IELTS Academic Reading Practice Test 494

For much of the twentieth century the coelacanth was known only from fossils. The group first appears in rocks about 400 million years old, and it was believed to have died out at roughly the same time as the dinosaurs, some 66 million years ago. Fossils show that it was once diverse, including forms far larger than the living animal. Museums held its thick, enamel-covered scales and its odd fins, and palaeontologists treated the lineage as long extinct. Then, on 22 December 1938, a trawler working off the south-east coast of South Africa brought up something that nobody had expected to see.

The captain of the trawler, Hendrik Goosen, had an arrangement with Marjorie Courtenay-Latimer, the curator of a small museum in the port of East London: he would set aside unusual creatures for her. Among the catch that day was a heavy, blue-grey fish about a metre and a half long, with scales like armour and fins that seemed to be mounted on muscular stalks. Courtenay-Latimer could not match it to any illustration in her reference books. Unable to keep the whole animal, she had the skin preserved and mounted by a local taxidermist, and waited for an expert to explain what she had found.

Help came from J. L. B. Smith, a lecturer in chemistry at Rhodes University who was a keen amateur ichthyologist. Shown a sketch, he recognised the fish from fossil drawings and travelled to East London early in 1939 to examine it. He named the species Latimeria chalumnae, after Courtenay-Latimer and the river near whose mouth it had been caught. The announcement caused a sensation, although some scientists wondered whether a single specimen might be an abnormality rather than a genuine survivor.

Smith then spent years searching for a second example, offering rewards to fishermen along the coast. The breakthrough came in 1952, when a fisherman in the Comoro Islands, between Madagascar and the African mainland, caught another. Those islands, with their steep volcanic slopes dropping into deep water, proved to hold a population of a few hundred fish. Coelacanths, it emerged, shelter in caves and on rocky ledges at depths of between about 100 and 500 metres, where they hang almost motionless in the current until darkness falls. The hunt for a living fossil had found not a relic in shallow water but a deep-sea animal.

The coelacanth's anatomy is peculiar in ways that fossil hunters had predicted. Its fins are supported by fleshy lobes containing bones, the arrangement that in land animals became arms and legs. Its skull is divided by a joint that allows the front half of the head to be lifted, a mechanism thought to help it seize prey. In place of the gas-filled swim bladder found in most fish it has a tube packed with fat, and its eyes are adapted to very dim light, while its brain is remarkably small for a fish of its size. The most surprising observations have come from film: in their caves, coelacanths appear to move their paired fins in a diagonal sequence, rather as a four-legged animal walks.

The label 'living fossil' is used loosely. The coelacanth is not a survivor from the distant past in the sense that its body has stayed the same: fossil coelacanths from 300 million years ago differ from the living species in the shape of the skull and the proportions of the fins.

Another puzzle concerned reproduction. For decades almost nothing was known about how the animals bred, because fishermen rarely landed females carrying eggs. It is now established that coelacanths give birth to live young, and that the embryos are nourished by large yolks inside the mother. Growth is slow and individuals appear to live for many decades, which leaves a small population dangerously exposed to accidental capture.

In September 1997 a second population was discovered, this time in northern Indonesia. A biologist and his wife, walking through a market in Manado on the island of Sulawesi, noticed an unfamiliar fish on a cart; photographs reached scientists, and within a year local fishermen had produced another specimen. The Indonesian fish differs slightly in colour and has been classified as a separate species, Latimeria menadoensis. Its existence, thousands of kilometres from the Comoros, suggested that coelacanths were once widespread in warm oceans and survive only where caves, currents and cold, oxygen-rich water occur together.

The popular image of the coelacanth as the ancestor of land-dwelling animals is mistaken. It belongs to an ancient branch of the lobe-finned fishes, but the line that produced the first four-legged creatures split away from the coelacanth's line hundreds of millions of years ago; lungfish, not coelacanths, are now regarded as the closest living relatives of tetrapods. When the coelacanth genome was sequenced in 2013, biologists found that parts of it had indeed changed unusually slowly, but the idea of a creature that had somehow opted out of evolution was an exaggeration.

