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Phascolosoma lurca (P. lurca) and Uca coarctata (U. coarctata) species and their organs were examined and studied to determine their osmoregulatory ability using the density of extracellular fluid (ECF) as a measure of ionic and osmotic concentration. Body fluids are denser than water which has a specific gravity of 1, plus solutes. Through the examination of the two organisms, P. lurca is an osmoconformer and U. coactarta is an osmoregulator.


Osmoregulation is the physiological process that all organisms of any habitat use to maintain their water balance and internal salt for several different reasons such as, compensating for water loss, avoiding excess water gain, and maintaining a proper osmotic concentration of the body fluids (Saladin, 2010). This process is derived from osmosis, however is much more complex. Osmosis is the diffusion of water down its cavity gradient through a semipermeable membrane.

The aim of the experiment was to determine whether P. lurca (Phylum Sipuncula) was an osmoconformer and U. coarctata, a fiddler crab (Phylum Arthropoda, Subphylum Crustacea) was an osmoregulator (School of Marine and Tropical Biology, 2010).

It was hypothesised that P. lurca is an osmoconformer with a soft-body while U. coarctata is an osmoregulator and has a thick cuticle. This was predicted due to the fact that both species inhabit local salt water creeks and salinity levels fluctuate throughout the year. Osmoconformity occurs within an animal if the osmotic concentration of the body fluids equals that of the medium (Miller & Harley, 2005). However, an animal that maintains its internal concentrations different to that of its surrounding environment is an osmoregulator. In order to find P. lurca and U. coarctata and their means of osmoregulatory ability, the specific gravity of the animal’s extracellular fluid (ceolomic fluid) needs to be found....
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