To conform or to regulate: the question of osmoregulation
of Uca pugilator
Jessica Mathis
October 24, 2007
Bio 412
Abstract
Osmoregulation of organisms is important to the ability to survive. Some animals
conform their internal environment to the external environment while others maintain a
constant internal environment no matter what the outside environmental changes might
occur. The fiddler crab, Uca pugilator, was used to examine the osmoregularity of
brackish-water animals. By using external water samples of the four salinities (25%,
50%, 75%, and 100%) and plasma Na+ concentrations in a flame photometer to obtain an
Optical Density of the concentration levels, then converting them to Na+ concentrations
to compare levels from the internal environment and the external to determine whether
the fiddler crab is an osmoconformer or osmoregulator. The external environment at
25% salinity was on average 63.4425 while the internal environment for the fiddle crab
was 134.5417; at 50% salinity the external was 114.3875 while the internal was
139.1898; at 75% salinity the external was 146.3875 while the internal was 138.9071;
and at 100% salinity the external was 169.6625 while the internal was 145.7543. The
external environment changed drastically with the increase of salinity while the internal
environment of the fiddler crab remained at a more constant level throughout the changes
of the internal supported that the fiddler crab is an osmoregulator (homeoosmotic
organism).
Introduction
Osmoregulation of organisms is important to the ability to survive.
Osmoregulation is a process of the body to maintain a homeostatic condition of the water
content within the organism. This is important to the survival of organism because the
lack of osmotic regulation could make it possible that organism will intake too much
water and become bloated or that water loss to the external environment would cause
severe dehydration. Either scenario would cause death among organism.
Osmoregulation can occur in two ways. Some animals conform their internal
environment to the external environment which is referred to osmoconforming while
others maintain a constant internal environment no matter what the outside environmental
changes might occur which is referred to as osmoregulating.
Osmoconformers can use
passive or active processes to regulate water and salinity exchange, while osmoregulators
use active processes to keep an internal constant salt concentration no matter the external
environment conditions. If organisms are osmoregulators, certain body organs and fluids
are important in maintain homeostasis. If the outside environment is hypotonic an
organism must release excess water or too water occurs and organisms retain too much
water. If the outside environment is hypertonic then water intake in needed to offset the
loss of water by osmosis. “The oligohaline or “freshwater species, U.minax, U.
spinicarpa and U. longisignalis, possess the lowest average hemolymph osmolality. On
the other hand, the euryhaline species, U. speciosa, U. panacea, U. pugilator, and U.
pugnax, have much higher average hemolymph osmolality. The Uca species are not
uniform in osmoregulatory abilities. There is considerable inter- and intraspecific
physiological variation associate with the ecological distribution of each species”
(Thurman 2003). The differences in regulatory systems between species can be
explained by adaptation of each to their own environment. The freshwater species and
the open ocean water species have more stable environments than the brackish-water
species like the Uca pugilator. Brackish-water organisms are exposed to the areas where
the freshwater and the ocean salt water merge together. High tides, low tides, and the
changing of the tides make it hard for organisms to conform to their external
environment. The wide changes in the salinity would keep a brackish-water organism
constantly changing their internal environment to match their external one. “The
excretory organs contributed to the osmoionic regulation of the hemolymph in crabs ...
by means of the partial reabsorption or excretion, respectively, of salts from or into the
urine” (Zanders and Rojas 1995). Crabs use specialized organs dedicated to the ridding
the body of waste to keep the salt concentration in a constant state within the body. “An
animal going from the ocean into freshwater usually encounters more variable
temperatures and of course enter a less saline medium. The bogy fluids become more
dilute or adjustments occur which prevent loss of salts from the body of the animal and
keep its blood at a density near that which it had in the ocean. Certain crabs tolerate
considerable variations in the salt content of their blood, which contains salts in about the
same quantities as the surrounding medium. Hence they readily migrate into brackish or
nearly fresh water” (Pearse 1927). Osmoregulators can migrate to different salinities
easier that osmoconformers. With the ability to maintain an internal constant salt
concentration and the maintenance to minimize water loss lets osmoregulators exploit
more habitats. The purpose of this lab is to examine Uca pugilator hemolymph exposed
to four different salinities and compare the Na+ concentration to the concentrations of the
four salinities of the external environments to determine whether the fiddler crabs are
osmoregulators or osmoconformers.
Materials and Methods
Uca pugilator were exposed to four different salinities; 100% to depict high tide,
75% and 50% to illustrate the changing tides, and 25% for low tide. Three crabs from
each salinity were examined. Using insulin syringes hemolymph samples were collected
by puncturing the soft integument (tissue) between the segments of the walking legs near
the body. After the samples were collected a dilution of 1:200 was made with distilled
water. Samples of each of the external water environment were collected, pulse
centrifuged to separate out any sediment and also diluted to 1:200 with distilled water.
