Another thing to worry about: jellyfish. In “They’re Taking Over!” Tim Flannery’s New York Review of Books critique of Lisa-ann Gershwin’s Stung!, he explains how the colorful stingers are perhaps becoming kings of the sea. An excerpt:

“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.

Jellyfish are voracious feeders. Mnemiopsis is able to eat over ten times its own body weight in food, and to double in size, each day. They can do this because they are, metabolically speaking, tremendously efficient, being able to put more of the energy they ingest toward growth than the more complex creatures they compete with. And they can be wasteful. Mnemiopsis acts like a fox in a henhouse. After they gorge themselves, they continue to collect and kill prey. As far as the ecosystem goes, the result is the same whether the jellyfish digest the food or not: they go on killing until there is nothing left. That can happen quickly. One study showed thatMnemiopsis removed over 30 percent of the copepod (small marine crustaceans) population available to it each day.

Jellyfish ‘can eat anything, and often do,’ Gershwin says. Some don’t even need to eat, in the usual sense of the word. They simply absorb dissolved organic matter through their epidermis. Others have algae living in their cells that provide food through photosynthesis.

The question of jellyfish death is vexing. If jellyfish fall on hard times, they can simply ‘de-grow.’ That is, they reduce in size, but their bodies remain in proportion. That’s a very different outcome from what is seen in starving fish, or people.”