OSMOREGULATION OF FRESHWATER AND MARINE INVERTEBRATES

INTRODUCTION

• Regulation of osmotic pressure of an organism body fluid is refer to as osmoregulation. It allow to maintain balance of water content, fluid content and electrolytes. Hence due to osmoregulation body neither become too dilute nor become too concentrated. Right concentration of solute and amount of water must be maintain by any aquatic [fresh or marine] and terrestrial organisms. It can be mostly achieved by excretion.
• Three terms are important to describe when talking about osmoregulation.
• Diffusion – it is a random movement of molecules along the concentration gradient from the region of high concentration to region of lower concentration.
• Osmosis – in osmosis selective diffusion of solvent is driven  by internal energy of solvent molecules. We can also say that osmosis is specialised  case of diffusion that involve passive transport of solvent from its region of higher concentration to its region of lower concentration through a semipermeable membrane.
• Osmotic pressure – it is define as pressure required to completely stop the entry of solvent of osmotically active solution.
• On the basis of above describe termed any living cell can be present in three condition with respect to its external environment.
• Isotonic solution– when concentration of ion of cell is equals to concentration in external environment or in solution.
• Hypotonic solution – when concentration of ion in cell is more than to concentration in external environment or in solution.
• Hypertonic solution – when concentration of ions in a cell is less than concentration in external environment or in solution.
• Based upon this properties only any organism is able to maintain is optimal ionic balance.

OSMOREGULATION OF FRESH WATER ORGANISMS

COELENTERATA

Hydra

• Condition in hydra favours an influx of surrounding water into its body cells. There is no evidence of presences of any special organ for osmoregulation.

FLATWORM

Dugesia

• Osmoregulation is controlled by cerebral organs and several other organs. Nephridia plays a vital role in osmoregulation of gases, particularly oxygen is exchange through body surface. Electrolytes can be lost through fluid excretion through excretory organs. They are able to compensate for salt absorbed by papillae.
• The main function of flame cells seems to be of osmoregulation rather than excretion. Flame cells and nephridia are best develop in freshwater species but less develop or absent in marine species. As excretory system plays an important role in getting rid of excess of water from the body, it is better to call it osmoregulatory system.

MOLLUSCA

Class – Bivalvia

• Body fluids are approximately isotonic  compare to surrounding medium. Hence there is little net flux of water or ion in and out.
• In case of extreme osmotic stress the bivalve as their initial step close their valve. This will not allow to exchange their ions and water to external environment including waste product such as ammonia. To avoid accumulation of waste they slow down their metabolism.

OSMOREGULATION OF MARINE INVERTEBRATES

SCYPHA, CRAB, SEA STAR

• Contractile vacuoles in ameboid cells are believe to take part in osmoregulation in case of Scypha.
• In crabs water is regualated by green gland. Each gland consist of small end sac, which connect to larger sponge like cavity, which is connected to bladder, that opens outside by small pore by the base of antennae. This glands are responsible for nitrogen excretion. Green gland eliminate water and salt alike, the loss of salt in urine is compensate by gills.
•  In star fish the tissues are perfused by sea water itself. Hence their internal osmotic pressure is equal to their external osmotic pressure. Hence they do not need any special osmoregulation.

CONCLUSION

• Every organism should adopt well to its environment to perform its day to day functions without any problem or hurdles. Some needs to regulate their temperature while other needs to regulate their ionic balance. Here we discuss about regulation of ions and water content in invertebrates which is known as osmoregualtion.
• There are two major types of osmoregulators

Osmoconformers :-

• Actively or passively matches their body osmolarity to their environment. This includes most marine invertebrates. In marine invertebrates the amount of internal salt are relatively constant to the environment. It needs to be intake and outflow the water and salt to be equal over a very large period.

Osmoregulators :-

• Their internal osmolarity may differ from a medium in which they are present. They can successfully maintain constant internal condition. Hence refer to as osmoregulators.
• They are more common in animal kingdom they can actively control salt concentration, independent of salt concentration of environment.