Marine Biology
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50 free Marine Biology trivia questions with answers — science & nature quiz, new questions added Oct 2026.
Marine biology is the study of life in salt water, from drifting cells too small to see to the largest animals that have ever lived. The sea covers most of the planet, and much of what lives in it still surprises the people who study it: fish that change sex, animals that glow in the dark, and creatures that have barely changed in hundreds of millions of years. This quiz covers what a first course in the subject would teach. Expect questions on how sea animals are classified and named, how reefs, coastal forests and the deep ocean work, the tricks animals use to hunt and hide, and the divers, writers and expeditions that opened up the underwater world. It is written for students revising for a class, quiz bowl and Science Olympiad teams, would-be marine biologists, and anyone who has stared into a rock pool. Every answer comes with a short explanation, so a wrong guess still teaches something.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Which metal, carried in the protein hemocyanin, turns the blood of squid and lobsters blue instead of red?
Copper
Copper does the job that iron does in human blood, binding oxygen and turning blue as it does so. The same pigment runs through snails, spiders and scorpions.
Q 02Which microscopic drifting organisms make about half of Earth's oxygen by photosynthesis in the sunlit sea?
Phytoplankton
Phytoplankton manage it while making up only about 1% of the planet's plant biomass. They also sit at the base of nearly every food chain in the sea.
Q 03In which fish does the male carry the eggs in a brood pouch until the young are born?
Seahorse
Seahorse fathers receive up to 1,500 eggs from the female and carry them for 9 to 45 days. The young emerge fully formed but very small.
Q 04Which marine mammal has no blubber and relies instead on the densest fur of any animal to stay warm?
Sea otter
Sea otter fur packs up to 150,000 hairs into each square centimetre. The animal grooms constantly, because the coat only keeps water out while it stays clean.
Q 05Which bright reef fish rasps algae off coral rock and excretes the remains as sand?
Parrotfish
Parrotfish digest the edible part and pass the rest as sand, which helps to build beaches and small islands.
Q 06Which large land animals are the closest living relatives of manatees and dugongs, despite the nickname sea cows?
Elephants
Elephants share the kinship with the small, furry hyraxes. Manatees even replace their molars the same way, with new teeth moving forward from the back of the jaw like a conveyor belt.
Q 07Which Arctic whale grows a spiral tusk, up to three metres long, that is really an overgrown canine tooth?
Narwhal
Narwhal males grow the tusk from the left side of the upper jaw. The scientific name, Monodon monoceros, is Greek for single-tooth, single-horn.
Q 08Which striped reef fish shelters among sea anemone tentacles and has males that turn female when the top female dies?
Clownfish
Clownfish live in a strict pecking order ruled by the largest fish, the breeding female. In return for shelter they clean and fan the anemone and chase off fish that would eat it.
Q 09Which sluggish Arctic fish is the longest-lived vertebrate known, with a lifespan estimated at several centuries?
Greenland shark
Greenland shark females may not breed until they are about 150 years old. The cold-water giant also has the slowest swimming speed for its size of any fish.
Q 10Which tide-pool predator pries open clams, then pushes its own stomach out of its body to digest them in the shell?
Starfish
Starfish need a gap of only a fraction of a millimetre between the two shells to slip the stomach in. Most can also regrow lost arms.
Q 11What is the term for light made by living organisms, such as deep-sea lanternfish and sparkling dinoflagellates?
Bioluminescence
Bioluminescence is far commoner in the deep than near shore: about 76% of the main groups of deep-sea animals in one Pacific survey could make light, against some 2.5% of coastal organisms.
Q 12What is the name for the way dolphins find prey by sending out clicks and reading the sound that bounces back?
Echolocation
Echolocation is the main sense of toothed whales, which can survive by it even when completely blind. The clicks pass out through a fatty organ in the forehead called the melon.
Q 13What flexible tissue, rather than bone, makes up the skeletons of sharks, rays and skates?
Cartilage
Q 21Corals, sea anemones and box jellies all belong to which phylum, named after the Greek word for nettle?
Cnidaria
Cnidaria arm their tentacles with cnidocytes, stinging cells that fire a coiled thread to envenom and capture prey. The phylum holds more than 11,000 species.
