1. In all the seas of the world,
the warm waters of the tropics
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2. contain the richest
and most colorful communities.
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3. Coral reefs.
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4. They may seem
like underwater paradise,
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5. but they are perpetual
battlegrounds for space.
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6. Even the corals
have to fight for it.
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7. In this crowded frenetic community,
every individual needs its place,
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8. its own way of surviving.
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9. None of these creatures would be
here if it were not for the coral.
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10. A coral larva
drifts in the open sea,
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11. floating in a soup
of young reef animals.
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12. If just one of these coral larvae
survives in a suitable spot,
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13. a new reef will be founded.
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14. In just a few days,
the larva changes form
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15. and becomes a polyp,
similar to a sea anemone.
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16. Identical copies bud off
and, gradually, a colony develops.
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17. Each polyp surrounds itself
with a hard skeleton,
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18. and, from this solid base,
begins to grow.
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19. It increases in length
by an impressive 15cm a year.
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20. This branching coral
is only two years old.
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21. A mature reef can be thousands.
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22. Corals provide the foundations
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23. on which the entire
reef community relies.
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24. Some organisms,
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25. like the Christmas tree worms,
actually live within the coral.
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26. Others climb out,
away from the reef,
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27. to filter their food
from the water.
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28. As the community grows,
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29. intimate relationships form,
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30. and different creatures
become dependent on one another.
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31. Even animals that spend much
of their time traveling in the ocean
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32. return to the reef for a clean.
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33. Coral reefs can be home
to astounding numbers of fish.
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34. Here swim the smallest
and the largest fish in the sea.
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35. Whale sharks are only visitors.
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36. When currents bring nutrient-rich
water up from the deep,
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37. they come here to feed.
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38. All these animals are here
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39. because of the coral.
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40. This extraordinary complex maze
is built, layer upon layer,
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41. by millions and millions
of individual animals -
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42. Polyps.
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43. Each polyp's flesh is supported
by a limestone skeleton.
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44. Below the gut...
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45. is the place
where most of the growth occurs.
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46. Here, the living tissue deposits
an intricate lattice of limestone.
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47. Beneath that,
the limestone skeleton is bare,
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48. having been vacated
by the living coral tissues.
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49. This is the hard structure
that forms the foundation of the reef.
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50. A single reef can extend
for many miles.
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51. Coral reefs are only found
in the clear, warm,
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52. shallow waters of the tropics.
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53. Sunlight is vital to them,
even though they are animals,
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54. because inside their flesh live
millions of tiny single-cell algae,
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55. plants.
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56. And all plants need sunlight
to photosynthesize sugars.
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57. 98 percent of the food the corals consume
is produced by the algae.
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58. Without them,
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59. the reef would not exist.
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60. Like any plant, algae need
just the right amount of light.
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61. Not too much, not too little.
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62. The corals regulate that
with pigments that we can only see
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63. when they are illuminated
by ultraviolet light.
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64. Most corals, for protection,
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65. spend the day withdrawn
into their stony fortresses.
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66. Even then they are not safe
from the jaws of these butterfly fish.
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67. At night, the corals take in water,
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68. expand their tentacles
and emerge to feed.
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69. They collect plankton.
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70. Each tentacle has stinging cells,
which fire on contact.
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71. Once the prey is caught, it's passed
down to the polyp's mouth.
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72. It is at night,
when the polyps are extended,
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73. that they add to the limestone
foundations beneath them.
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74. inevitably, the corals
begin to overgrow each other,
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75. and that means trouble.
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76. When neighbors get too close,
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77. they detect
one another's presence chemically.
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78. The aggressor on the right
prepares for battle.
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79. The polyps extrude their guts
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80. and simply
digest their rivals alive.
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81. A no-man's-land -
a band of white skeleton -
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82. is the only evidence
of the night's border dispute.
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83. Some corals are targeted
by yet more deadly predators.
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84. Predators that can crawl
in search of their victims.
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85. Crown of thorns starfish -
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86. poisonous,
invincible eating machines.
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87. They also extrude their gut
and digest coral wholesale.
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88. But some corals have help.
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89. Small crabs living within their
branches resist these onslaughts
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90. and defend their home.
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91. From beneath, they launch an attack
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92. on the vulnerable underside
of the starfish.
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93. Even the crown of thorns
will retreat
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94. from such a determined attack.
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95. This coral is left unharmed.
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96. Humphead parrotfish,
nearly a meter and a half in length.
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97. Their jaws are so powerful
they can bite through rock.
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98. When they descend to feed,
the reef itself is under threat.
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99. They are indiscriminate feeders,
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100. taking both rock and coral alike
in their quest for algae.
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101. These fish play a large part
in the erosion of the reef.
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102. The rock and coral emerges later
as a fine sand.
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103. On a single reef, they can produce
tons of it every year.
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104. This soft sand
forms the tropical beaches
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105. that we find so alluring.
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106. Over time, the sand builds up
to form an island,
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107. which is then colonized
by animals and plants.
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108. Trees take root.
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109. Birds arrive.
