Walk along a beach or pier in summer or fall and, if you're lucky, you may spot a comb jelly or two in the shallows. The luminescent drifters are named for the eight rows of shimmering combs that line their translucent bodies. The teeth of these combs are tiny vibrating hairs that propel the creatures through the water. At night, comb jellies--also known as ctenophores--glow bright green or flash eerie blue light when disturbed by boaters or swimmers. Like fireflies on land, they produce their otherworldly glow by a chemical reaction involving a light-emitting enzyme. Over eons, many shallow-water marine organisms have shifted their home ranges to and from the deep sea despite the differences between these habitats, including light availability, temperature and pressure. Such habitat shifts required dramatic genetic and physiological changes. Relationships among comb jelly species indicate that at least five such transitions have occurred, with some ctenophores staying in shallow waters and others evolving to thrive in the deep sea.
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