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...