Step 1: Understanding the Concept:
Animal migration is a widespread biological phenomenon, but in the world of ichthyology (the study of fish), migration patterns are often tied to the extreme differences between marine and freshwater environments.
Fish that migrate between these two vastly different habitats are called Diadromous fish.
This journey is incredibly difficult because it requires the fish to completely reconfigure their osmoregulatory systems – their gills and kidneys must switch from excreting excess salt in the ocean to conserving salt in the rivers.
There are two primary directions for this migration:
1. Anadromous migration: Where fish grow up in the saltwater of the sea but return to the "upward" (ana = up) freshwater rivers to lay eggs (spawn).
2. Catadromous migration: Where fish live in freshwater but move "downward" (kata = down) to the sea to reproduce.
These behaviors are often driven by the need to find safer environments for vulnerable fry (young fish) or to access specific nutrient-rich feeding grounds.
Step 2: Detailed Explanation:
Let's evaluate the fish species listed in the options based on their life histories:
- Hilsa (Tenualosa ilisha): This is a member of the herring family and is culturally and economically vital in South Asia. Hilsa is the perfect example of an anadromous fish. It spends most of its adult life in the marine environment of the Bay of Bengal and the Arabian Sea. During the breeding season, usually triggered by monsoon rains, they form large schools and migrate hundreds of kilometers into rivers like the Padma, Ganges, and Godavari. They spawn in the freshwater, and the hatchlings (jatka) slowly migrate back to the sea to mature.
- Anguilla (Eel): This represents the opposite strategy. The freshwater eel lives in rivers but makes an epic journey to the Sargasso Sea in the Atlantic Ocean to spawn. Thus, it is Catadromous.
- Labeo (Rohu): This is a species of carp. It is an "obligate" freshwater fish, meaning it lives, eats, and breeds entirely in freshwater rivers and lakes. It does not go to the sea.
- Channa (Snakehead/Murrel): These are freshwater air-breathing fish. While they can move short distances over land between ponds, they do not perform sea-to-river migrations.
Other famous examples of anadromous fish globally include Salmon and the Sea Lamprey.
Step 3: Final Answer:
Among the given options, Hilsa is the correct example of an anadromous fish that migrates from the sea into fresh water for the purpose of spawning.