Caught in the act

I happened to glance at the aquarium at just the right moment to see an anemone spawning.

Pink-tipped green anemone, releasing sperm!

I've checked several videos, all showing fogs of sperm floating out in a slow, continuous stream. This little guy spit out three (that I saw) quick bursts of cloudy material, with a few seconds interval between. I had time to run for the camera and catch the third. There was no fourth. (Hooray for autofocus!)

Anemones in aquaria usually reproduce by splitting in half, creating two separate animals that each go in their own way. This is generally the case in my tank; there are two pink-tipped greens half-way through a split today and another couple of new pairs starting to move apart.

Sexual reproduction is rarer, at least in tanks. The anemones, although they were "born" by cloning, still manage to become either male or female, not necessarily the gender of their parent individual. This one is male.

A study on Anthopleura elegantissima in north San Francisco Bay, California shows that gonads build in size through spring and summer, and spawning occurs in late summer/early autumn.  Gonad indices peak coincidentally with high surface seawater temperatures. ... Eggs are released in brown mucousy masses, while spermatozoa are released in milky-white masses. (From A Snail's Odyssey)

Note: the schedule may be different because my tank didn't suffer the more extreme winter that outside waters would.

There is some evidence that males begin spawning earlier than females, and perhaps this stimulates the females to spawn. (Same site, Research study 2)

I'll be monitoring the anemones more frequently in the next few days. I have no idea which are female; I hope some are.

There's not much chance of this producing more anemones here; the fertilized eggs swim in the current for a while, and will probably be caught in the filter. The critters are smart to use the splitting strategy in captivity.