Marine Biology

Floatation by reducing over weight

For

For reducing the weight of the organisms so as to live as plankton in the surface layers of the water, these organisms simply change their body fluid composition in order to make the body less dense than the seawater without affecting the salt balance/ osmotic conditions of the planktonic organisms. The most common way for this is the  replacement of  heavy ions in the body fluids with lighter ones. This is seen in Noctiluca, by having ammonium chloride in its internal body fluid   with the specific gravity of 1.01 against the seawater value of 1.025. Similarly, plankton species like salps, ctenophores, and heteropods are known to eliminate heavy ions like SO₄^2- from their bodies and replace them with osmotically similar but lighter chloride ions.

               Some plankton will have more water contents in their body to reduce the specific gravity, thereby increasing the buoyancy e.g. jelly fish. Some animals like foraminiferans have many perforations in their test or shells so as to reduce the weight of the animal to increase the buoyancy. Some are having oil globules or droplets in their body to reduce the specific gravity so as to increase the buoyancy e.g. fish eggs and diatoms.

           Another mechanism that is employed to reduce density is the possession of a gas float of some sort. Some siphonophores have air filled sacs called pneumatophores in its body. In the case of the blue-bottle (Physalia physalis, commonly known as Portuguese man- of- war), its float is so large which  projects above the surface of the sea  like a boat-sail and trails its tentacles below. This type of animal floating at the air-sea interface is termed neuston.