A rui (rohu) can spend its life in a pond in Bengal without drinking much water. A bangda (Indian mackerel) in the Arabian Sea faces the opposite challenge: surrounded by saltwater, it must prevent too much water from leaving its body.

Put them on ice at a fish market and the difference may seem purely culinary. One might end up in a Bengali jhol; the other could become a crisp fry or a coconut-based curry. But beneath their scales is a biological divide that has shaped how they breathe, drink, eat and survive.

The difference begins with the chemistry of the water around them.

India’s fish live across an extraordinary range of habitats — Himalayan rivers, floodplain wetlands, village ponds, reservoirs, mangroves, estuaries, coastal lagoons and nearly 7,500 km of coastline. For fish living in these environments, survival depends on constantly maintaining the right balance of water and salt inside their bodies.

The body versus the water

The central principle is osmosis — the movement of water across a selectively permeable membrane in response to differences in the concentration of dissolved substances.

A freshwater fish such as rohu, catla, mrigal, mahseer or magur has body fluids that are saltier than the water around it. As a result, water naturally tends to enter its body.

That creates a problem. Cells need a stable concentration of salts and other dissolved substances to function. If too much water enters, that balance can be disturbed.

Freshwater fish have evolved an efficient way to deal with this. They generally drink very little water. Instead, they produce large quantities of dilute urine to remove excess water, while specialised cells in their gills actively take up essential salts from the surrounding water.

Marine bony fish face the reverse situation.

Seawater is generally saltier than the body fluids of species such as Indian mackerel, sardines and many pomfrets. Water therefore tends to move out of their bodies. To compensate, many marine fish drink seawater, absorb the water they need and use their gills and kidneys to remove excess salts.

In simple terms, a freshwater fish is trying to prevent itself from becoming too diluted. A marine fish is trying not to lose too much water.

This continuous balancing act is called........

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