An Introduction to the Invertebrates, Second Edition - tiera.ru

BUOYANCY143thereismore food.Buoyancycontrolis achievedbyaltering the proportions ofgasand water in the shell compartments (see diagram a).When first formed, thechambers are full of sea water.The siphuncle takes up ions from the sea water byactive transport (see Box 7.1), water follows by osmosis and enters the bloodspace within the siphuncular tube (see diagram b) to be swept down into theanimal.Inthe chambersgas bubbles outof solution, replacing the water removed.The animalwillthen be more buoyant and float higherinthe sea.Whenthe siphunclepumps ions in the opposite direction, from the blood to the chamber, waterwill follow and there will be less gas in the chambers; the animal will therefore beless buoyant and sink. In cuttlefish the chambers of the internal shell are linedby a membrane functioning very much as does the siphuncle in Nautilus, andbuoyancyis regulatedin muchthe same way.Many fish also undergo daily vertical migration. They alter the pressure ofgasinthe swimbladder, rather thanthe proportions ofgas andwater; thisprocessrequires more energy and cannot give neutral buoyancy as quickly or as completelyasis possible in Nautilus.The control of buoyancy inNautilus: (a) gas and water inthe shell compartments;(b) the siphuncular tube enlarged,with an arrow indicating thedirection of movements of ionsand hence water, causing anincrease in the proportion of gas.