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

SEA, FRESH WATER AND LAND87Box 7.1Sea, fresh water and landSeaAnimal life began in the sea.The marine habitat is favourable, being a large bodyof water fairly constant in quantity, movement and salt concentration. It isbuoyant, relatively rich in potential food and it enables animals to disperse.Many marine invertebrates shed their eggs and sperm directly into the seaand fertilisation is external. The eggs contain little yolk and hatch early, usuallyinto larvae feeding in the plankton (surface waters). Above all, the body fluidsof marine invertebrates are similar in salt concentration to that of the sea inwhichtheylive, and there is no osmotic problem.Units and definitionsThe osmotic pressure of a solute is usually measured by the depression ofthe freezing point () caused by the solutes. Pure water freezes at 0.00 °C;sea water (with 35 parts of salt per1000) at 1.86 °C.Osmoles measure the osmotic effect of a solute whose molecular weightin grams is dissolved in a litre of sea water. For sea water the value is 1osmole(¼ 1000 milliosmoles or mosm) and this provides a useful standard forcomparison.Most marine invertebrates are osmoconformers, and are fully permeableto water, i.e. the body fluid concentration is close to one osmole.Osmotic regulation by contrast is the maintenance of the internal bodyfluids at a concentration different from that of the environment.Euryhaline animals are those which can tolerate a wide range of salinities,usually by a combination of osmoconforming and osmoregulating processes.Stenohaline animals are those (like most marine invertebrates incapableof osmoregulation) that are restricted to a narrow salinityrange.Ionic regulation, selection of particular ions at concentrations different fromthose in the sea, does however occur even in the simplest marine animals.A jellyfish floats easily because it contains relative to the surrounding seafewer heavyions (magnesium, sulphate) and more lighter ions (sodium, chloride).A marine lobster (Homarus sp.) maintains concentrations of magnesiumions at14% and sodium at111% sea water, butitisincapable of osmoregulation.Fresh waterColonisation of fresh water from the sea raises problems. The medium is muchmore variable in many ways: it may over-heat or freeze, it may flow very fast,it may dry up, and above all it is much more dilute (5 10 mosm) than internalfluids can be.Water will therefore tend to enter by osmosis, and salts (ions) toleave by diffusion. If these movements were unchecked, animals would swellup and burst or all the salts would leak away. Small animals, with relativelymore surface area in proportion to their volume, have a worse problem thanlarger ones.