Bio140 14J-3
William Hu
36802031
Terry Wong
David Vu
Gurvinder Dhalinwal
The effect of salinity on the movement rate of marine isopod,
Gnorimosphaeroma oregonensis
Abstract:
The effect of salinity on the movement rate of marine isopod, Gnorimosphaeroma
oregonensis was examined in a controlled experiment involving different salinity levels.
Test organisms were put into three separate dishes that have salinity ranging from 5 ppt
to 40 ppt for a period ranging from one to two minutes. It was found in trial 1 the
movement tends to increase as the salinity increases. The mean speed in 5 ppt is 0.63
cm/s and it increased to a maximum of 0.82cm/s at 30ppt. In trial 2 there was a gradual
increase in movement from 0.12cm/s at 20ppt, 0.24cm/s at 30ppt to 0.6cm/s at 40 ppt.
No significant changes were observed in both trials because the 95% confidence intervals
overlap one another. The change in movement rate as salinity shifts can be described by
analyzing the internal function of the endopodites through the process of sodium
potassium active transport and its relation with ATP. We conclude that salinity change
does not exert an influence on the movement of Gnorimosphaeroma oregonensis.
Introduction
Gnorimosphaeroma oregonensis, also known as the Oregon pill bug, is a species of
intertidal isopod that is oval-shaped and is approximately one centimeter long (Snively
1943). It is visibly segmented. The body is divided into two parts containing a thorax of
seven segments and an abdomen of six segments (Snively 1943). Its overall gray color
makes it ideal for concealments under the shore rocks. Gnorimosphaeroma oregonensis
has a variety of modified appendages, such a pair of antennae on the head as feelers and
seven pairs of thoracic legs for movement in shallow waters (Headstrom 1979). When
disturbed, the isopod rolls itself into a ball for protection (Standing and Beatty 1978).
Gnorimosphaeroma oregonensis is one of the many inhabitants living along the western
seashores of North America, and eastern shore of Asia (Rees 1975). It is often found in
larger numbers living under mussels or with barnacles in the mid to low intertidal zone
(Snively 1943). Because the organism is not in a closed environment, it is exposed to
many factors that affect its growth, survival, reproduction, and/or behaviour. One of these
factors is salinity. Salinity range limits the demographic distribution and the population
size of the Gnorimosphaeroma oregonensis. It is affected mainly by the water movement
and by the rate of condensation relative to the temperature. The relationship between a
similar species of isopod and salinity has been established in the study by Charmantier
and Daures (1944) on Sphaeroma serratum. It is suggested that because most
sphaeromids live under rocks close to the shore, in 5 to 80cm deep water, and they
usually move in a range of only a few meters, as such, individual of this species,
including the young stages, are subjected to large variation in salinity. Research has
shown that salinity has direct correlation with the organism’s lethal temperature (Nair et
al. 1992). Furthermore, Bliss and Vernberg (1983) suggest that changing salinity has an
impact on the organism’s respiration. Gnorimosphaeroma oregonensis is an
osmoregulator. Osmoregulator regulates the concentration of dissolved ion in its body
fluids regardless of changes in the surrounding (Campbell and Reece 2002).
Gnorimosphaeroma oregonensis detect the salinity change through the chemoreceptors
within the antennas (multi-citation). When it detects a change in salinity, it will try to
balance out the concentration of salt in its body by uptaking or excreting salts. The
regulatory process is powered by the sodium potassium ion pump (Lucu, Towle, 2003).
Furthermore, Isopods use their pleopods and attached flattened endo and exopodites for
osmoregulatory active ion uptake (Postel et al., 2000). The posterior endopodites located
in the abdominal section of the isopod are gill-like appendages where most of the
absorption is done (Postel et al., 2000). The objective of this study is to define how
salinity affects the movement rate of Gnorimosphaeroma oregonensis. Because the
sodium potassium ion pump uses ATP (Lucu and Towels 2002), we expect a decrease in
Gnorimosphaeroma oregonensis’ movement as it is placed in extreme salinities. We have
formulated the pair of hypothesis with null hypothesis being that salinity change does not
exert an influence on the movement of Gnorimosphaeroma oregonensis, and the alternate
hypothesis stating that the factor does contribute to the changing movement rate.
