Q.How do marine animals adopt to hypertonic seawater ?

Marine animals cope with the water-drawing (hypertonic) effect of seawater through different osmoregulatory strategies depending on the group — osmoconformity in most invertebrates, active salt excretion with seawater drinking in bony fishes, and urea retention in elasmobranchs.

Seawater is hypertonic relative to the body fluids of most marine animals, tending to draw water out of the body by osmosis and letting salts diffuse in. Different groups have evolved distinct adaptations:

1. Marine invertebrates (osmoconformers): Most marine invertebrates (e.g. sponges, cnidarians, echinoderms, many molluscs) keep their body fluid osmotic concentration equal to that of the surrounding seawater (osmoconformity), so there is no net osmotic gradient and hence little need for active regulation.

2. Marine bony (teleost) fishes (osmoregulators, hypoosmotic to seawater): Their body fluids are less concentrated than seawater, so they continuously lose water by osmosis through the gills. To compensate, they:

   • Drink seawater almost continuously to replace lost water.
   • Actively pump out the excess salts (Na+, Cl-) absorbed from the swallowed seawater and across the gills using specialised chloride secretory cells.
   • Excrete only a small volume of concentrated urine through well-developed kidneys, to minimise further water loss. …