Jellyfish -- The Dominant Species

A  Jellyfish have become the curse of beach holidays, permeating every ocean on the globe, thriving in the Arctic and the tropics. In an ever-changing world where other species struggle to endure, jellyfish populations are on the rise.

To the untrained eye, these creatures drift aimlessly on the oceans’ currents and appear benign. In addition, they lack sharp claws, piercing teeth or even a brain. Despite this, they are armed with an amazing arsenal of weapons, especially the stinging power of their tentacles. As a result, jellyfish are among the most-feared, least-understood creatures in the seas.

B  According to Dr Monty Graham, a jellyfish scientist at the University of South Alabama, US, ‘Jellyfish are a pretty good group of animals to track coastal ecosystems. When you start to see jellyfish numbers grow, that usually indicates a stressed system.’ While populations appear to be down this year, Dr Graham sees ‘a statistically solid increase’ over the longer term.

This increase first gained attention in the 1980s when a huge number of jellyfish, Atlantic Ocean natives named Mnemiopsis leidyi, devastated the Black Sea, an ecosystem already weakened by overfishing of anchovies. Scientists believe that this species of jellyfish came in on the bottom of a ship and then rapidly multiplied, feeding on anchovy eggs and the plankton that young fish rely on.

C  Dense jellyfish aggregations can be a natural feature of healthy ocean ecosystems, but a clear picture is now emerging of more severe and frequent jellyfish outbreaks worldwide. Dr Anthony Richardson, from the University of Queensland, Australia, explains that once jellyfish gain a foothold, if conditions are right they can establish a massive population at the expense of other ocean life. The problem is that parts of the ocean might switch from being dominated by fish to being dominated by jellyfish.

D  A study done by Richardson and his colleagues explores the causes behind jellyfish infestation, and the need for swift, decisive action to stem jellyfish takeover. Jellyfish outbreaks are linked directly to human actions, including overfishing, the input of fertilizer and sewage into the ocean, and climate change.

Overfishing has removed fish from marine ecosystems at astounding rates. According to Richardson, this has made it possible for jellyfish to take their place. ‘This is because small fish appear to keep jellyfish in check by predation (on jellyfish when they are very small) and competition (when feeding). So, once we remove fish, jellyfish can proliferate.'

Eutrophication is another human-caused change in the ocean that has likely contributed to jellyfish explosions. Eutrophication is an increase of nitrogen and phosphorus in the ocean, largely caused by fertilizer and waste run-off. This leads to algae blooms, which lower oxygen in the marine ecosystem, creating so-called ‘dead zones’, which have been increasing dramatically around the world. According to Richardson, these low-oxygen waters give jellyfish the advantage. ‘Fish avoid low-oxygen waters but jellyfish, having lower oxygen demands, not only survive but can thrive in these conditions as there is less predation and competition from fish.'

E  Furthermore, Richardson and his colleagues speculate that climate change may expand the traditional geographical range of jellyfish. ‘As water warms, tropical species are moving towards the poles. Many venomous jellyfish species are tropical and could move into more densely populated subtropical and temperate regions.'

F  Once jellyfish appear en masse in an ecosystem they can make it very difficult for fish to stage a comeback. By feeding on fish, the jellyfish successfully prevent fish from returning to their normal population numbers, says Richardson. ‘One can thus think of two alternate states with each being stable: one dominated by fish and the other by jellyfish. Unfortunately, where there is a jellyfish-dominated state then this does not support the nutritional needs of other fish, marine mammals, and seabirds.’ In other words an ecosystem that loses fish also loses the species that depend on fish for survival.

This state has been defined as a ‘monoculture of jellyfish’, an apt analogy since the situation shares similarities with other monocultures. When the rich biodiversity of tropical forests is replaced by plantations growing a single species of tree, an area of rich variety becomes a desert in terms of biodiversity, as do ocean ecosystems when jellyfish become the dominant species.

One result of large jellyfish populations is the economic effect it has had on the fishing industry. In the Gulf of Mexico, shrimp fishermen are struggling with a jellyfish boom that fills nets, causing them to break and resulting in millions of dollars in losses.

G  Experts say that a greater understanding of jellyfish, including their ideal water temperature and feeding habits, is necessary to determine with certainty what is causing the recent massive invasion, and to come up with ways to combat it.

