#coronatae

Deep-sea crown jellies of the genus Atolla (image 4) are the most common jellies in the deep sea. They use their bright red coloration as camouflage (red is the first color to lose its hue in the absence of sunlight, thus causing the jelly to appear pure black). When approached by predators, it will display a sudden burst of brilliant blue bioluminescence (see image below). This burst of light in the dark water not only disorients predators but also attracts larger predators to the area. The threat of a bigger predator scares off any immediate danger, allowing the jelly to swim to safety. The Atolla Jelly does all this without eyes or a brain.

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The Thimble Jellyfish (Linuche unguiculata) is a tiny jellyfish with a straight-sided, flat-topped bell. It is the most common cause of “seabather's eruption”, a hypersensitivity reaction to the immature nematocysts of larval-stage jellyfish, in Florida or the Caribbean. Symptoms of seabather’s eruption generally arise later, after showering, and are identified by severe itching around small red papules 1mm to 1.5 cm in size on areas of skin that were covered by water-permeable clothing or hair during ocean swimming. Initial swimmer exposure to the free-floating larvae produces no effects, as each animal possesses only a single, undeveloped nematocyst which is inactive while suspended in seawater. However, due to their microscopic size and sticky bodies, large concentrations of larvae can become trapped in minute gaps between skin and clothing or hair. Once the swimmer leaves the ocean, the organisms stuck against the skin die, and automatically discharge their nematocysts when crushed, dried out, or exposed to freshwater. This is why symptoms usually do not appear until the swimmer dries, or takes a freshwater shower, without removing the affected clothing. So, it sucks for both human and baby jelly, and why you should always be mindful of where and when you are swimming. In Florida, most cases occur between March and August, to swimmers who unknowingly swim into a cloud of Thimble Jellyfish larvae.

The Thimble Jellyfish Linuche aquila (image 2) has symbiotic zooxanthellae (single-celled photosynthetic organisms) which live within its tissues, and essentially allow it to photosynthesize. These supply a major part of its energy requirements.

The Helmet Jellyfish (Periphylla periphylla) (image 1 and gif above) is one of the rare examples in Scyphozoa whose life-cycle lacks a polyp stage. Their eggs are also the largest sized eggs within all Cnidaria, and females only produce a small number of eggs at a time. The jellyfish release their eggs on the surface of the water, where they rapidly sink to a depth that limits visibility of predators. These develop directly into medusae, whose development rests entirely upon the egg's high yolk supply, not even developing an open mouth until the sixth stage of their development.

The Helmet Jellyfish is photophobic (intolerant of light) and inhabits deeper parts of the oceans, but may be found at the surface on dark nights. However, they seem to communicate with each other via bioluminescence, with red flashes.