Q. Which of the following are detritivores?

1. Earthworms
2. Jellyfish
3. Millipedes
4. Seahorses
5. Woodlice

Select the correct answer using the code given below. (UPSC Prelims 2021)
Answer: 1, 3 and 5 only
Notes: The correct answer is [C] 1, 3 and 5 only. Detritivores are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts, as well as feces). They play a vital role in ecosystems by recycling nutrients.

• Earthworms (Statement 1 – Correct): These are classic examples of detritivores. They ingest soil, extract nutrients from decomposing organic matter, and excrete nutrient-rich castings that improve soil fertility.
• Millipedes (Statement 3 – Correct): Unlike centipedes (which are predators), most millipedes are detritivores. They feed on decaying leaves and other dead plant matter found in moist soil or leaf litter.
• Woodlice (Statement 5 – Correct): These terrestrial crustaceans (also known as roly-polies) feed primarily on dead plant material. They break down complex organic polymers, facilitating further decomposition by fungi and bacteria.
• Jellyfish (Statement 2 – Incorrect): Jellyfish are carnivorous predators. They use stinging cells (nematocysts) on their tentacles to capture zooplankton, small fish, or other aquatic animals.
• Seahorses (Statement 4 – Incorrect): Seahorses are also carnivorous predators. They use their elongated snouts to suck in small crustaceans, such as mysis shrimp, and other tiny plankton.

Detritivores differ from decomposers (like fungi and bacteria) in that detritivores physically ingest and digest chunks of organic matter, whereas decomposers secrete enzymes to break down matter externally.