Today the African coelacanth is listed as critically endangered. Its total population is probably numbered in the hundreds, and animals are caught accidentally at night by fishermen using handlines in deep water. A hooked coelacanth is a valuable curiosity, so conservation depends less on protection in law than on persuading fishing communities that a live animal is worth more than a dead specimen. The Indonesian population appears even smaller.

Frost is often a surprise. A gardener may find the lawn white at dawn while a thermometer a metre above it records several degrees above freezing. Heat is lost from the surface mainly by radiation towards a clear sky, so the ground becomes colder than the air. Frost forms when water vapour changes directly into ice on a surface below the frost point, the temperature at which air can hold no more vapour. No liquid stage is involved. The process, known as deposition, needs a surface that is losing heat faster than the surrounding air can replace it.

Meteorologists distinguish an air frost from a ground frost, because the two are measured differently. Air temperature is read from a thermometer in a louvred white box, a screen, about 1.25 metres above the ground, where it is shaded and ventilated. Ground frost is recorded by a thermometer lying on short grass in an open place. On a clear, still night the grass can be several degrees colder than the air in the screen, which is why frost whitens lawns and car roofs while puddles stay unfrozen.

Three conditions favour a heavy formation. The sky must be clear, so that heat escapes freely into space. The night must be long, giving the surface time to cool, and the air must be reasonably moist, since vapour is the raw material of the crystals. A gentle breeze supplies fresh vapour and thickens the deposit. A strong wind, by contrast, mixes the cold layer near the ground with warmer air above and can prevent frost altogether. Crystals grow most readily on surfaces fully exposed to the sky, so the tops of leaves whiten first.

The commonest result is hoarfrost: white, feathery crystals standing out from the surface they grow on. Because the ice forms directly from vapour, it can build outward into delicate needles and scales. Hoarfrost is typical of clear winter nights, when the ground cools quickly after sunset, and it is thickest in hollows and on exposed edges rather than in the middle of a field. A heavier deposit does not necessarily mean a colder night; it usually means a longer one, or more moisture in the air.

Rime is a different substance with a different origin. It forms when droplets of water in fog or cloud remain liquid below zero, a state known as supercooling, and freeze on contact with any object they strike. The result is a hard, granular, greyish deposit on the windward side of trees, fences and radio masts. Rime is common on hills, where cloud often covers the ground in winter, and can add substantial weight to structures. Unlike hoarfrost, it needs cloud rather than a clear sky.

Indoors, frost can appear on glass even in a heated room. Warm, moist air from a kitchen meets a pane chilled by the air outside, and vapour is deposited on the inner surface. The patterns that result, often called fern frost, depend on the temperature of the glass, the humidity of the room and the microscopic scratches and dust on the pane. Rapid cooling produces many small crystals; slow cooling lets a few large ones spread into long feathered fronds.

On a calm night the air touching a hillside is chilled, becomes denser and slides downhill — katabatic drainage — collecting on the valley floor. Such frost hollows can be several degrees colder than the slopes above. Orchards are planted on gentle slopes where possible, and on cold nights a large fan may mix the warmer air above with the cold layer near the ground. Overhead sprinklers offer another defence: freezing water releases latent heat, keeping blossom near freezing point even under a coat of ice.

In soil, freezing water does damage of a different kind. When the upper layer of ground freezes, an ice lens may form just below the surface, and more water is drawn upwards by capillary action. The lens grows, lifting the soil above it; when it melts, the surface collapses into the space left behind. Repeated winter after winter, this cycle breaks up roads, heaves fence posts out of the ground and loosens the roots of plants. In cold regions it also sorts stones and soil into the patterned ground studied by geologists.

One form of frost resisted explanation for a century. On humid nights, thin, silky hairs of ice, a fraction of a millimetre wide, push out from dead wood — a phenomenon called hair ice. Their shape is so regular that early observers assumed the wood itself produced it. In 2015 researchers reported that the hairs form only on wood containing a particular fungus, which appears to prevent the ice from recrystallising into coarser grains. How the fungus controls growth, and why the hairs keep a constant diameter, is still not understood.

Forecasting frost remains difficult in detail. Computer models of the atmosphere divide the world into grids tens of kilometres across, far larger than an orchard. The temperature of one patch of grass must therefore be estimated from local experience. Climate records add a complication: as global temperatures rise, frosty nights are becoming less frequent in many regions, yet a single severe frost can still ruin a crop, and growers want warnings reliable to within a degree.

