Which one of the following is the correct sequence of a food chain?

Diatoms are microscopic phytoplankton, so they are the producers at the base of the chain. Crustaceans such as copepods graze on them, making them primary consumers. Herrings feed on these zooplankton and sit one level higher as secondary consumers. So the correct sequence is Diatoms, Crustaceans, Herrings, which is option A.

• Option B reverses the first two links. Crustaceans cannot be the base because they depend on phytoplankton for energy.
• Option C places herrings before crustaceans. Herrings are higher in the chain than the zooplankton they eat, so this order is wrong.
• Option D starts with a consumer and ends with a producer, which reverses the direction of energy flow entirely.

Core concept: trophic levels and the unidirectional flow of energy from producers upward, with roughly 10 per cent transfer between levels. This links to Mains questions on marine fisheries, the role of phytoplankton in oceanic productivity, and why overfishing of forage fish disrupts the whole ecosystem.

Trap: candidates get distracted by herrings because they are the most visible organism in the list. Always trace energy from the autotroph upwards.