Crystal jellies.

One of the jellies we saw during our February visit to the Monterey Bay Aquarium is especially important to biologists. The crystal jelly (Aequorea victoria) is not only an interesting critter in its own right, but also serves as a source of green fluorescent protein (GFP), used to mark genes.

Like the cross jellies, the crystal jellies are fairly transparent and tend towards the small, with bells around 3 inches in diameter (although occasionally they get as big as 10 inches). Proportionately, their tentacles are a little bit longer.

Their preferred food, copepods, comb jellies, and other hydromedusae, is a bit bigger than the zooplankton the cross jellies eat. Indeed, the crystal jelly can get its mouth (underneath the bell) open wide enough to ingest critters half its size. Try that sometime -- it's not easy!

The exciting thing about crystal jellies is that they can light up the tank with the light-producing organs on their outer bells. These organs use aequorin, a photoprotein, and green fluorescent protein (GFP). In the presence of Ca²⁺ ions, the aequorin emits a blue-green light. The GFP piggybacks on the aequorin's bioluminescence, accepting energy from the aequorin and then using that energy to emit green light.

The Monterey Bay Aquarium website doesn't say anything about what kind of adaptive advantage bioluminescence might confer on these jellies. Indeed, at this point crystal jellies have to contend with a new predator -- humans harvesting them as a source of aequorin and GFP for use in biological research.

As Sandra Porter notes, these jellies are not actually glow-in-the-dark fluorescent green. (You probably already guessed that from my photos.) Still, the substances they contain can be used to do pretty cool stuff -- including winning a Nobel Prize in Chemistry.