Few wildlife experiences are sold as insistently as the chance to swim with wild dolphins. Boats carry holidaymakers to bays where bottlenose or spinner dolphins come to rest, tourists slip into the water, and the animals may circle within a few metres. Campaigners argue that the practice harasses creatures already under pressure from fishing nets, pollution and the loss of coastal habitat. Operators reply that their livelihoods depend on the animals and that they have every reason to protect them. The question, then, is not whether tourists should be allowed to see dolphins, but whether they should be allowed to enter the water with them.

The strongest biological objection concerns rest. Several species of dolphin spend the day in shallow, sheltered bays, where the water is calm and predators are few, and they sleep there in short bouts. Sleep in a dolphin is not like sleep in a human: only half of the brain rests at a time, and the animal must keep swimming in order to breathe. A boat that approaches, and especially a swimmer who enters the water, interrupts this rest. Researchers watching dolphin-watching trips have recorded shorter rests, tighter groups, more diving and animals leaving the bay. Mothers with young calves appear to be particularly sensitive to disturbance.

Not all encounters are alike. Spinner dolphins, which rest in shallow sandy bays during the day and feed offshore at night, are especially exposed, because the same few bays are used by the animals and by tour boats. Bottlenose dolphins, which range more widely, may be less tied to a single site, though some populations are resident and are visited daily. The species, the size of the group and the number of boats involved all influence how much disturbance an encounter creates.

Whether these short-term responses matter for the population is much less clear, and this is where the argument becomes difficult. In Western Australia, researchers monitoring bottlenose dolphins over many years found that the number using an area fell after wildlife-viewing boats began operating there. The finding is often quoted as proof of harm. Its authors were more cautious, noting that the decline might also reflect changes in the supply of food or in the wider environment. Few studies have followed a population for long enough to separate the effects of tourism from other pressures.

Those who run the trips make three arguments. The first is economic: in places where fishing has declined, dolphin-watching brings income to boat owners and guesthouses, and gives local people a reason to keep the animals and their bay intact. The second is educational: passengers, it is said, leave with a stronger commitment to conservation. The third is strategic: if licensed operators are forbidden to enter the water, visitors will simply go out with unlicensed boats that ignore the rules, and the animals will be worse off. The claim is plausible, though it is rarely supported by evidence.

Regulators have taken a middle path. In several countries, commercial viewing requires a permit, which sets out how close a boat may come, how many vessels may be present at once and how long each may stay. Some authorities have gone further, closing particular bays to boats during the hours when dolphins rest, or forbidding in-water encounters with mothers and calves. New Zealand, which has regulated marine mammal watching for decades, requires permits for commercial operators. There is no international standard, and neighbouring countries may impose quite different limits on the same population of animals. Such rules are only as good as the surveillance behind them.

Enforcement is the weak point. Patrols are expensive, the coast is large and violations are hard to prove: a boat that drifts too close and then moves away leaves no record. Where tourism is a major earner, governments are reluctant to restrict it. Self-regulation by the industry has a poor record, partly because operators who keep their distance lose custom to those who do not. Surveys of compliance at popular sites have reported breaches of the rules, though the rate varies greatly from place to place.

There are also risks for the people who take part. Dolphins are large, powerful predators; a female protecting a calf may charge, butt or bite, and injuries to swimmers have been recorded. Close contact raises the possibility of passing infections in either direction, since some dolphin diseases can affect humans and some human ones can affect dolphins. Hand-feeding, which a few operators still use to keep animals near the boat, changes behaviour more profoundly than swimming ever could, tying animals to vessels and increasing the chance of collisions with propellers.

My own view is that neither a universal ban nor unrestricted access is defensible. The evidence does not justify ending all in-water encounters with every species everywhere, but neither does it justify the assumption that the activity is harmless until proven otherwise. Where dolphins use a bay for rest or for nursing calves, swimming should be prohibited outright, and the prohibition should be enforced. Elsewhere, licences should be granted to a limited number of boats, conditional on monitoring, and renewed only if the animals stay. The burden of proof should rest with the operator, not with the animals. A ban that is widely ignored protects nothing, and a licence that is never reviewed protects very little.