The samples were then vortexed and transferred from large test tubes to smaller ones.
The flame photometer was blank to 0.00 using distilled water. Standards of 12.5, 25, 50,
100, and 200 µg/L were analysized. Then each of the samples analysized by using the
steps using the flame photometer as described in the Lab Manual (Dr. Crain 2007). Data
was recorded by the class for a bigger data set. Crab concentration, water concentration,
and crab sex was recorded by all groups and collaborated. Linear regression analysis in
Excel was used to determine correlations between environmental and internal Na+
concentrations.
Results
The hemolymph from each crab was diluted with distilled water and the number
gotten from the flame photometer was the optical density. These numbers in return were
put into Excel and the standard curve was obtained. The equation for the points that each
group was different because of the standard recording with each change of group with the
flame photometer. The standard curve equation that my group got was
y = 0.0573x + 2.2375
and each of the points were plugged into the equation as y to solve for x. The
manipulated equation was
x = y-202375
0.0573
Figure 1 shows the difference in the concentration of the internal hemolymph
concentration of the fiddler crab compared to the external water salinity concentration.
The external environment at 25% salinity was on average 63.4425 while the internal
environment for the fiddle crab was 134.5417; at 50% salinity the external was 114.3875
while the internal was 139.1898; at 75% salinity the external was 146.3875 while the
internal was 138.9071; and at 100% salinity the external was 169.6625 while the internal
was 145.7543. Overall there was an average internal Na+ concentration of 139.5982 and
an external Na+ concentration of 123.47. A linear regression analysis in Excel was done
to see if there was a correlation between environmental Na+ concentration and plasma
Na+ concentration. Figure 2 shows the correlation; there is a relationship, but not an
extremely strong one to which the external environment would influence the internal
regulation of the fiddler crab.
180
160
140
120
100
80
60
40
20
0
Concentration
(microg/L)
Environmental concentation
(microg/L)
200
Internal(left bar) vs external(right bar) concentration
Figure 1: Hemolymph concentration of fiddler
crabs in comparison to environmental salt
concentration.
150
100
50
y = 8.9809x - 1130.2
2
0
132
R = 0.815
134
136
138
140
142
144
Figure 2: Correlation between Na+ concentation of
environment to fiddler crab plasma (with trendline.
Without a more equalized collection between males and females an accurate difference
cannot be calculated.
146
Plasm a concentration (m icrog/L)
148
Discussion
The purpose of this lab was to examine Uca pugilator hemolymph exposed to
four different salinities and compare the Na+ concentration to the concentrations of the
four salinities of the external environments to determine whether the fiddler crabs are
osmoregulators or osmoconformers. Uca pugilator is an osmoregulator. The results
suggest that while the external environment changes drastically when the tides changes,
the fiddler crabs maintain a more constant internal temperature. There was a wide
difference between the numbers of females to males. “Greater experimental variability is
found between different batches of crabs than between the sexes of the two species”
(Green et al 1959). Other studies of the regulation of the Uca pugilator also support the
osmoregulation process. “The ability of these crabs to maintain their sear hypo-osmotic
to the medium in both normal and concentrated sea water as shown by the osmotic
concentration” (Green et al 1959). Because of the drastic changes in there habitats with
the changing tides and salinity level differences, Uca pugilator osmoregulate their
internal Na+ concentrations to maintain a homeostatic internal environment. After
looking at the information from the data collected, osmoconformers tend to need a
constant environment so the organism can conform to the external environment. Extreme
shifts in water versus salt concentrations would be dangerous to an osmoconformer
because of the drastic changes that a homeostatic system would have to undergo on a
continuous basis to keep the body in check. The hemolymph of the fiddler crab, Uca
pugilator maintains a relatively constant internal Na+ concentration regardless of the
changing environmental surroundings and is an osmoregulator (homeoosmotic
organism).
Literature cited
Crain D.A. 2007. Bio 412: Animal Physiology Laboratory Manual. 36 pgs.
Green J.W., Harsch M, Barr L, and Prosser C.L. 1959. The Regulation of Water and
Salt by the Fiddler Crabs, Uca pugnax and Uca pugilator. Biological Bulletin
116:76-87.
Pearse A.S. 1927. The Migration of Animals from the Ocean into Freshwater and Land
Habitats. The American Naturalist 61(676): 466-476.
Thurman C. 2003. Osmoregulation in fiddle crabs (Uca) from temperate Atlantic and
Gulf of Mexico coasts of North America. Marine Biology 142 (1): 77-92.
Zanders I.P, and Rojas W.E. 1995. Osmotic and ionic regulation in the fiddler crab
Una rapax acclimate to dilute and hypersaline seawater. Marine Biology 125(2):
315-320.