses aren’t normal. First of all, they have only one acrosomal membrane. (The figure on the left also shows the plasma membrane, which covers the entire sperm; it isn’t part of the acrosome). Underneath the acrosomal membrane, there is a screw-like structure.
Researchers Zhe Li and colleagues at Zhejiang University and Ningbo University wanted to know what exactly happens during sperm activation to the acrosomal membrane and the acrosomal screw.
17 male octopuses were collected, allowed to recuperate in aquariums, and then were cut open to remove their mature sperm.
The rest was a matter of mimicking the conditions for fertilization (calcium is important) and then putting the sperm under an electron microscope to see what happens.
At first, as you can see from the image below, the screw isn’t visible (panel A in the picture). In the first step of the reaction (B), the acrosomal membrane starts to bubble up with vesicles, possibly to bring enzymes or other proteins to the surface. This eventually eats away the membrane, showing the full acrosomal screw at the end of the second step (D-E).
Sadly, we still don’t know what the screw does during fertilization. I’d like to see those images. Does it just bore through the egg’s membranes, or does it actually screw in, turning as it enters? These are important questions that science needs to answer.
Li Z, Zhu JQ, & Yang WX (2010). Acrosome reaction in Octopus tankahkeei induced by calcium ionophore A23187 and a possible role of the acrosomal screw. Micron (Oxford, England : 1993), 41 (1), 39-46 PMID: 19729317