Due to the difficulty of turning ecosystems around once jellyfish have become dominant, Richardson and his colleagues propose focusing on ‘prevention rather than cure’. They recommend a halt to overfishing small fish that are vital to keeping jellyfish in check, reducing the amount of fertilizer and sewage running off into the oceans, and finally, if possible, confronting climate change.

Understanding symbols

Judy DeLoache describes her research into the difficulties children experience with symbols

A  About 20 years ago I had one of those wonderful moments when research takes an unexpected but fruitful turn. I had been studying toddler memory and was beginning a new experiment with two-and-a-half- and three-year-olds. For the project, I had built a model of a room that was in my laboratory. The real space contained basic furniture such as a couch and table. The miniature version was as similar as possible to its larger counterpart: the furniture was the same shape and material and was arranged in the same position. For the study, a child watched as we hid a miniature toy -- a plastic dog we called Little Snoopy -- in the model. We then encouraged the child to find ‘Big Snoopy’, a large version of the toy ‘hiding in the same place in his big room.

B  The three-year-olds were very successful. After they observed the small toy being placed behind the miniature couch, they ran into the room and found the large toy behind the real couch. But the two-and-a-half-year-olds failed abysmally. They cheerfully ran into the room to retrieve the large toy, but most had no idea where to look, even though they remembered where the tiny toy was hidden in the miniature room and could readily find it there.

C  Their failure to use what they knew about the model to draw an inference about the real room indicated that they did not appreciate the relation between the model and the room. I realised my memory study was instead a study of symbolic understanding and that the children’s failure might be telling us something about how and when children acquire the ability to understand that one object stands for another.

D  The first type of symbolic object that infants and young children master is the picture. No symbols seem simpler to adults, but infants initially find pictures perplexing. The problem stems from the duality inherent in all symbolic objects: They are real in themselves and also representations of something else. A few years ago I became intrigued by anecdotes suggesting that infants do not appreciate this duality-stories of a baby trying to pick up a depicted toy or fit a foot into a photograph of a shoe. We assumed such behaviour would be rare and therefore difficult to study.

E  Fortunately we were wrong. We began testing infants’ understanding of pictures by putting a book of highly realistic colour photographs of individual objects in front of nine month-olds. To our surprise, every child in the study reached out to feel or scratch the pictures. The confusion seems to be conceptual not perceptual. Infants can perfectly well perceive the difference between objects and pictures and given the choice they will choose the real thing. But they do not yet fully understand what pictures are and how they differ from real objects. However, when depicted objects bear little resemblance to the real thing -- as in a black and white drawing -- infants rarely explore them. By 18 months, babies have come to appreciate that a picture merely represents an object; instead of manipulating the paper, they point to pictures and name objects. Nevertheless, it takes several years for the nature of pictures to be completely understood.

F  Studies have shown that, until the age of four, many children think that turning a picture of a bowl of popcorn upside down will result in the depicted popcorn falling out of the bowl.

G  Pictures are not the only source of confusion for very young children. In the third experiment, we brought a group of toddlers (18- to 30-month-old children) into a room containing a child’s slide, a chair, and a toddler-sized car. The children played with these for a while. Then we secretly replaced each object with an identical miniature version. Most children attempted to perform the same actions with the miniature items that they had with the larger ones. Some tried to sit on the chair, others attempted to climb into the car. Interestingly, most of the children showed little or no reaction to their failed attempts. We think this probably reflects the fact that toddlers’ daily lives are full of unsuccessful attempts to do one thing or another. This confusion has implications for educational practice. Teachers everywhere use blocks or other objects to represent numerical quantity. However, if young children do not understand the relation between an object and what it represents, this could be counterproductive. To demonstrate this, we taught six-and seven-year-olds a difficult subtraction problem using blocks.

H  We taught an identical comparison group the same concept, but using pencil and paper. Both groups learned to solve the problems equally well, but the group using blocks took three times as long to do so.

I  Dual representation also comes into play in many children’s books. Modern children’s books are often 3D, with features that encourage children to interact directly with the book itself, for example flaps that can be lifted to reveal pictures. Graduate student Cynthia Chong and I reasoned that these features might distract children from the information. Accordingly, we used different types of books to teach letters to 30-month old children. One was a simple old-fashioned alphabet book with each letter accompanied by an appropriate picture. The other was a 3-D version. The children using the traditional book subsequently recognised more letters than those using the 3-D book.

J  Presumably, the children could concentrate more easily with the plain 2-D book, whereas with the other one they were distracted by the 3-D activities. Less may be more when it comes to educational books for young children.