Oceans and Sea Life

Salt in the sea comes mostly from minerals washed off rocks by rain and carried down rivers.

Tides rise and fall mainly because the Moon's gravity pulls on Earth's oceans as the planet rotates.

Waves are moving energy rather than moving water, since the water itself mostly circles in place.

Coral reefs are built by tiny animals that leave behind hard skeletons of calcium carbonate.

Coral turns pale when the colourful algae living inside its tissues are expelled during warm spells.

Whales are mammals, so they breathe air at the surface and nurse their young with milk.

Blue whales are the largest animals known to have lived, bigger than any dinosaur yet found.

Humpback whales sing long, repeating songs that can travel for many kilometres through deep water.

Dolphins use clicking sounds and echoes to locate fish and navigate in murky water.

Sharks have skeletons made of cartilage, the same flexible material found in human ears and noses.

Shark skin feels like sandpaper because it is covered in tiny tooth-like scales called denticles.

Jellyfish have no brain, heart, or bones, and drift by pulsing their bell-shaped bodies.

Jellyfish stings come from stinging cells that fire tiny harpoons when something brushes past.

Octopuses have three hearts and blue blood, which helps them carry oxygen in cold water.

Squid can change colour in an instant by stretching pigment cells in their skin.

The deep sea is permanently dark below the reach of sunlight, so many animals there make their own light.

Bioluminescence lets many sea creatures glow, using light to attract prey or confuse predators.

Anglerfish dangle a glowing lure above their heads to draw curious prey within reach.

Seashells are made by molluscs, which build them layer by layer from minerals in seawater.

Hermit crabs borrow empty shells for protection and swap them as they grow larger.

Sea otters use stones as tools to crack open shellfish resting on their chests.

Seagrass meadows grow on the seabed and provide shelter for young fish and small crustaceans.

Kelp forests rise from the seafloor in cold coastal water and sway with the moving tide.

Plankton drift with the currents and form the base of nearly every ocean food chain.

Phytoplankton are microscopic drifters that photosynthesise and supply a large share of the world's oxygen.

Flying fish glide above the waves on wide fins to escape predators chasing from below.

Sardines swim in enormous schools that twist and turn together as if one animal.

Tuna swim constantly and never fully stop, since they need water flowing over their gills.

Salmon return from the sea to the rivers where they hatched in order to spawn.

Eels migrate thousands of kilometres across the ocean to reach their spawning grounds.

Seahorses are fish that swim upright, and the male carries the eggs until they hatch.

Clownfish live among stinging anemones, protected by a mucus layer that keeps them safe.

Parrotfish graze on coral and produce fine white sand as a by-product of their feeding.

Manta rays glide through open water with wide wings, filtering tiny prey as they swim.

Stingrays bury themselves in sand on the seabed, leaving only eyes and tail exposed.

Orcas hunt in coordinated groups and pass hunting techniques down through family lines.

Baleen whales filter seawater through bristly plates, trapping krill and small fish inside their mouths.

Krill swarm in vast numbers in cold polar seas and feed many larger animals.

Penguins are flightless birds that use their wings to swim rather than to fly.

Albatrosses glide for hours over the ocean, using wind rising off the waves to stay aloft.

Turtles return to the beaches where they hatched to lay their own eggs years later.

Deep ocean trenches plunge far deeper than the height of the tallest mountains on land.

Hydrothermal vents on the seabed spew superheated water, supporting strange communities without any sunlight.

Sound travels much faster in water than in air, which is why whale calls carry so far.

Tsunamis are caused by sudden shifts in the seafloor, often after a strong underwater earthquake.

Gulf Stream currents carry warm water across the Atlantic and moderate the climate of nearby coasts.

Underwater currents circle the globe, moving heat from the equator toward the poles.

Sailors once measured depth by lowering a weighted rope, a practice known as sounding the sea.

Shipwrecks on the seabed slowly become artificial reefs covered in coral, sponges, and barnacles.

Barnacles glue themselves headfirst to rocks, ships, and whales, and feed by filtering passing water.

The Dead Sea is so salty that swimmers float easily on its surface.

Salt water freezes at a lower temperature than fresh water, so polar seas stay liquid longer.