To understand why jellyfish are taking over, we need to understand where they live and how they breed, feed, and die. Jellyfish are almost ubiquitous in the oceans. As survivors of an earlier, less hospitable world, they can flourish where few other species can venture. Their low metabolic rate, and thus low oxygen requirement, allows them to thrive in waters that would suffocate other marine creatures. Some jellyfish can even absorb oxygen into their bells, allowing them to “dive” into oxygen-less waters like a diver with scuba gear and forage there for up to two hours.
Jellyfish reproduction is astonishing, and no small part of their evolutionary success: “Hermaphroditism. Cloning. External fertilization. Self fertilization. Courtship and copulation. Fission. Fusion. Cannibalism. You name it, jellyfish [are] ‘doing it.’” But perhaps the most unusual thing is that their eggs do not develop immediately into jellyfish. Instead they hatch into polyps, which are small creatures resembling sea anemones. The polyps attach to hard surfaces on the sea floor, and are particularly fond of man-made structures, on which they can form a continuous jelly coating. As they grow, the polyps develop into a stack of small jellyfish growing atop each other that look rather like a stack of coins. When conditions are right, each “coin” or small jellyfish detaches and swims free. In a few days or weeks, a jellyfish bloom is observed.
One of the fastest breeders of all is Mnemiopsis. Biologists characterize it as a “self-fertilizing simultaneous hermaphrodite,” which means that it doesn’t need a partner to reproduce, nor does it need to switch from one sex to the other, but can be both sexes at once. It begins laying eggs when just thirteen days old, and is soon laying 10,000 per day. Even cutting these prolific breeders into pieces doesn’t slow them down. If quartered, the bits will regenerate and resume normal life as whole adults in two to three days.