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110. The guano from thousands of terns
which have chosen to nest here
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111. enriches the sandy soil,
which then can support more plants.
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112. But these terns,
like other seabirds,
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113. depend on the ocean for their food.
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114. Below water, there is not only
competition for living space,
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115. but a continual contest
between predators and prey.
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116. it's the arms race between them
that over millions of years
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117. has produced today's
extraordinary diversity of form.
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118. Jacks are one of the key predators
on the reef.
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119. Their weapon is speed.
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120. They seek silversides,
whose defense is to congregate
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121. in confusing shoals
of shimmering silver.
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122. The jacks deal with that by herding
the silversides onto the reef.
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123. Here, the jacks
have a better chance
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124. of separating individual fish
from the shoal.
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125. The jacks can now catch
the isolated individuals
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126. with lightning attacks.
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127. It's far safer to be hidden
on the reef itself,
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128. within the tunnels of a sponge,
for example.
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129. These tiny shrimp are no bigger
than grains of rice.
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130. These shrimps are unique.
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131. It's recently been discovered
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132. that they have
a highly-sophisticated social system,
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133. similar to that of bees.
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134. All members of the colony
are the offspring of one female.
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135. She is the queen
and the only one to produce eggs.
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136. As in a colony of bees,
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137. different individuals are
specialized for particular tasks.
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138. Some are guards and are armed
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139. with particularly
large and powerful claws.
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140. They are on watch at all times,
ready to tackle intruders.
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141. A polychaete worm. For it, a sponge
is an excellent hunting ground.
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142. in such a maze of tunnels,
attack can come at any time
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143. from any quarter.
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144. Once the guards are alerted,
the worm loses its advantage.
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145. Better to retreat intact
than risk serious injury.
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146. The sponge not only makes
a safe home for the shrimps,
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147. it also supplies them with food,
so they never need venture outside.
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148. An establishment that provides
all their needs
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149. is clearly well worth defending.
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150. Just as shrimps guard their home,
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151. other animals defend
their hunting grounds.
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152. Glassfish make tempting prey
for the redmouth grouper.
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153. its strategy is to swim
slowly amongst them
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154. till they no longer see it
as a threat.
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155. There are other fish here, too.
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156. Lionfish are ambush predators,
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157. taking their time
and watching for the right moment.
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158. But there isn't room here
for two predators.
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159. The grouper, braving the lionfish's
poisonous spines,
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160. tries to evict its rival.
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161. But lionfish are persistent.
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162. This grouper spent many hours
simply defending his hunting patch.
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163. Some animals prefer to avoid
conflict whenever possible.
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164. These harlequin shrimp,
having captured a starfish,
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165. are taking it back to a safe house
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166. beyond the reach
of competitors and danger.
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167. The problem is that starfish
have minds of their own
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168. and five large sticky arms.
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169. By the time the shrimps
have prized off one arm,
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170. another has reattached itself.
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171. Only by maneuvering
the starfish onto its back
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172. can they have any hope
of gaining the advantage.
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173. Even so, getting it back home
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174. is a major undertaking.
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175. The starfish
is now a living larder.
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176. if the shrimps can hang onto it,
it will feed them for days to come.
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177. The top of the reef
is usually covered
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178. by a thin layer of green algae,
another living larder.
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179. And many fish depend on it.
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180. Powder blue tangs
defend their right
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181. to graze on a particular patch.
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182. But, for a larder as well stocked
as this, there is always competition.
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183. When a shoal of convict tangs
decide to graze,
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184. little can stop them.
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185. The powder blue tangs
try to keep them off..
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186. but they are overwhelmed
by sheer numbers.
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187. The territory is stripped of algae
in minutes.
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188. The blue tangs appear
to be fighting a losing battle.
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189. But, eventually,
they begin to get the upper hand.
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190. They persist with their attacks
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191. until the marauders
are well on their way.
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192. When night falls, some very strange
creatures creep out of crevices
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193. and crawl over the reef.
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194. This moving bush is an animal -
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195. a basket star -
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196. which spreads out its arms
to catch the night's plankton.
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197. The reef becomes
uncannily tranquil.
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198. fish retire, hiding themselves
where they can.
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199. Marbled rays come out to hunt
for prey buried in the sand...
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200. using electro-receptors
to scan the seabed.
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201. Their activity attracts sharks.
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202. White tips.
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203. At night, when vision is of little use,
sharks have a real advantage.
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204. They can still use both smell
and electro-reception to track fish.
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205. These sharks are also hunting
for fish concealed within the reef.
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206. Their slender shape
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207. There is nowhere to hide.
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208. Few animals are safe
during these feeding frenzies.
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209. Night after night, the reef animals
are subjected to these raids.
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210. But life on the reef
is not just about food,
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211. it's also about sex.
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212. There are many different
breeding strategies,
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213. but each is aimed at maximizing
the number of young that will survive.
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214. Every afternoon for two months,
brown surgeonfish can be seen
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215. streaming across reefs
in the Red Sea.
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216. They all head for the same place,
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217. usually some prominent feature.
