Jellyfish are very diverse. They range in size from a millimeter long to giants with bells over a meter across that can weigh almost half a ton. Common names give some idea of the diversity and appearance: moon jellies, lion’s manes, sea walnuts, snotties, agua vivas, agua mala, blubbers, Portuguese men-o-war, and long stingy stringy thingies. These last two types are not, strictly speaking, organisms at all. Instead they are made up of collections of jellyfish species, the individuals of which are referred to as “persons” (as in food-catching persons, digestive persons, defensive persons, etc.) that function collectively like, and indeed appear to be, a single individual. And they can be enormous—up to 150 feet long. If you’re confused by this you’re in good company. As Gershwin explains, such entities are “not quite an individual. Not quite a colony…. For over 150 years, many of the greatest minds in evolutionary biology have debated [their] proper status.”
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 that Mnemiopsis 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. And when food becomes available again, jellyfish simply recommence growing. Some individual jellyfish live for a decade. But the polyp stage survives pretty much indefinitely by cloning. One polyp colony started in 1935 and studied ever since is still alive and well in a laboratory in Virginia.
One kind of jellyfish, which might be termed the zombie jelly, is quite literally immortal. When Turritopsis dohrnii “dies” it begins to disintegrate, which is pretty much what you expect from a corpse. But then something strange happens. A number of cells escape the rotting body. These cells somehow find each other, and reaggregate to form a polyp. All of this happens within five days of the jellyfish’s “death,” and weirdly, it’s the norm for the species. Well may we ask of this astonishing creature, “Sting, where is thy death?”
Despite their marvelous biology, jellyfish populations have been held in check ever since complex life evolved half a billion years ago. So why are they expanding now? In Part 2 of Stung!, titled “Jellyfish, Planetary Doom, and Other Trivia,” Gershwin attempts to answer this, and to tell us what it means for the oceans.
It’s clear from Gershwin’s book that it has taken a mighty effort by other living creatures to hold jellyfish down. An important part of that effort has involved the maintenance of complex ecosystems, with their abundant predators and competitors of jellyfish. It’s no accident that prodigious jellyfish blooms have occurred in areas like the Black Sea and off South Africa, where anchovies once swarmed. Overfishing anchovies, which compete with jellyfish for food, has doubtless helped them take over. That alone might not have been enough to allow the jellyfish to gain the march on us, but we’ve overfished virtually every resource in the oceans, causing the outright collapse of many ecosystems, thus opening vast new resources to the jellyfish.
Our waste, such as plastic bags, and fishing methods like drift nets and long lines are busy destroying the few jellyfish predators, such as sea turtles. We are also creating the most splendid jellyfish nurseries. From piers to boat hulls, oil and gas platforms and industrial waste and other floating rubbish, we’re littering the oceans with the kind of artificial hard surfaces that jellyfish polyps love.
Then there is the amount of oxygen dissolved in seawater. Oxygen is created by plants using photosynthesis, and high oxygen levels allow fish and other complex creatures to compete successfully with jellyfish. But the oxygen in water can be depleted far more quickly that it can be replaced. Where humans add nutrients to seawater (such as fertilizer runoff from farms), areas with depleted oxygen, known as eutrophied zones, form. They can occur naturally, but are spreading quickly as the oceans become filled with excess phosphorus and nitrogen derived from a variety of agriculture and industrial human activities. In river estuaries, and in confined waters such as the Baltic, the Black Sea, and the Gulf of Mexico, eutrophied zones have spread to a frightening extent, and they appear to be permanent. Nothing that needs even moderate amounts of oxygen, including fish, shellfish, prawns, and crabs, can survive in them. But the jellyfish thrive.
Our changing climate is also having many impacts on jellyfish. As the oceans warm, the tropical box jellyfish and the Irukandjis are likely to extend their ranges, while other species will benefit from the lowered oxygen levels that warmer waters contain. Remarkably, jellyfish may have the capacity to accelerate climate change. This can happen in two ways. Jellyfish release carbon-rich feces and mucus (poo and goo) that bacteria prefer to use for respiration. As Gershwin puts it, “jellyfish blooms turn these bacteria into carbon dioxide factories.” But jellyfish also consume vast numbers of copepods and other plankton. These creatures migrate vertically through the water column, taking in carbon-rich food at the surface and releasing it as fecal pellets, which fall to the sea floor and are buried. The plankton are thus a major means of taking carbon dioxide out of the atmosphere and oceans. If their loss occurs on a large enough scale, it will hasten climate change.
There is one final impact that must be considered: acidification of the oceans. This results from carbon dioxide being absorbed into seawater. Already our oceans are 30 percent more acidic than they were thirty years ago, and creatures with shells are suffering. In recent years, there has been mass failure of oyster spawning off the American Northwest, and tiny snails in the Arctic and Antarctic oceans are having their shells eaten away by the acid. Jellyfish lack hard parts: they, it seems, will pull through the acidification crisis admirably.
How could jellyfish take over the ocean? “One bite at a time” Gershwin says. And there may be no way back. A new balance may be struck, one in which jellyfish rule:
We are creating a world more like the late Precambrian than the late 1800s—a world where jellyfish ruled the seas and organisms with shells didn’t exist. We are creating a world where we humans may soon be unable to survive, or want to.
At the same time that Gershwin asserts that jellyfish are taking over the oceans “one bite at a time,” she offers a slender hope that we might eat our own way through the problem. Ancient Chinese texts show that jellyfish have been part of the human diet for over 1,700 years. Recently, the global jellyfish harvest has risen to 321,000 tons, most of which is consumed in China and Japan. But unless we all develop an Asiatic zeal for the gelatinous creatures it’s hard to imagine we humans making much of a dent in the jellyfish multitudes.
As I came toward the end of this astonishing, if dismaying, book my spirits were lifted briefly when I discovered that Congress seems to be aware of the jellyfish menace. On November 2, 1966, it passed the Jellyfish Control Act (16 U.S.C. §§ 1201–1205; 1966, amended 1970 and 1972). This seemingly prescient legislation authorized the secretary of commerce to “conduct studies, research and investigations to determine the abundance and distribution of jellyfish and other pests and their effects on fish, shellfish and water-based recreation.” Up to $1 million annually was spent in the 1970s. Regrettably, today Gershwin and the handful of jellyfish experts in the world struggle for access to what is clearly pitifully inadequate funding.
Gershwin leaves us with a disturbing final rumination:
When I began writing this book,… I had a naive gut feeling that all was still salvageable…. But I think I underestimated how severely we have damaged our oceans and their inhabitants. I now think that we have pushed them too far, past some mysterious tipping point that came and went without fanfare, with no red circle on the calendar and without us knowing the precise moment it all became irreversible. I now sincerely believe that it is only a matter of time before the oceans as we know them and need them to be become very different places indeed. No coral reefs teeming with life. No more mighty whales or wobbling penguins. No lobsters or oysters. Sushi without fish.
Her final word to her readers: “Adapt.”