My Science School

PLANKTON

The sunlit surface waters of many oceans teem with life, most of it microscopic that drifts with the currents. The whole drifting community is called the plankton. It is made up of plant-like phytoplankton, which uses the energy of sunlight to make food from carbon dioxide and water, and zooplankton — animals that feed on both the phytoplankton and each other.

• DIATOMS The phytoplankton consists of microscopic organisms such as diatoms and cyanobacteria. Diatoms have shells of glassy silica that fit together like tiny boxes with lids, and they exist in a dazzling variety of forms. They thrive in cool seas, where they turn the water grey-green and often multiply into vast cloudy “blooms” that are visible from space.


• CRAB LARVA Among the members of the zooplankton are the eggs and young of animals that have very different shapes and lives when adult. They include the eggs of reef corals and infant fish, molluscs, and crustaceans like this crab larva. Drifting in the plankton provides them with food and helps them disperse through the oceans to find new places to live.


• CYANOBACTERIA Once known as “blue-green algae”, these simple organisms were among the first forms of life to appear on Earth, more than 3.5 billion years ago. They still flourish in the oceans where, like diatoms, they turn carbon dioxide and water into sugary carbohydrates.


• COPEPODS Many animals spend their entire lives as members of the zooplankton. They include the tiny shrimp-like copepods, which form dense swarms in many seas, providing food for shoals of fish and giant filter-feeding whales.


• ARROW WORMS These long, almost transparent animals prey on the other creatures of the zooplankton, including copepods. They are named for the way that they shoot forward through the water to catch their victims.

Salmon have one of the most amazing homing instincts in the natural world. The fish are born in rivers and streams often far inland where they live for their first two years. Then they swim down-river and out into the sea, which becomes their home for the next two to four years.

After growing plump and strong from the rich feeding in the sea, the salmon turn round and head back to the freshwater spawning ground where they were born. What is incredible is that every fish tries to find its way back to the very stream where it started life as an egg. Only those that are caught by fishermen or die in some other way on the journey fail to make it. The rest find their way upstream to the very tributaries where they were hatched. Here the females lay their eggs. The males fertilize them, and the eggs are buried in the gravel until they hatch and start a life of their own.

Experiments have shown that salmon manage to pinpoint the place they were born and find their way to it thanks to a highly developed sense of smell (or taste). Salmon that have had their nostrils blocked with cotton wool have been lost. If this is true, it explains how they find their home once they are in the river. But not how they navigate in the open sea. Perhaps they get their bearings from the sun and the stars. Maybe changes in water temperature help guide them. We still do not know for certain how they do it. Not that that worries the salmon. They just carry on coming home as their ancestors have been doing for millions of years.

 

Picture Credit : Google

When the wind blows across an open area of water it causes waves. You can see this on the edge of lakes and ponds as well as the sea-shore, calm weather when winds are light, the waves are small. As winds strengthen, the waves grow larger. In stormy weather huge waves build up over hundreds of miles of ocean and eventually crash against the shore with terrifying force. But no matter how strong a wind is, the water itself does not move - the waves pass through it. In 1933 an American ship crossing the Pacific Ocean during a hurricane recorded a wave that measured thirty-four metres from its crest at the top to the trough at the bottom.

Huge as this is, earthquakes have produced even larger waves. These are called tsunamis, the Japanese for harbour waves'. Tsunamis are low waves that travel very fast - up to 790 kilometres per hour - across the ocean until they approach the shallow waters near land. There they grow much higher. In 1771 a tsunami estimated to be eighty-five metres high appeared off the Ryukyu Islands of southern Japan. The force of the wave was so great that it threw a boulder weighing 750 tonnes more than two and a half kilometres!

 

Picture Credit : Google

Millions of years ago when the earth was still young, the land that now forms the Himalayas was an ocean floor. A lot has changed since then and today the Himalayas are the world's highest mountains. They rise to a peak at the summit of Mount Everest - not far short of nine kilometres above sea level. So what turned an ocean floor into a mountain range? The answer is a huge build-up of pressure.

The earth's surface is formed from gigantic rafts of land, known as tectonic plates. Sixty million years ago the plate on which India lies started moving. It moved northwards towards the plate that the rest of central Asia sits on. In between India and Asia at that time was the sea. And when the two plates crashed together the ocean bed under the sea was crushed. As the Indian plate kept pushing north, the sea floor buckled and folded. One layer of rock was squeezed up and piled on top of another. After sixty million years the effect is dramatic. The sea has long since disappeared and in its place stands a mighty mountain range separating India from the rest of central Asia. The process is still going on and experts think the Himalayas are being pushed slowly upwards, perhaps growing as much as five centimetres a year.

And what about the sea shells? Buried in some of the rock high up in the mountains, climbers have found the remains of shells that once lay on the floor of that ancient sea.

 

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Even though whales look like fishes, they are related to you and me. We are all mammals, as are porpoises and dolphins. Whales are warm-blooded like us. They don’t have furry coats to keep warm like many land animals. Instead whales have a thick layer of oily blubber under their skin. Whales have hearts like yours and mine too – double hearts with four chambers.

The thing that really distinguishers whales from fish is the way they breathe. Fish get the oxygen they need from passing water through their gills. Whales, on the other hand, have lungs similar to ours. They can hold their breath for far longer than we can. But when they need to breathe again they have to come to the surface, they spout out a jet of mist which looks like a fountain or geyser. As it leaves their nostrils, the breath is a warm vapour. But as soon as it is released into the colder air, it condenses to form the mist we can see. Once the sued air is expelled, the whale can take another breath and dive again.

 

Picture Credit : Google

Deep down in the oceans it is pitch-black. You cannot see a thing without artificial light, because sunlight cannot penetrate further than 460 metres below the surface. This could be a serious problem for fish living so deep, if they did not have lights of their own.

Some, like the angler fish, use light to catch their prey. The rat-tail fish sends out a cloud of light to dazzle an enemy while it makes a speedy getaway. Then there are those who use light just to keep in touch with friends and find a mate. They have ‘recognition lights’, rather like ships.

The light produced in these fish is like no other light on earth. For one thing, it’s cold. Almost every other sort of light we know of produces heat. Being cold makes it very energy efficient. Some fish produce their light by mixing chemicals with a little oxygen from their blood. These meet in glands in the fish’s skin. The chemicals form a combustion and light is produce in the gland, which turns into a lamp.

In other oceanic fish bacteria produce the light. The bacteria live and feed on tissue in the fish’s skin, which the fish is constantly replacing. However, the bacteria help the fish by producing substances that glow with bright light. So while the fish helps them, they help the fish.

 

Picture Credit : Google