I am an attractive male sea squirt (a Styela plicata, in case you were wondering) in the prime of life. I live alone on the underside of a nice dock, I’ve got plenty of tasty phytoplankton to eat, and my siphons have extremely handsome pleats. But I’m worried, because every time I broadcast spawn, my sperm are so small. My tunicate-trash neighbors live together in a big clump, and they have really big sperm. I’m worried that none of my tiny sperm are making it to an egg, but I don’t want to live in a licentious tangle like my neighbors. Help!
Desperate on the Dockside
Though you are an unusually moral tunicate – most of your kind are content to spawn the whole week long – you don’t need to worry about your sperm. It’s supposed to be small. A recent paper in PNAS has just discovered that the sperm of tunicates living in high densities is bigger and lives longer than the sperm from single tunicates such as yourself. Why do apartment-dwelling tunicates have better sperm than tunicates living in McMansion-like isolation? The researchers suggest that big sperm are slow, so there’s less likelihood of all the sperm mobbing the same egg and killing it. (It’s called polyspermy.)
In contrast, DOTD, your sperm may be small, but they are quick and numerous. The more sperm you put out there, the greater the chances one will encounter an egg. And don’t worry – the ladies are doing their part too. Female tunicates living in low densities make bigger eggs than female tunicates living high densities, so the target is easier to hit.
Of course, DOTD, when you have reached the appropriate size, you will turn into a female. Does the fact that you make teeny tiny effective sperm mean that you make teeny tiny ineffective eggs? Nope. Gamete size seems to be entirely determined by your environment – in other words, it is phenotypically plastic. You will make the right sized gametes for your environment.
So fear not, little lone tunicate. You do not have to live in a tangle of big fat sperm and teeny tiny eggs like your dissolute neighbors to make a go of it. Enjoy your low-density lifestyle!
Style shamelessly stolen from my hero, Dr. Tatiana.
A. J. Crean, D. J. Marshall (2008). Gamete plasticity in a broadcast spawning marine invertebrate Proceedings of the National Academy of Sciences, 105 (36), 13508-13513 DOI: 10.1073/pnas.0806590105