This thing called life

Sex cells: sperm that crawl instead of swim

So much work for so little reward. yousuckbla

I have recently learnt that human sperm don’t swim, they crawl. Who knew?

This actually makes sense, once you think about it, despite all the videos and cartoons that depict sperm swimming. Our internal organs do not have tubes like garden hoses. Passages in the human body are more potential than actual, with tissues on either side touching most of the time.

So it turns out that human sperm, when placed inside a narrow tube, hit one wall or another and then squirm their way along the surface. Worming, not swimming.

But this has me wondering, what do fish sperm do? Obviously, in species with external fertilisation, such as salmon, the sperm have to do quite a bit of swimming. That would happen when the female deposits her eggs and males deposit their sperm directly onto them. Salmon sperm must move through water, without the aid of passageways to guide them to their goal.

But plenty of fish have internal fertilisation as well, and their passages may well permit their sperm to crawl. And I am thinking about the poor crawling sperm inside the fish called the Amazon molly, because they are metaphorically on their knees. I have written about the Amazon molly before in my sex cells series. They are an all female species that nevertheless requires the act of mating for reproduction. The sperm in this case come from another, closely related species. And none of them get to fertilise the egg. This type of reproduction is called gynogenesis.

I had always imagined the poor rejected sperm in the reproductive tract of the Amazon molly as bouncing off the egg after an energetic swimming spree. I would have explained it as two bubbles bumping into each other: they could combine into one big bubble, or bounce away and never join up. Now it seems that the sperm are crawling up to the egg and nudging it, without success.

Of course most sperm are unsuccessful. Even when there is an egg at the end of the passage, and it is of a compatible type (the same species), only one sperm among millions is privileged enough to fertilise any given egg.

As Aldous Huxley wrote:

            A million million spermatozoa,
            All of them alive:
            Out of their cataclysm but one poor Noah
            Dare hope to survive.
            And among that billion minus one
            Might have chanced to be
            Shakespeare, another Newton, a new Donne –
            But the One was Me.

In the case of the Amazon molly, there is not even one. This seems like an incredible waste of sperm for the males who mate with the molly. Why would they do it? Actually, given a choice, males prefer to mate with their own species and have fry of their own. But female fish can also lure the males by releasing hormones that indicate their receptiveness. These wily females are far from virginal, even though they produce virgin born offspring.

Why would evolution produce such an all female species? Every individual produced can give birth to more young, doubling their reproductive potential. There is a catch, however, in the case of sperm dependent species. If they out-compete the sexual species with which they must mate, they would then go extinct themselves. They use the males of another species, but they are also dependent upon those males.

But this doesn’t make things any better for the lowly sperm who crawl all that way to die. On the other hand, they could succeed in fertilising the egg only to have every single one of their genes rejected by the growing embryo. That sneaky scenario can happen, and I can’t decide if it is more kind or more cruel.

But that’s another fish, and another story.