Quiz of the Day
Ocean Life
Tuesday, 14 April 2026
National Dolphin Day — marine biology, ocean facts, sea creatures
1. What percentage of the Earth's surface is covered by oceans?
- 81%
- 51%
- 61%
- 71%
Despite covering 71% of the planet, more than 80% of the ocean remains unmapped and unexplored — we know more about the surface of Mars.
2. What is the largest animal ever to have lived on Earth?
- Blue whale
- Megalodon
- African elephant
- Argentinosaurus
Blue whales can reach 30 metres long and weigh 200 tonnes. Their hearts are the size of a small car and can be heard beating from over 3 km away.
3. How do dolphins sleep without drowning?
- They float on the surface
- They sleep on the ocean floor
- They hold their breath for hours
- They sleep with half their brain at a time
Dolphins practice 'unihemispheric sleep' — one half of the brain rests while the other stays alert to control breathing and watch for predators. They keep one eye open too.
4. What is the deepest point in the ocean?
- Tonga Trench
- Java Trench
- Puerto Rico Trench
- Mariana Trench
The Challenger Deep in the Mariana Trench reaches about 10,935 metres deep. If you placed Mount Everest at the bottom, there would still be over 2 km of water above its peak.
5. Which sea creature has three hearts?
- Sea horse
- Jellyfish
- Octopus
- Starfish
Two of an octopus's hearts pump blood to the gills, while the third pumps it to the rest of the body. Their blood is blue because it uses copper-based haemocyanin instead of iron.
6. What is a group of dolphins called?
- A pod
- A herd
- A school
- A pack
Dolphin pods can range from a handful to over 1,000 individuals. Dolphins in a pod have been observed giving each other unique 'signature whistles' — essentially names.
7. Which ocean is the largest?
- Atlantic
- Indian
- Arctic
- Pacific
The Pacific Ocean covers about 165 million square kilometres — it's larger than all the land on Earth combined and contains more than half of the world's free water.
8. What gives coral reefs their vibrant colours?
- Symbiotic algae called zooxanthellae
- Reflected sunlight
- Bioluminescent bacteria
- Minerals in the water
When corals are stressed by warm water, they expel these algae and turn white — a process called 'coral bleaching'. Without the algae, corals eventually starve and die.
9. How long can a sea turtle hold its breath underwater?
- 1 hour
- 24 hours
- 4-7 hours
- 15 minutes
While sleeping, sea turtles can hold their breath for up to 7 hours by slowing their heart rate to conserve oxygen — sometimes to just one beat every 9 minutes.
10. What is the fastest fish in the ocean?
- Sailfish
- Bluefin tuna
- Great white shark
- Swordfish
Sailfish can reach speeds of 110 km/h (68 mph), making them the Usain Bolt of the ocean. They use their distinctive sail-like dorsal fin to herd schools of fish.
11. How many species of shark are there?
- About 50
- About 150
- About 500
- About 1,000
There are over 500 known shark species, ranging from the 20 cm dwarf lanternshark to the 12-metre whale shark. Only about a dozen species are considered potentially dangerous to humans.
12. What ocean phenomenon is El Niño associated with?
- Deep ocean volcanic eruptions
- Warming of Pacific surface waters
- Tidal waves
- Arctic ice melting
El Niño events occur every 2-7 years and can raise Pacific surface temperatures by up to 3°C, affecting weather patterns worldwide — from droughts in Australia to floods in South America.
13. Which marine creature can regenerate lost arms?
- Dolphin
- Starfish
- Seahorse
- Clownfish
Some starfish species can regrow an entire body from a single severed arm, as long as part of the central disc is attached. The process can take up to a year.
14. What is the Great Barrier Reef made of?
- Living coral organisms
- Rock formations
- Volcanic lava
- Compressed sand
The Great Barrier Reef is the largest living structure on Earth, stretching over 2,300 km along Australia's coast. It's so big it can be seen from space.
15. Which deep-sea creature produces its own light through bioluminescence?
- Anglerfish
- Whale
- Dolphin
- Manta ray
About 76% of deep-sea creatures produce bioluminescence. The anglerfish uses a glowing lure dangling from its head to attract prey in the pitch-black depths.