You may imagine the upper layers of the sea as a kind of water meadow. Instead of grasses, there is phytoplankton - millions of tiny drifting plants. Most live in the top 100 meters (330 ft) where sunlight is strong enough for them to grow. Diatoms are the most abundant kind. Seen through an electron microscope they reveal amazing shapes. Some resemble stretchers, some others pencils and still some others like wheels.
Feeding on such tiny plants and on each other - are the tiny drifting creatures comprising zooplankton. Some of these are no bigger than a grain of rice. Some are the tiny young of barnacles, crabs, winkles, worms and starfish. Fish eggs and fry also form part of the zooplankton.
Phytoplankton and zooplankton together make the upper sea a nourishing 'soup' on which small fishes and other creatures feed. Fishes like mackerel and herring are among many plankton eaters. The surgeon fish browse on plants. The trigger fish munches mollusks and crustaceans. The blue demoiselle fish and clown anemone fish snap up various small sea animals. Again these fishes are the fodder of other big fishes and creatures. Predators, from the surface as also from the water, target these fishes for a feast. While seabirds swoop to snatch up herrings, pilchards, anchovies and sprats. Tuna and other bony fishes also prey on small creatures of their kind. Again bony fishes are hunted by sharks and whales and the larger squids. This is the food chain for marine animals. all along the feeds and fodders are all well arranged.

 The Deep:

The ocean depths have their own animals. The floors of shallow seas off northern continents are feeding grounds for familiar fishes, such as haddock, cod and plaice. Below warmer waters, huge moray eels lurk in rocky crevices. 

 True, deep-sea fishes seldom reach the fishmonger's slab. These tend to be bizarrely designed in ways that help them cope with life in everlasting darkness. Snipe fishes and most other deepwater fishes are only centimeters long, but they have enormous jaws and stomachs to have most of those few large meals that chance to come their way. Many produce their own light. Among these are the lantern fish and hatchet fish. Both kinds fall victim to the angler fish which sprouts a rod-like fin with an illuminated lure.


Corals play a great role in beautifying the sea world by adding a variety of color underwater. It is on the rock hard skeletons of the corals that coral reefs are built. By providing food and shelter these reefs have been one of the most significant habitats for a large variety of invertebrates and fishes. While the skeletons act as the bricks, it is the coralline algae and the limey shells of other marine invertebrates that act as the cement. Many of these corals look like stags' horns, fans or human brains. In tune with the colorful coral land many fishes of the reef display their brilliant colors as birds do to warn the rivals off their chosen territory. Fishes like the colorful triggerfish, butterfly fish, parrot fish feed on coral polyps. Certain butterfly fishes pluck polyps from their cups.
Hermit crab, a crustacean, is a slow moving creature under its whelk shell home and hence prone to predators' attack. But Sea anemones which grow its shells seek to paralyze small creatures with poisoned 'darts' fired from their tentacles. Immune to their attack, clown fish actually hide among the tentacles of certain sea anemones. The anemones give protection to the hermit crabs and in turn are carried from one feeding ground to another.

The porcupine fish of the warm seas has spines that usually lie flat. If something scares the fish, it gulps water to distend its body. This makes the spines jut out. This mechanism saves this slow swimmers from predators.

Other seabed creatures include giant clams, sea cucumbers, sea horses and starfish - including one that preys on coral polyps. Octopus is yet another mollusk with eight legs and looks like a different kind of a big spider.  All the eight limbs, called tentacles, come out of a lump of jelly-like flesh with the eyes and mouth.  It preys fishes and crustaceans with its tentacles fitted with millions of suckers. The suckers have a tube-like opening at the surface of the legs. Octopuses prefer to lurk at the seabed behind the plants, or among the boulder gaps. If themselves feel threatened, they flee at a jetset speed by darting out a strange liquid that makes the water around murky. 

Unlike most fishes, sharks and rays have skeletons made of cartilage instead of bone. The roughly 200 species of sharks range in size from the meter long dogfish to the whale shark which can stretch up to 20 meters and weighs as much as seven elephants. Strangely this is a harmless monster, browsing on plankton and small fishes which it tilters from the water with special combs on its gills. 
Most other sharks are superbly streamlined hunters relying mainly on smell to find their prey. The killer ones are the hammerheads, and the white and tiger sharks with their razor-sharp teeth.

Rays have a flattened body. The huge manta-ray with a 7 meter(32 ft) span eats planktonic fishes and crustaceans and most rays live on seabed.

Giants of the Sea:
Besides all the above variety there are some giants roaming in the vast ocean world. 
These giants include  whales, squids, whale sharks, dolphins, and certain deep sea variety of fish.

However, of all these, whales are the largest ever animal on planet earth . These are giant sea mammals that hunt largely among the upper levels of the oceans. Like any land mammal, a whale must surface to breathe air. 

Otherwise whales are superbly built for life in the sea. In many ways they look like fish. They have a streamlined, neck less body. Their limbs are on the other side of the are shaped as fin-like flippers. They even have a fish-like tail, but flattened horizontally, unlike the fishes. Beneath the skin is blubber, a thick layer of fat. This helps whales to stop losing body heat even a long stay in cold water. The biggest animals on Earth are blue whales. These huge creatures eat chiefly small, shrimp-like animals known as krill. They sieve these from the sea by means of fringed whalebone strips which hang down from their open jaws. 
Whales that feed like this are whalebone whales. The toothed whales include dolphins, the not-so-large white whales, the killer whales and sperm whales. These hunt larger prey such as fishes, squids and seals. Scars found on sperm whales reveal their underwater battles with giant squids. Killer whales hunt in packs. With a battery of big, oval teeth it has a reputation as the fiercest hunter of the ocean.

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