Many animals need to control the water content of their body because the concentration of their body fluids is different to that of their environment. The process by which the amount of water and concentration of soluble ions in the body are regulated is called osmoregulation. In most cases the organs which are responsible for osmoregulation are also involved in the excretion of metabolic waste products.
In this practical you will conduct an experiment to determine the osmoregulatory ability of two organisms. You will also examine the organs responsible for osmoregulation and excretion of both invertebrates and vertebrates.
Phascolosoma lurca (Phylum Sipuncula) and Uca coarctata, fiddler crab (Subphylum Crustacea) occur throughout the year in the mudflats of Suva Points, Nasese, and generally around all river mouths and in the mudflats throughout Fiji. In these environments the water salinity can vary dramatically from time to time. Salinity can be close to freshwater (low salinity) after heavy rains, especially around the river mouths and estuaries. During the dry season the salinity would be same as seawater salinity. In addition the environment also undergoes daily tidal changes. By occupying these habitats, P. lucra and U. coarctata obviously tolerate a wide salinity range. However, it is anticipated that P. lucra being soft-bodied and related to annelids, tends towards osmoconforming; while U. coarctata, having a thick cuticle, tends towards osmoregulating over a range of salinities.
This experiment is designed to test the hypothesis that P. lurcra is an osmoconformer and U. coarctata is an osmoregulator.
The method used the density of extrcellular fluid (ECF) as a measure of ionic and osmotic concentration. All body fluids are more dense than water since they consist of water (SG=1) plus solutes. Increased solute concentration causes an increase in density and thus density provides a relative measure of solute concentration and osmotic pressure.
1 (Lab BI108)
Groups of P. lurca and U. coarctata have been kept for at least 2 days in 1/8, ¼, ½, ¾, 1 and 1¼ seawater concentration (acclimation media). Lowered similarities were produced by diluting the seawater and 1¼ seawater by addition of NaCl. The experiment tests whether the animals adjust their ECF according to the salinity of their environment or they maintain their ECF at a certain concentration regardless of their environmental salinity. During the practical ECF will be collected from the animals and the specific gravity will be measured. ECF from the Sipunculid will be collected buy slitting the posterior body wall and drawing into a clean dry beaker. The fluid is centrifuged before being given to you for measuring the SG.
Crab ECF is extracted using a syringe inserted at the base of the walking legs and chelipeds. You also need to measure the SG of the acclimation media i.e. the different seawater concentrations in which the animals were kept during the experiment. Use a salinometer/refractometer to measure the SG of all the media and the ECF of P. lurca and U. coarctata. (Procedure will be demonstrated).
Do each measurement in triplicates
Record your results in the table.
On the table, give the average value
2. Demonstration Material
Examine the dissection of the earthworm (Phylum Annelida, Class Oligochaeta). Observe the metanephirida in each segment. In the T.S. of the earthworm, identify the nephridiopore and excretory tubule of the metanephridium.
Examine the dissection of cockroach and identify the malpighian tubules. What is the excretory product created by the tubules?
Examine the mode of the kidney. Identify, the cortext, medulla, Bowman’s capsules and Loops of Henle. What is the function of the Bowman’s capsule. DRAW.
Examine the demonstration of a contractile vacuole of Paramecium. 2 (Lab BI108)
Examine the dissection of the Sipunculid, identify the nephridia.
PRESENTATION (Practical Report)
Write up this experiment as a scientific report of no more than 800 words, i.e. with a title, introduction, methods (no need to describe, just state “as per notes”), results and discussions. The results should include a table of your own data giving SG readings for the P. lucre ECF, U. coarctata ECF and acclimation media. Plot the class results for P. lucre and U. coarctata on a graph of SG of ECF (of P. lurca and U. coarctata) versus SG of medium (not seawater dilution – these are approximate dilutions). Plot P. lucre and U. coarctata SG separately but on the same graph.
Also, include an isosmotic line in the graph. Don’t try to fit a curve on your graph – join the points on the graph with straight lines (why?). Discuss your results. Was the initial hypothesis confirmed? How do these animals osmoregulate? Relate your results to the Biology of animals. What sources of error were present? Can the experiment be improved in the context of a student laboratory class?
REFERENCES (some suggestions)
Lockwood, A.P.M. (1963) Animal body fluids and their regulation. London: Scholarship Series, Heinemann.
Potts, W.T.C. & G. Parry (1964) Osmotic and ionic regulation in animals. Oxford, Pergamon Press.
Villee, C.A., W.F. Walker Jr. & R.D. Barnes (1973) General Zoology, Sixth edition. Philadelphia: Saunders 97, 205, 566, 587.
(plus any general or comparative text or animal physiology).
3 (Lab BI108)