Method
To test the effect of salinity on the movement rate of Gnorimosphaeroma oregonensis,
the two trials, was conducted each separated by the length of a week.The organism’s
length and specific characteristics were observed and measured for both trials to monitor
the variation. In the first trial, 18 Gnorimosphaeroma oregonensis were randomly
assigned, one replicate at a time, to one of three 100mm wide dishes containing salinity
levels ranging from 5ppt, 20ppt to 30ppt. In trial 2, we changed the tested salinity to
20ppt, 30ppt, and 40 ppt. In both trials our control was 30 ppt as this is the salinity
average that Gnorimosphaeroma oregonensis is exposed to in its natural environment
(Rees 1975). It is also the salinity that they have been acclimated to in the lab. The
procedure in both trials was basically the same. From the previous work of Rees(1975)
we knew that in general the average temperature G. oregonensis reside in is around 30
ppt therefore we have used it as the treatment control. Clear covers were put onto each of
the three dishes once the organisms were inserted into dishes. Each organism was
allowed 1 minute to adapt to its surrounding. With an acetate sheet placed on top of the
dishes’ clear covers, we traced the movement of G. oregonensis for 30 sec. We were able
to find out the length traveled during the allotted time using the string ruler method. In
the second trial we altered the testing range by replacing salinity level of 10ppt with 40
ppt. In addition, we have doubled the time required for adaptation and tracing. We
analyzed the result of both trials separately using 95% confidence interval
Result
In trial 1 there was unclear trend between salinity and the movement rate of the
Gnorimosphaeroma oregonensis (figure 1). The mean movement rate decreased from
0.63 cm/s at 5 ppt to a minimum mean rate of 0.42 cm/s at 20 ppt, and then increased to a
maximum of 0.82 cm/s at 30ppt (figure 1). However, none of these differences were
significant. There was considerable variation in all treatment levels, with the least
variation (+/- 0.25 cm/s) at 30 ppt, and the greatest variation (+/- 0.43 cm/s) at 5 ppt.
Gnorimosphaeroma oregonensis shown a decline in movement at salinity level 5 to 20
ppt. The decrease in distance is shown from 19cm at 5 ppt to 12.6 cm at 20 ppt. At 30 ppt
the distance traveled is 24.6cm. Hence the general trend observed is the increase in
movement as the salinity level increases. A different pattern was observed in trial 2
(figure 2). In the second trial, the mean rate of movement increased as salinity increased.
More specifically the mean movement rate increased from 0.12cm/s at 20ppt, 0.24cm/s at
30ppt, to a maximum of 0.6cm/s at 40 ppt. There was also considerable variation in all
treatment levels, with the least variation (+/- 0.15 cm/sec) at 20ppt, and the greatest
variation (+/- 0.36 cm/s).
1.2
Mean speed(cm/s)
1
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
35
Salinity (‰)
Figure 1. Mean speed in centimetres per second of Gnorimosphaeroma
oregonensis after 90 seconds at 5, 20, and 30 ppt. Bars represent 95% confidence
intervals, n = 6 for each treatment.
1.2
Mean speed(cm/s)
1
0.8
0.6
0.4
0.2
0
-0.2
10
15
20
25
30
35
40
45
Salinity (‰)
Figure 2. Mean speed in centimetres per second of Gnorimosphaeroma
oregonensis after 180 seconds at 20, 30, and 40 ppt. Bars represent 95% confidence
intervals, n = 6 for each treatment.
