Sea-run sticklebacks are described as anadromous. What does this mean?

• A. Live entirely in freshwater
• B. Migrate only within the ocean
• C. Born in freshwater, spend most of life in the ocean, return to freshwater to breed
• D. Never leave freshwater but breed in saltwater

Answer: (C)

Anadromy describes a life cycle where fish are born in freshwater, migrate to the ocean to grow, and return to freshwater to breed. Sea-run sticklebacks follow this pattern: they start life in freshwater, spend most of their adult years in the sea, and then move back into freshwater to reproduce. This is why the described option is the best fit.

Other patterns don’t match: staying entirely in freshwater would be freshwater-resident, not anadromous; migrating only within the ocean means never returning to freshwater to spawn; and never leaving freshwater but breeding in saltwater wouldn’t align with the defining journey between saltwater and freshwater that anadromy involves.