Student: Osmosis and Costa et Al. Essay

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Volume regulation in a marine polychaete worm at various salinities
Alexandra Lindsay
Lab Partners: Lindsay Seebeck and Alexander Paul
Due Date: December 1, 2011


The control of tissue osmotic pressure defines osmotic regulation (Moyes & Schulte, 2008). Osmotic regulation establishes the movement of water across the membrane (Moyes & Schulte, 2008). It requires the altering of the osmotic gradient by the movement of solutes across a membrane (Moyes & Schulte, 2008).
Glycera dibranchiata, commonly known as the bloodworm, is a marine invertebrate. The bloodworm lives in seawater with concentrations ranging from 1366 to 374 mosm/kg (Costa et al., 1980). Polychaete worms can be found in environments where the salinity fluctuates daily for example in estuaries and intertidial zones (Lab #4, 2011). Therefore polychate may be exposed to more dilute waters or increased osmolaity and must regulate cell swelling when solution is dilute and cell shrinking when solution is saline (Lab #4, 2011). The bloodworm is a polychaete that uses euryhaline osmoconforming for osmotic regulation (Costa & Pierce, 1982). Osmoconformers will demonstrate internal osmolarity that are similar to the external osmolarity, even when external conditions change (Moyes & Schulte, 2008). Bloodworms regulate their volume when exposed to a hypotonic solution (Costa & Pierce, 1982). This is done by regulation of the hemoglobin filled coelomocytes found within the coelomic fluid of the bloodworm (Costa & Pierce, 1982). Osmotic swelling of these cells occurs when they are exposed to a hypotonic solution (Costa et al., 1980). This swelling is generally followed by partial recovery of the original cell volume (Costa & Pierce, 1982). A rapid reduction in the solute concentration of the intracellular fluid produces the volume recovery (Costa & Pierce, 1982).
The purpose of this investigation is to show the process by which bloodworms osomoconfrom to their external environment. Measuring the mass of the bloodworms placed in different mediums containing a known concentration of salt will demonstrate whether there is an influx of water thus resulting in the swelling of cells.
The raw data (refer to appendix 1-1) was manipulated using a formula in order to obtain percentage change in weight. The formula (refer to appendix1-2) used the time at a specific point minus the initial time divided by the initial time for various salinities in order to calculate the percentage change in weight. The percentages were then organized in a chart and the average percentages were calculated (refer to appendix 1-3). This data was then used in order to create a graph in excel (refer to graph 1). The statistical methods used in order to analyze the data included a Post hoc test as well as a t-test. The calculated percentage change in weight after 75 minutes (refer to appendix 1-3) was than used in a post hoc test. The post hoc test was done on the program SPSS, the data was inputted resulting in a chart (refer to appendix 1-4). The data in the chart was than examined in order to determine whether there was a significant difference regarding the change in weight for each salinity. The mean difference is significant at the 0.05 level therefore any values less than 0.05 were identified as being significantly different. A t-test was than performed on the SPSS program in order to make a comparison between the mean percentage change in weight at 75 min and after the recovery period of 45 minutes. The data was inputted resulting in a chart summarizing the t-test results (refer to appendix 1-5).


Recovery period is from 90min – 120 min.

The results of the lab demonstrate that as the salinity of the medium deceases the Glycera dibranchiata takes in more water therefore the percentage change in weight increases. During the recovery period the bloodworm is transferred