Questions on Aquatic Organisms (Ecology)
Multiple-Choice Questions: Aquatic Organisms
1. What defines an aquatic organism?
A) Any organism that lives underground
B) An organism that feeds only on plants
C) An organism that lives in water for all or most of its life
D) An organism that breathes air through lungs
E) An organism that flies above oceans
2. Plankton are primarily:
A) Large aquatic reptiles
B) Floating or drifting organisms
C) Sea-bottom dwellers
D) Deep-sea fish
E) Freshwater amphibians
3. Phytoplankton are:
A) Animal-like protists
B) Microscopic aquatic plants
C) Floating worms
D) Deep-sea bacteria
E) Parasites in fish
4. Zooplankton are:
A) Small floating animals or protozoa
B) Rooted plants in lakes
C) Non-living particles
D) Airborne insects near water
E) Coral polyps only
5. Which aquatic organisms are bottom-dwelling?
A) Nekton
B) Plankton
C) Benthos
D) Pelagic
E) Algae
6. The term nekton refers to:
A) Floating microscopic plants
B) Sessile filter-feeders
C) Actively swimming aquatic animals
D) Photosynthetic bacteria
E) Decomposing fungi
7. Which of the following is an example of a nektonic organism?
A) Jellyfish
B) Octopus
C) Algae
D) Coral
E) Diatom
8. Benthos organisms are usually found in the:
A) Surface water
B) Pelagic zone
C) Open ocean
D) Bottom of aquatic environments
E) Atmosphere
9. Coral reefs are formed by:
A) Jellyfish colonies
B) Floating algae
C) The accumulation of calcium carbonate from coral polyps
D) Sediment from rivers
E) Plankton excretion
10. Which aquatic organisms perform most of the photosynthesis in aquatic ecosystems?
A) Jellyfish
B) Phytoplankton
C) Zooplankton
D) Mollusks
E) Crustaceans
11. Which of the following is a sessile aquatic organism?
A) Tuna
B) Squid
C) Sea sponge
D) Eel
E) Seahorse
12. Which group includes many filter feeders in the ocean?
A) Sharks
B) Crustaceans
C) Bivalves (like clams and mussels)
D) Echinoderms
E) Amphibians
13. Which aquatic organism can fix nitrogen and is a primary producer?
A) Blue-green algae (cyanobacteria)
B) Copepods
C) Sea cucumbers
D) Amphipods
E) Krill
14. Anadromous fish, such as salmon, are characterized by:
A) Living only in freshwater
B) Living only in saltwater
C) Migrating from saltwater to freshwater to spawn
D) Being blind and bottom-dwelling
E) Swimming only in estuaries
15. Euryhaline organisms are able to:
A) Survive without oxygen
B) Tolerate a wide range of temperatures
C) Tolerate a wide range of salinities
D) Live on both land and sea
E) Live only in frozen water
16. Which of the following is a marine mammal?
A) Tuna
B) Penguin
C) Sea otter
D) Jellyfish
E) Clam
17. A detritivore in an aquatic ecosystem feeds on:
A) Live fish
B) Sunlight
C) Dead organic matter
D) Plankton only
E) Coral polyps
18. Which of these is a major role of aquatic decomposers?
A) Producing light in the deep sea
B) Filtering water through gills
C) Breaking down dead organisms
D) Building coral reefs
E) Eating zooplankton
19. Marine invertebrates include all of the following EXCEPT:
A) Starfish
B) Squid
C) Sea anemones
D) Dolphins
E) Jellyfish
20. Photosynthesis in aquatic ecosystems is limited mainly by:
A) Salinity
B) Depth and light availability
C) Water current
D) Predator populations
E) Temperature
- Questions on Nitrogen Cycle
- Questions on Biomes and Climate
- Questions on Terrestrial and Aquatic Biomes
Answer Key with Full Explanations
1. C – An organism that lives in water for all or most of its life
Aquatic organisms are adapted to life in water environments.
2. B – Floating or drifting organisms
Plankton drift with water currents and are a crucial part of aquatic food chains.
3. B – Microscopic aquatic plants
Phytoplankton are photosynthetic and form the base of many food webs.
4. A – Small floating animals or protozoa
Zooplankton consume phytoplankton and are often microscopic animals.
5. C – Benthos
Benthos includes organisms like crabs, sea stars, and worms that live on the bottom.
6. C – Actively swimming aquatic animals
Nekton includes fish, whales, and other swimmers.
7. B – Octopus
Octopuses are active swimmers and part of the nekton.
8. D – Bottom of aquatic environments
Benthos live on or in the ocean/lake floor.
9. C – The accumulation of calcium carbonate from coral polyps
Coral reefs grow from the hard skeletons secreted by corals.
10. B – Phytoplankton
These are the primary producers in most aquatic systems.
11. C – Sea sponge
Sessile means fixed in place; sponges do not move.
12. C – Bivalves (like clams and mussels)
These organisms filter particles from the water.
13. A – Blue-green algae (cyanobacteria)
These photosynthetic bacteria fix nitrogen and produce oxygen.
14. C – Migrating from saltwater to freshwater to spawn
Anadromous fish live in saltwater but reproduce in freshwater.
15. C – Tolerate a wide range of salinities
Euryhaline organisms like estuarine fish can survive in varied salinity.
16. C – Sea otter
Sea otters are warm-blooded mammals that live in marine environments.
17. C – Dead organic matter
Detritivores help recycle nutrients by feeding on detritus.
18. C – Breaking down dead organisms
Decomposers like bacteria and fungi are vital for nutrient cycling.
19. D – Dolphins
Dolphins are marine mammals, not invertebrates.
20. B – Depth and light availability
Photosynthesis depends on access to sunlight, which decreases with depth.
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