Making babies like a stack of plates

posted by Perrin Ireland / on December 10th, 2009 / in Jellies, lifecycles


Our own lifecycles are pretty simple. Making babies requires sex. Sex creates offspring with new unique combinations of genes. Many organisms are also capable of asexual reproduction, which doesn’t involve sex (as the name implies) and involves only one parent. In most types of asexual reproduction, genes aren’t reshuffled and the offspring are genetic clones of their parent.

Unlike ourselves, many species have lifecycles that combine both  sexual and asexual reproduction. Take the moon jelly, for example. Moon jellies, also known as Aurelia aurita, are perhaps the quintessential jellyfish, with a typical umbrella-like medusa that travels on ocean currents. There is more to their lifecyle, though, than this swimming organism. The swimming medusa does use sex to make babies—but the babies don’t grow directly into swimming medusae. Medusae release their eggs and sperm into the water and these combine to form a zygote (the fertilized egg). The zygote then develops into a planula larva. The planula eventually sinks to the ocean floor and develops into a polyp, an organism that looks nothing like a medusa. Polyps are attached to the ocean floor, usually on a rock or other hard surface, and stay in one place their whole life. They have a mouth surounded by tentacles, just like the more familiar polyps of sea anemones and Hydra. These polyps, however, are incapable of having sex—they cannot make eggs and sperm. Instead, they reproduce asexually. They can asexually produce other polyps, but they can also asexually produce miniature medusae called ephyra. These are pinched off from the polyp’s mouth as if they were a stack of plates, with the most mature medusa on top. The ephyra then swim away, grow into mature medusae, and complete the lifecycle.