Q 22Sea urchins, sand dollars and brittle stars belong to which phylum, named from the Greek for hedgehog skin?
Echinodermata
Echinodermata are built on a five-part radial plan as adults, yet their free-swimming larvae are two-sided like most other animals.
Q 23Which class of mollusks, whose name is Greek for head-feet, includes squid, cuttlefish and the nautilus?
Cephalopoda
Cartilage gives the whole class its name: Chondrichthyes comes from the Greek words for cartilage and fish, in contrast to the bony fish, Osteichthyes.
Q 14Coral reefs are built by colonies of which tiny sac-like animals that secrete hard skeletons of calcium carbonate?
Polyps
Polyps are usually only a few millimetres across, and every one in a colony is genetically identical. Scholars argued for millennia over whether corals were plants or animals.
Q 15Which gas, absorbed from the air by seawater, is making the oceans more acidic and threatening shell-building animals?
Carbon dioxide
Carbon dioxide from human activity lowered the average pH of the sea surface from about 8.15 to 8.05 between 1950 and 2020. Corals and mollusks suffer because they build with calcium carbonate.
Q 16In sea turtles, which condition of the sand around a buried nest decides whether hatchlings become male or female?
Temperature
Temperature sets the sex partway through incubation: cooler nests hatch mostly males and hotter nests mostly females, so warming beaches may tip whole populations female.
Q 17Which ocean giant, the largest animal known to have ever existed, feeds almost entirely on tiny krill?
Blue whale
Blue whale hearts are the largest of any animal: one from a stranded North Atlantic individual weighed 180 kg. The animal itself can reach about 30 metres and 190 tonnes.
Q 18What comb-like plates of keratin do humpback and right whales use to strain small prey from mouthfuls of seawater?
Baleen
Baleen is made of the same substance as human fingernails and hair. The whale gulps water, then pushes it out through the plates, which hold the food back.
Q 19Which brown seaweed forms towering underwater forests and can grow about half a metre in a single day?
Giant kelp
Giant kelp can grow more than 45 metres long, anchored to the seabed by a root-like holdfast and lifted toward the light by gas-filled bladders at the base of its blades.
Q 20Which salt-tolerant trees fringe tropical coasts, standing above the tide on tangled, stilt-like prop roots?
Mangroves
Mangroves filter out salt and take in air through pores in their bark, and some drop ready-grown seedlings that can float for up to a year before taking root.
Cephalopoda are sometimes called inkfish by fishers, after their shared trick of squirting ink. The class includes the colossal squid, the heaviest living invertebrate.
Q 24Which simple animals, classed as phylum Porifera, pump water through bodies full of pores and have no nervous system?
Sponges
Sponges with soft, fibrous skeletons were used for thousands of years as padding and cleaning tools, which is where the bath sponge gets its name.
Q 25In the great white shark's scientific name, Carcharodon carcharias, which taxonomic rank does the first word give?
Genus
Genus comes first, and the second word marks the species. The great white is the sole living species of Carcharodon; Linnaeus had first filed it under Squalus with every other shark.
Q 26Seals, sea lions and the walrus make up which group of marine mammals, named from the Latin for fin and foot?
Pinnipeds
Pinnipeds include the walrus, the only surviving member of its family, alongside the eared seals and the earless true seals.
Q 27Which land mammal is the closest living relative of whales and dolphins, a kinship first shown by blood proteins?
Hippopotamus
Hippopotamus ancestors and early whales shared a semiaquatic forebear and parted ways about 55 million years ago, which is why whales now sit among the even-toed hoofed mammals.
Q 28Which apex predator, a toothed whale, is the largest member of the oceanic dolphin family?
Orca
Orca groups pass their own calls and hunting techniques down the generations, behaviour that scientists regard as a form of animal culture.
Q 29Which helmet-shaped beach animal, little changed in 250 million years, is closer kin to spiders than to any crustacean?
Horseshoe crab
Horseshoe crab blood contains a substance that reacts to bacterial toxins, so the animals are caught and bled to make a test for contamination.
Q 30Which animals, cemented head-down to rocks and ship hulls, did Charles Darwin study for eight years?
Barnacles
Barnacles so filled Darwin's days at home that his young son, visiting a friend, asked where that boy's father did his. Darwin published the results as a series of monographs.