Ocean Life Trivia: The Strangest Things in the Deep

We have mapped the surface of Mars in more detail than the floor of our own ocean. Most of the planet is water, most of that water is deep, and most of what lives down there we have barely met. That makes the ocean one of the richest — and strangest — sources of quiz material on Earth.

The Record Holders

• The blue whale is the largest animal that has ever lived — up to about 30 metres long, bigger than any dinosaur we know of. A newborn calf can weigh more than two tonnes.
• Octopuses have three hearts and blue blood. The copper-based haemocyanin that carries their oxygen is what gives it the colour. They are also remarkably intelligent: octopuses can open jars and escape from tanks.
• Sharks predate trees. They have been around for more than 400 million years and have survived several mass extinctions.
• The Greenland shark may live for centuries. Scientists estimate that some individuals are well over 250 years old, making it the longest-lived vertebrate known.
• A starfish can regrow a lost arm; in some species, a single arm can regrow the whole starfish. And starfish are not fish at all — scientists prefer "sea stars".
• The Great Barrier Reef, off northeast Australia, is the largest coral reef system in the world. Coral may look like rock or plants, but it is made of tiny animals called polyps.

The Layers of the Ocean

Oceanographers divide the ocean into zones by depth, and each one is a different world:

• The sunlight zone (to about 200 metres) has enough light for plants and plankton to photosynthesise. Most familiar sea life lives here.
• The twilight zone (200 to 1,000 metres) is dim. Many animals here rise to the surface at night to feed and sink again by day — the largest daily migration on Earth.
• The midnight zone (below 1,000 metres) is completely dark and very cold.
• The abyss and the trenches reach down to almost 11 kilometres, with crushing pressure and temperatures just above freezing.

Light in the Dark

Below a few hundred metres, sunlight simply stops. What fills the gap is bioluminescence — animals producing their own light through chemistry. It is used to lure prey, to confuse predators and to find a mate, and it is not rare. In the deep ocean it is close to standard equipment.

The anglerfish, with its glowing lure hanging in front of its jaws, is the famous one. In some species, the tiny male bites onto the much larger female and fuses with her body for life. It is far from the only strange creature down there: the vampire squid, the gulper eel with its enormous mouth, and the blobfish — which only looks like a sad blob when it is pulled up from the deep. At home, under high pressure, it looks like a fairly ordinary fish.

Pressure and Cold

The deepest point of the ocean is the Challenger Deep in the Mariana Trench, in the western Pacific, nearly eleven kilometres down. The pressure there is more than a thousand times that at the surface. Only a handful of people have been there. The first were Jacques Piccard and Don Walsh in the bathyscaphe Trieste in 1960; film director James Cameron made the first solo dive in 2012.

Even there, there is life. In 1977, scientists exploring near the Galápagos Islands discovered hydrothermal vents — cracks in the sea floor gushing hot, mineral-rich water. Around them lived whole ecosystems of giant tube worms, clams and shrimp, powered not by sunlight but by chemicals. The discovery changed what biologists thought life required, and it shapes how scientists search for life on other worlds.

Why the Ocean Matters

• The ocean produces around half of the oxygen we breathe, mostly from microscopic phytoplankton.
• It covers about 71% of the planet and holds about 97% of Earth's water.
• It absorbs huge amounts of heat and carbon dioxide, helping to regulate the climate.
• Only a fraction of the sea floor has been mapped in high detail, though international projects are working to complete the map.

Roughly seventy percent of the planet is covered by water, and we have properly explored a very small fraction of it. The strangest facts are probably still down there. Until then, test what you know in the free Animals & Nature quiz.