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218. Here, they wait
for the light to fade.
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219. Suddenly, females within the group
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220. make a dash away from the reef
to release their eggs.
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221. They're followed
by the quickest and closest males,
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222. all of whom are striving
to fertilize the eggs.
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223. inevitably, others come here
to feast on such easy food.
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224. As the surgeonfish spawn,
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225. fusiliers move in above
to eat the nutritious eggs.
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226. These are just the first
of many predators which will feed
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227. on the eggs and developing larvae
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228. as they drift
during the next few weeks.
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229. Other fish are less casual
about their eggs.
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230. Banded pipefish stay close
to a small chosen area on the reef.
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231. Every morning at sunrise,
the female leaves her sleeping site
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232. and swims to find her partner.
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233. For ten minutes or so,
they remain together,
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234. reaffirming the bond that is
essential to their partnership.
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235. They swim together
around his territory
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236. in a simple greeting dance.
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237. Throughout the summer,
when the female's eggs are ripe,
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238. courtship begins in earnest
in the early morning.
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239. It takes time. After about two hours,
they rise off the seabed,
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240. entwining their two bodies.
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241. The male rubs himself
against the female,
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242. stimulating her
to release her eggs.
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243. And now, swiftly,
the male takes them.
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244. The eggs, now stuck to his belly,
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245. are patted down
to ensure that they stay there.
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246. The female then leaves him,
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247. but, every morning, returns for
a session of synchronized swimming
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248. and so ensures
that their bond is maintained.
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249. Ten days later, under the cover of
darkness, the male shakes his body
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250. and the young pipefish are born.
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251. Only now are they
independent of their parents.
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252. Since the male takes charge
of the eggs as soon as they are laid,
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253. the female can start
producing the next batch.
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254. Without his help, the pair could
only breed every 20 days,
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255. rather than every ten.
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256. By sharing the work, they double
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257. the number of young
they can produce in any one year.
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258. A flamboyant cuttlefish.
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259. Unlike most cuttlefish, this one
spends much of its time walking,
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260. rather than jetting
across the seabed.
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261. This is a male.
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262. He is using his colorful display
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263. to try and seduce the larger female,
who seems unimpressed.
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264. Eventually, she concedes.
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265. The final event, the transfer
of sperm, is very quick.
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266. A singing male humpback whale.
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267. Humpbacks are only visitors
to the reef.
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268. After a pregnancy
that lasted a whole year,
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269. the females come here to give birth
and suckle their newly-born young.
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270. Their investment in their single
offspring is considerable,
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271. for each female
will continue to nurse it
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272. for a further six to 12 months.
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273. But the males are here to mate.
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274. The lone males sing to establish
their relative seniority.
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275. The louder and longer the song,
the bigger and stronger the singer.
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276. The better the song,
the larger the male,
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277. the more mating opportunities
he will get.
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278. All these different mating
strategies have the same aim -
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279. to ensure that the greatest
possible number of offspring
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280. will live long enough
to breed themselves.
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281. Corals also reproduce sexually,
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282. but being fixed to the seabed,
they can't move to find a mate.
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283. Somehow, they must synchronize
their sexual activity.
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284. They do so using the rising
water temperatures of spring
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285. and the phases of the moon.
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286. A few days after the full moon
in late spring,
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287. when tidal currents
are at their weakest,
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288. the corals of the Great Barrier Reef
are ready to spawn.
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289. Some corals are male
and release clouds of sperm.
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290. Nearby, a female
will be releasing eggs.
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291. Other species of coral
are both male and female.
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292. These release packages of eggs
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293. already pre-wrapped in sperm.
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294. Bundles of eggs and sperm
float to the surface
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295. to mix with others
from further along the reef.
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296. Each kind of coral
times its release
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297. to a certain hour
on a certain night.
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298. That maximizes the chances
for cross-fertilization.
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299. The fertilized eggs
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300. drift away from the reef.
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301. The stormy season brings real
danger to the animals of the reef.
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302. Lobsters in the Caribbean
sense a change in the water.
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303. The temperature drops and powerful
swells disturb the sand.
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304. Under the cover of darkness,
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305. they emerge to run before the storm
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306. and risk crossing
the exposed sand flats
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307. to seek shelter in deeper water.
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308. Every year they make this journey.
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309. From all over the reef,
lobsters come to join the march.
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310. They conserve energy by traveling
in one another's slipstream.
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311. And there is the added benefit
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312. of safety in numbers.
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313. By daybreak, they've reached
the edge of the deep reef
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314. and down they go.
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315. For the rest of the stormy season,
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316. they will remain in the shelter
of deep water,
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317. out of harm's way.
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318. Sometimes, during the stormy
season, a hurricane builds.
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319. Then the very structure
of the reef itself is under threat.
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320. An entire reef can be destroyed
by just one big storm.
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321. Hundreds of years of growth
gone in a few hours.
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322. Out in the ocean,
new life continues to develop.
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323. in time, coral larvae will return
to colonize the rubble
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324. and a new reef will grow
on the wasteland.
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