Hula Skirt Siphonophore This jellyfish relative, which can grow to about 16 inches, controls its elevation in the water with a gas-filled float. Gas is released through a pore in the bottom of its float, which is refilled through secretions from a special gland. Its luminescence is a mystery. Scientists think it startles predators, but it’s just a best guess. Image: Steve Haddock/Monterey Bay Aquarium Research Institute.