Discussion
In trial 1, we are unable to provide support to our alternate hypothesis because the 95%
confidence intervals for all 3 treatment levels overlapped one another. Contrary to what
we had expected, the Gnorimosphaeroma oregonensis does not display significant
change in the rate of movement. Similarly, in trial 2 the organism does not show a
response significant enough to support the alternate hypothesis. Although the variation
between the treatment levels (20 and 40 ppt) and the controlled level (30ppt) is greater
there is still overlapping of all 3 confidence intervals. In both trials the individuals tested
in hypotonic environment, on average, moved at a slower rate then the control group.
This is expected because the organism is removed from known salinity range of 9.06 to
30.90 ppt (Rees 1975) to extreme salinity levels that pose an environmental stress.
Gnorimosphaeroma oregonensis uses several mechanisms to cope with the induced stress.
If it is placed in a hypotonic solution, G. oregonensis will reduce the water permeability
across body surface and increase the amount of water excreted as urine (Mantel and
Farmer 1983). In addition to the water regulation, G. oregonensis will activate the sodium
potassium ion pump to increase the intake of salt (Lucu and Towels 2002). The sodium
potassium ion pump is located within the endopodites. It is a single layer of epithelial
cells located under cuticle (Mantel and Farmer 1983). The mitochondria on its lining
suggest active transport of ions and salts across the membrane (Postel et al. 2000). Na
and Cl ions are absorbed from the external medium across the surface of the epithelial
cells in a carrier-mediated mechanism (Mantel and Farmer 1983). The movement of Na
from the cell to the hemolymph is an active exchange of Na and K. Na+/K+ ATPase is a
supportive enzyme that is active during the transepithelial absorption of NaCl (Postel et
al. 2000). Phillipst (1972, cited in Mantel and Farmer 1983) suggest the activity of the
enzyme in the gills increase as the organism is in diluted solution. The lack of movement
observed in G oregonensis at low salinity of 5 ppt in trial 1 and 20 ppt in trial 2 can thus
be explained as a way to conserve the ATP required to sustain enzyme formation and to
ensure the continuation of active transport. Conversely, the rate of movement at higher
salinity is high in both trials. In a hypertonic solution, G. oregonensis does not need to
worry about loss of water because the water permeability is controlled by the hormone
secreted from the antennal gland (Mantel and Farmer 1983). Charmantier and Trilles
(1977, cited in Mantel and Farmer 1983) find an increase in the water uptake in
Sphaeroma serratum when the gland is removed. Active transport is not required because
the salinity of the environment is higher than the hemolymph of the organism hence
allowing the organism to save its energy for other purposes. One possible explanation
might be that G oregonensis is trying to search for areas of suitable salinity. Although
individuals in hypotonic environment moved more slowly, on average, the means for
both trials are not significantly different. This inconsistency can be caused by factors
such as biotic variations. We pick specimens randomly for the 2 trials. The bigger
organisms might be more active than the smaller ones since they have fully developed
their osmoregulatory system. In addition, because they are big, it might seem like they
travelled longer distance than the smaller ones. Another reason contributing to this
inconsistency is the mean calculated. The mean can be easily offset if there are extremes.
During our first trial at 5 ppt there were some Gnorimosphaeroma oregonensis that
remained stationary in the salinity and there were some that ran continuously. Experiment
errors can also offset the result. The variation created in the first trial might partly be the
difference in lighting because we did not place the dishes in one spot. Our statistical
analysis did not project the result we had expected. During the period of adjustment, the
organism might show its behavioural response instead of the biological response. If we
had lengthened the acclimation time we might have observed the biological trend as the
organism adjust its behaviour oregonensis’ movement rate.
Conclusion
The change in salinity does not have any affect on the movement of Gnorimosphaeroma
oregonensis
Acknowledgements
I am grateful to L. Norman for proofreading/editing of my paper and offering valuable
comments. D. Vu for providing the group with valuable information and T. Wong for
relaying messages to group members.
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