Deep-sea life

Why are sea organisms living deep in the sea not affected by the high water pressure?

K. AKHIL

Hyderabad

Organisms living in the sea are affected by the high water pressure but they can resist it. In fact, life has evolved in water.

We live on the surface of the earth and are subjected to the atmospheric pressure of about 100,000 pascals. This is called one bar pressure and is roughly equivalent to keeping one kilogram weight on one square centimetre.

The pressure under water is much higher due to the high density of water. It increases approximately by 1 bar for a depth of every 10 metres. Hence a fish at a depth of 20 metres is subjected to three bars pressure.

This type of pressure in the fluid is called hydrostatic pressure. Even though it increases with depth, at a given point it is same in all directions and acts perpendicular to a surface immersed. Such type of loading on any body creates only compressive stress, i.e. there are no tensile (pulling) or shear stresses inside the body.

If I am carrying a school bag, the stresses due to the bag and my own weight will be added to this one bar compressive stress with proper sign and direction. The additional stresses can also be tensile and shear depending on several factors like the body shape and how the load is applied, and in some cases can be very high. But in case of hydrostatic stresses, irrespective of th/e body shape and the properties of the composite materials like bone and muscle, the interior stresses are just the applied compressive stress.

Most materials are stronger in resisting compressive stress but not so strong in tension and specially shear. The tensile and shear strength values for failure of the human bone are about 65 per cent and 30 per cent, respectively of the compressive strength. Also, applied shear deforms the body. Imagine an invertebrate like a jellyfish subjected to shear. It will deform and may tear off if the stress is high. But now it is subjected to only compressive stress and which is exactly equal (no more) to the value of pressure at that depth irrespective of its shape and material properties. The individual cells in the body are subjected to this value which they can resist. This is how the organisms survive the high pressure.

PROF. MOHAN D. DESHPANDE

M.S. Ramaiah School of Advanced Studies

Bangalore