by Ashley Hay
Jellyfish are very diverse. They range in size from a millimetre long to giants with bells over a metre 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’.
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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 characterise 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 13 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 per cent 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?’
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Despite their marvellous 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!, entitled ‘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 than it can be replaced. Where humans add nutrients to seawater (such as fertiliser run-off 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 faeces 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 faecal pellets, which fall to the sea floor and are buried. The plankton are thus a major means of taking CO2 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 CO2 b
eing absorbed into seawater. Already our oceans are 30 per cent more acidic than they were 30 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.
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How could jellyfish take over the ocean? ‘One bite at a time,’ says Gershwin. 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 1700 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 2 November 1966, it passed the Jellyfish Control Act (16 U.S.C. § 1201–1205; 1966, amended 1970 and 1972). This seemingly prescient legislation authorised 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 US$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.’
Planet of the vines
Antarctic ice: Going, going …
From Alzheimer’s to zebrafish
Michael Lardelli
The Irish rock band U2 sang that ‘a woman needs a man like a fish needs a bicycle’. In the same way, it is perhaps initially difficult to see how a small, freshwater fish from northern India – the zebrafish – could contribute to our understanding of Alzheimer’s disease. But it can.
The zebrafish has some remarkable characteristics that make it especially suitable for studying how genes work normally and in disease situations. Like humans, zebrafish are vertebrates – animals with backbones – and they share many of the same genes. The fish are small, easy to breed and can be kept in small tanks at fairly high density. This makes them relatively cheap to care for which is important when considering how best to spend precious research dollars.
For laboratory-based genetics research, it is important that an organism has a short generation time and produces large numbers of offspring. Zebrafish can produce a new generation in only three months and one female can produce tens of thousands of offspring in a lifetime. But the main advantage of zebrafish is that their embryos are transparent and develop outside the mother’s body. This means that the development of living zebrafish embryos can be observed in real time. We can alter the activity of a gene in zebrafish embryos and then observe how this changes the function of other genes and the way that the embryo develops.
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Over 60 000 papers reporting scientific research on Alzheimer’s disease have been published. However we still do not have a good understanding of why the disease occurs and what is actually happening in the brains of people with the disease.
The first thing to know about Alzheimer’s disease is that it is actually a disease. It is not simply a natural consequence of old age. A change of state of the brain is definitely involved. Age is the greatest risk factor for Alzheimer’s disease but it is also possible to develop the disease when young. About 1 per cent of Alzheimer’s disease cases are regarded as ‘early onset’ because they affect people less than 65 years of age – and sometimes people as young as their late 20s. The early onset cases are usually due to mutations passed down through families.
Mutations in three different genes can cause early onset Alzheimer’s disease – the APP gene that codes for Amyloid Precursor Protein and two that code for Presenilin proteins, PSEN1 and PSEN2. These three genes are functionally related since they produce proteins that interact within cells. Stated simply, the PSEN1 and PSEN2 genes produce two similar Presenilin proteins that are both able to cut the protein produced by APP. When the Amyloid Precursor Protein is cut it releases a small protein fragment (a peptide) known as amyloid beta or Aβ.
In brains affected by Alzheimer’s disease, the Aβ peptide accumulates and can form small clumps. In the past it has been thought that the accumulation and clumping of Aβ is what drives the disease. Many researchers have regarded the Aβ peptide as toxic and pharmaceutical companies have tried to find drugs that can reduce the amount of it in the brain. But all of the drugs tested so far have failed, and some have even made the disease worse! We now know Alzheimer’s disease develops over decades and it is only when damage to the brain is severe that the symptoms such as loss of memory start to show. For this reason, researchers are now looking at testing their drugs on people who are not yet showing the disease. However a growing group of researchers now doubt that Aβ is the actual cause of Alzheimer’s disease and suspect it may be just a symptom.
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Studying Alzheimer’s disease in humans is not easy since examining the brain in detail can really only be done post-mortem. In the laboratory it is possible to grow living cells from Alzheimer’s disease patients in culture systems. However, the environment in which these cells find themselves is so unnatural that their behaviour (including that of their genes and proteins) tends to be abnormal. Also, it is difficult to manipulate gene and protein activity in cultured cells in anything but a very crude fashion. This is where zebrafish embryos can help.
A fertilised zebrafish egg is basically a single, huge cell visible to the naked eye. It behaves normally when alone in an aqueous medium and its patterns of gene and protein activity can be manipulated easily and subtly. For example, we can force an embryo to make various amounts of a particular protein by injecting various amounts of messenger RNA (essentially a template for protein synthesis) into a fertilised egg. As the fertilised egg subsequently divides, the embryo’s cells receive the messenger RNA and translate it into protein.
One example of how we have been using zebrafish embryos to gain a greater understanding of Alzheimer’s disease involves studying the effects of insufficient oxygen (also known as hypoxia). Evidence has been accumulating for some time that hypoxia may be an important factor in Alzheimer’s disease. By placing zebrafish embryos in water low in oxygen we found that the APP, PSEN1 and PSEN2 genes in the embryos were all activated, which would cause increased production of Aβ. In fact, increased production of Aβ under hypoxia appears to be a widespread phenomenon among vertebrate animals. This means that Aβ production is probably an advantageous (protective) response to lack of oxygen. So the accumulation of Aβ in the brains of p
eople with Alzheimer’s disease may be a sign that their brains are starved of oxygen and the Aβ may actually be protecting their brains from harm rather than causing the disease.
Could mutations in APP, PSEN1 and PSEN2 cause early onset Alzheimer’s disease by disabling this protective mechanism? We don’t know, but these small and useful fish might just help us to find out.
The CAVE artists
Massimo’s genes
Joseph Jukes’ epiphanies
Iain McCalman
In the spring of 1842, the British Admiralty gave orders to the naval corvette the Fly to survey the northern end of the Great Barrier Reef and the surrounding waters and reefs of the Torres Strait. The Admiralty wanted particular attention paid to this area because so many British vessels trading in the South Seas or with India had come to grief trying to navigate the uncharted coral reefs and the Strait’s perilous narrow entrances. Joseph Beete Jukes, the ship’s 31-year-old naturalist, was officially charged with investigating the geological character of the Great Barrier Reef and the structure, origins and behaviour of reef-growing corals – the first scientist ever to be specifically assigned such a task.
Naturally the Admiralty’s concern was more practical than scholarly. By the 1840s it was widely recognised that corals were not inert rocks but living organisms, although little was known about the cause, extent and speed of their development. It was thought that dangerous new reefs might suddenly appear in places where previous surveys had shown nothing. The Admiralty hydrographer Francis Beaufort urged the Fly’s captain to remember that he would be dealing with submarine obstacles ‘which lurk and ever grow’.
It was also expected that a geologist would offer advice on suitable sites for future harbours and settlements, and when the captain gave Jukes responsibility for producing the official journal of the voyage, the geologist stressed that he would approach the task as a down-to-earth scientist, conveying ‘plain fact’ and ‘simplicity and fidelity’. He claimed he would eschew any ‘selecting for effect’, or ‘heightened recollections’, or ‘brilliancy, elegance, or graces of style’.
