Ocean Life
FROM THE SEA SHORE to the deepest depths, oceans are home to some of the most diverse life on Earth. Animals live either on the seabed or in midwater where they swim or float. Plants are only found in the sunlit zone where there is enough light for them to grow either anchored to the bottom or drifting in the water. Animals are found at all depths of the oceans, but are most abundant in the sunlit zone where food is plentiful. Not all free-swimming animals stay in one zone-the sperm whale dives to over 1,650 ft (500 m) to feed on squid, returning to the surface to breathe air. Some animals from cold, deep waters, such as the Greenland shark in the Atlantic, are also found in the cold, surface waters of polar regions. Over 90 percent of all species dwell on the bottom. One rock can be home to at least 10 major types, such as corals, mollusks, and sponges. Most ocean animals and plants have their origins in the sea, but some like whales and sea grasses are descended from ancestors that once lived on land.
TIME AND TIDE
Anyone spending time by the seaside or in an estuary will notice the tides. Tides are caused by the gravitational pull of the Moon on the Earth's mass of seawater. An equal and opposite bulge of water occurs on the side of the Earth away from the Moon, due to centrifugal force. As the Earth spins on its axis, the bulges (high tides) usually occur twice a day in anyone place. The highest and lowest tides occur when the Moon and Sun are in line causing the greatest gravitational pull. These are the spring tides at new and full Moon.
SQUISHY SQUID
Squid are among the most common animals living in the ocean. Like fish, they often swim around in shoals for protection in numbers. Their torpedo-shaped bodies are streamlined so they can swim fast.
SHORE LIFE
Often found on the shore at low tide, starfish also live in deeper water. Sea life on the shore must either be tough enough to withstand drying Ollt, or shelter in rock pools. The toughest animals and plants live high on the shore, but the least able to cope in air are found at the bottom.
THE OCEAN'S ZONES
The ocean is divided up into broad zones, according to how far down sunlight penetrates, and water temperature and pressure. In the sunlit zone, there is plenty of light, much water movement, and seasonal changes in temperature. Beneath this is the twilight zone, the maximum depth where light penetrates. Temperatures here decrease rapidly with depth to about 410P (SoC). Deeper yet is the dark zone, where there is no light and temperatures drop to about 34-36°P (I-2°C). Still in darkness and even deeper is the abyss and then the trenches. There are also zones on the seabed. The shallowest zone lies on the continental shelf. Below this are the continental slope, the abyssal plains, and the seafloor trenches.
GIANT AMONG SEA FIRS
Standing about 3 ft (1M) tall above the seabed, this giant sea fir was first discovered in the 1875 voyage of HMS Challenger when a specimen was trawled up from the ocean floor off the Japanese coast. In 1985, the first observations on living specimens were made from the Japanese submersible, Shinkai 2000. The sea fir catches food drifting by in its long tentacles and can even tackle tiny fish, up to 1 in (2 em) long. Specimens have been found in the Pacific Ocean at 3epths from 165 to 17,500 ft (50 to ),300 m), as well as in the Atlantic Ocean. Unlike other sea firs (pp. 20-21), the giant sea fir is a solitary individual, not a branching colony.
MAGNIFICENT WEED
Growing up from the bottom, the giant kelp has a central, stemlike stalk, covered with leaflike blades. At its base, each blade has a gas- filled air bladder, which keeps the kelp afloat. Bv spreading out its blades, the kelp absorbs the maximum amount of sunshine for making toad by photosynthesis. Giant kelps are among the fastest growing plants in the world, growing over 1 ft (0.3 m) a Off North America's Pacific coast, kelp forests provide a home for such animals as sea otters and sea urchins. They are also harvested for jellylike alginate, used to make ice cream and other products.
JEEP-SEA SHARK
Most people think of sharks as dangerous predators, but cat sharks are quite harmless. This one is from the deep Pacific Ocean. Sharks living in deep water do not have problems with buoyancy, because unlike some bony fishes they do not have gas- filled air bladders. Instead, all sharks have oil-rich livers, which help reduce their weight in water.
ICY OCEAN
There are two main types of sea ice-pack ice that forms on the surface of the open sea (as here in Canada's Hudson Bav) and fast ice that forms between the land and the pack ice. Because of its salt content, seawater freezes at lower temperatures tAan freshwater. Since cold water sinks and is replaced by warmer water, it takes a long time before the water is cold enough for ice to form. Icebergs are huge chunks broken off the polar ice sheets and glaciers, formed from freshwater on land.