Big love among the ostracods


How can anyone resist an article titled “Sexual Intercourse Involving Giant Sperm in Cretaceous Ostracode”? You can’t, I tell you. It’s like a giant brain magnet, you open the journal to the index, and there’s that title, and you must read it before you can even consider continuing on to anything else.

Some organisms have evolved immensely long sperm tails — Drosophila bifurca, for instance, has sperm cells that are about 60mm long, or 20 times longer than the length of the entire adult body. The excessively long sperm tail is obviously not a structure that has evolved for better swimming; instead, it is thought to act as a tangled barrier in the female reproductive tract to prevent other males from fertilizing the female, and there is also some very interesting evidence that sperm coevolves with the female reproductive tract, so some sexual selection at the level of the gametes is going on.

At the same time, sperm morphology is extremely diverse, and seems to evolve very rapidly. Perhaps these mega-sperm are a transient fad? Not all species of Drosophila exhibit the phenomenon, and those that do vary considerably from species to species. What we’d like to know is if there are any lineages that maintain these patterns of giant sperm over long periods of evolutionary time…so what do we need to do? We need to go spelunking for sperm in fossils!

That’s what this short letter in Science is about: the authors looked at ostracodes, a class of tiny crustacea that invests heavily in reproduction. About a third of their volume is their reproductive system, with males building giant (relative to their size) sperm pumps, and females having large seminal receptacles for sperm storage. The individual sperm are also large, often longer than the body length of the adult, and are also aflagellate — no flagellar tail at all, just a long, threadlike cell body. You can tell if a female ostracod is a virgin just by looking at those seminal receptacles, since they inflate hugely with all the giant sperm tucked inside.

So, if you look at the large orange blobs, the seminal receptacles, in this 3-D scan of a fossil female ostracod (bottom right of this image), you can tell that she was inseminated before she died, and that her mate had very large sperm. Her condition was also very similar to that of modern ostracodes (bottom left).

(Click for larger image)

Partial reconstruction of E. virens (extant) and H. micropapillosa (fossil). Anterior is to the left. Orange structures indicate central tubes of Zenker organs in males or seminal receptacles in females; brown, esophagus; turquoise, mandible; purple, upper lip; pink, lower lip; green, valves; and gray scales, whole-body reconstruction. All scale bars indicate 100 µm. (A) Lateral view of male E. virens with several organs included for comparison. (B) Male H. micropapillosa in lateral view with several organs in context of whole-body reconstruction. (C and D) Ventral views of several organs including tubes of Zenker organs of male H. micropapillosa. (E) Lateral view of female E. virens with several organs included for comparison. (F) Female H. micropapillosa in lateral view with several organs in context of whole-body reconstruction, including seminal receptacles.

So, the conclusion is that boinking with giant sperm is an enduring property of at least some lineages: they’ve been going at it for a hundred million years. The authors also suggest that this kind of technique could be useful for measuring sexual selection by assessing pre-mating parental investment in fossil invertebrates.

Matzke-Karasz R, Smith RJ, Symonova R, Miller CG, Tafforeau P (2009) Sexual Intercourse Involving Giant Sperm in Cretaceous Ostracode. Science 324(5934):1535.

Miller GT, Pitnick S (2002) Sperm-Female Coevolution in Drosophila. Science 298(5596):1230-1233.