Osmoregulation as a Homeostatic Mechanism using the Comparitive Melting-Point Method for two Crab Species.

Topics: Melting point, Osmoregulation, Melting Pages: 5 (1819 words) Published: April 14, 2014
Osmoregulation as a Homeostatic Mechanism using the Comparitive Melting-Point Method for two Crab Species.

Two crab species, Plagusia and Cyclograpsus, were collected from a local estuary in the littoral and deep water zone for osmoregulation studies. To examine differences in osmoregulatory mechanisms among the species, haemolymph of the specimens was extracted once they were acclimated to varying concentrations of seawater. Using the comparative melting-point, capillary tubes were filled with small samples of seawater and blood then frozen and melted in a -15˚C ethanol bath. The melting time of each was observed thereafter. Subject’s time range fell over 17 minutes of which the majority of the most salinated samples melted last and the remaining concentrations melting according to the different data sets. The study revealed the Plagusia crab as the osmoconformer and the Cyclograpsus as the osmoregulator. Introduction

For an organism to keep its body in a state of equilibrium it makes use of homeostatic mechanisms. These mechanisms are caused by fluctuations in extracellular fluid (Richardson, 2005). They act on a system of negative feedback in order to preserve or re-establish the ideal state. The relevant homeostatic mechanism tested in this investigation is osmoregulation. Osmoregulation is vital for organisms to be able to maintain a constant, ideal osmotic pressure within their body. It is a homeostastic mechanism in which they keep an optimal concentration of solutes and quantity of water in the bodily fluids (Canalon, 2009). Osmoregulation in crustaceans has been subjected to study by numerous authors (Morris, 2001) (Charmantier et al., 1988) (Thurman, 2003) typically in terrestrial and estuarine species (Bursey, 1976). For crab species living in estuaries, they are exposed to varying levels of salinity. In order to survive this environment, they cope by using one of two major “sub-mechanisms”; osmoregulation and osmoconforming. “Regulators” (via active control) maintain body osmolarity constantly. They display “extracellular-fluid anisosmotic regulation” (Florkin, 1962). “Conformers” (via passive control) control their body osmolarity according to their environment (O’Driscoll et al., 2008). They display “intracellular-fluid isosmotic regulation” (Florkin, 1962). Osmoregulators are generally freshwater and terrestrial animals that can live in diverse habitats while osmoconformers are usually seawater animals that are isosmotic to their environment. Therefore each species had distinctly different ion levels. The temperature at which a certain solid will melt into liquid form is referred to as the melting-point. The melting-point gives a good impression of how pure a substance is. This is due to the fact the small amounts of impurities (varying seawater concentrations in the species’ blood) can affect the time at which a substance melts. They cause defects in crystalline structure of the substance, causing the melting-point to decrease by a few degrees. In pure substances (distilled water), the molecules are identical thus packed tightly together therefore causing the melting-point to increase (Brittain, 2009). Consequently, it is expected that the melting times of the distilled water will be lower than the samples containing seawater. The comparative melting-point method will be used to investigate the hypothesis. It relies on a visual detection and the fact that the freezing point of a fluid decreases as the ionic concentration increases. In view of the above, the aim is to determine whether the two crab species, Plagusia and Cyclograpsus, are osmoconformers or osmoregulators based on the comparative melting-point method. It was expected that the method will reveal the Cycolgrapsus to be the osmoregulators. Materials and Methods

a) Species Collection
An intertidal crab species and infratidal species, Cyclograpsus and Plagusia respectively, were collected from a local estuary and acclimatised to...

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