READING PASSAGE-3
You should spend about 20 minutes on
Questions 27-40,
which are based on Reading Passage 3 below.
When viewed in the wild, jellyfish are perhaps the most graceful and vividly
coloured of all sea creatures. But few people have seen a jellyfish living in its
natural habitat. Instead, they might see a dead and shapeless specimen lying on the
beach, or perhaps receive
a painful sting while swimming, so it is inevitable that
jellyfish are often considered ugly and possibly dangerous. This misunderstanding
can be partly traced back to the 20
th
century, when the use of massive nets and
mechanical winches often damaged the delicate jellyfish that scientists managed to
recover. As a result, disappointingly little research was carried out into jellyfish, as
marine biologists took the easy option and focused on physically stronger species
such as fish, crabs and shrimp. Fortunately, however, new techniques are now
being developed. For example, scientists have discovered
that sound bounces
harmlessly off jellyfish, so in the Arctic and Norway researchers are using sonar to
monitor jellyfish beneath the ocean’s surface. This, together with aeroplane
surveys, satellite imagery and underwater cameras, has provided a wealth of new
information in recent years.
Scientists know believe that in shallow water alone there are at least 38 million
tonnes of jellyfish and these creatures inhabit every type of marine habitat,
including deep water. Furthermore, jellyfish were once regarded as relatively
solitary, but this is another area where science has evolved. Dr Karen Hansen was
the first to suggest that jellyfish are in fact the
centre of entire ecosystems, as
shrimp, lobster, and fish shelter and feed among their tentacles. This proposition
has subsequently been conclusively proven by independent studies. DNA
sequencing and isotope analysis have provided further insights, including the
identification of numerous additional species of jellyfish unknown to science only a
few years ago.
This brings us to the issue of climate change. Research studies around the world
have recorded a massive growth in jellyfish populations in recent years and some
scientists have linked this to climate change. However, while this may be credible,
it cannot be established with certainty as other factors might be involved. Related
to this was the longstanding academic belief that jellyfish had no predators and
therefore there was no natural process to limit their numbers. However,
observations made by Paul Dewar and his team showed that this was incorrect. As
a result, the scientific community now recognises
that species including sharks,
tuna, swordfish and some salmon all prey on jellyfish.
It is still widely assumed that jellyfish are among the simplest lifeforms, as they no
brain or central nervous system. While this is true, we now know they possess
senses that allow them to see, feel and interact with their environment on subtle
ways. What is more, analysis of so-called ‘upside-down jellyfish’ shows that they
shut down their bodies and rest in much the same way that humans do at night,
something once widely believed to be impossible for jellyfish. Furthermore, far
from ‘floating’ in the water as they are still sometimes thought to do, analysis has
shown jellyfish to be the most economical swimmers in the animal kingdom. In
short, scientific progress in recent years has shown that
many of our established
beliefs about jellyfish were inaccurate.
Jellyfish, though, are not harmless. Their sting can cause a serious allergic reaction
in some people and large outbreaks of them – known as ‘blooms’ – can damage
tourist businesses, break fishing nets, overwhelm fish farms and block industrial
cooling pipes. On the other hand, jellyfish are a source of medical collagen used in
surgery and wound dressings. In addition, a particular protein taken from jellyfish
has been used in over 30,000 scientific studies of serious diseases such as
Alzheimer’s. Thus, our relationship with jellyfish is complex as there are a range of
conflicting factors to consider.
Jellyfish have existed more or less unchanged for at least 500 million years.
Scientists recognise that over the planet’s history there have been three major
extinction events connected with changing environmental conditions. Together,
these destroyed 99% of all life, but jellyfish lived through all three. Research in the
Mediterranean Sea has now shown, remarkably, that in old age and on the point of
death, certain jellyfish are able to revert to an earlier physical state, leading to the
assertion that they are immortal. While this
may not technically be true, it is
certainly an extraordinary discovery. What is more, the oceans today contain 30%
more poisonous acid than they did 100 years ago, causing problems for numerous
species, but not jellyfish, which may even thrive in more acidic waters. Jellyfish
throughout their long history have shown themselves to be remarkably resilient.
Studies of jellyfish in class know as scyphozoa have shown a lifecycle of three
distinct phases. First, thousands of babies known as planulae are released. Them,
after a few days the planulae develop into polyps – stationary lifeforms that feed
off floating particles. Finally, these are transformed into something that looks like a
stack of pancakes, each of which is a tiny jellyfish. It
is now understood that all
species of jellyfish go through similarly distinct stages of life. This is further
evidence of just how sophisticated and unusual these lifeforms are.
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