Stimuli Essay, Research Paper
INTRODUCTION-
Since its itroduction to the New Jersey shore in 1988, the western Pacific shore
crab Hemigrapsus sanguineus has spread to inhabit rocky intertidal locations along the
Atlantic coast from Massachusetts to North Carolina (McDermott 1998). Many reasons
have been proposed to explain the rapid spread of this non-indiginous species. For
example, it has been shown that H. sanguineus has longer spawning periods along the
mid-Atlantic coast than it does in its natural habitat in the western Pacific Ocean, due to
a more favorable climate (Epifanio et al 1998). For this reason, these crabs are able to
spawn more times per season than indigenous crabs, providing one possible hypothesis
for their population explotion.
For this species to expand its range along the Atlantic coast, it will need to have
wide tolerances to temperature and salinity. In 1998 Epifanio found that
The purpose of this study is to show the tolerance and behavioral responses of H.
sanguineus to varying water and air temperatures, and water salinity concentrations. It is
believed that these crabs will be very tolerant to the various extreme conditions that they
will be put through. It is the ability of these crabs to survive in these unfavorable
situations that is key to their success. This experiment was also designed to prove the
hypothesis that the tolerance of H. sanguineus to various environmental factors increases
with size.
METHODS AND MATERIALS-
In February 2000, a field trip was taken to Crane Neck Point to collect live
specimens for the experiment. The field trip was conducted at low tide. The water
temperature was approximately 3 degrees Celcius, with the air temperature slightly above
freezing (0-1 degree Celcius). Live crabs were obtained by overturning rocks in the
intertidal zone. Hemigrapsus sanguineus was found at all levels of the intertidal zone,
although their numbers increased as one moved toward the waterline. The crabs were
collected with no distinction toward size. The sizes of the specimens collected were
found to range from 0.5 to 4.2 cm. The crabs were collected in a plastic five gallon
bucket. Water was added to the bucket to keep the crabs from dehydrating. The crabs
were taken back to the lab, where they were kept in the plastic five gallon buckets for a
few weeks until the experiment began. Air hoses were added to the buckets in order to
oxygenate the water. The water was changed, as necessary.
The first experiment conducted was the experiment regarding water temperature
and salinity tolerances. The objective was to conduct an experiment that would provide
measurable data on the tolerance of H. sanguineus in various water salinities, over a
range of temperatures.
To conduct this experiment, 8 one gallon acrylic tanks were obtained. Four were
used for the cold temperature experiment, and four were used for the room temperature
experiment.
Next, water of varying salinities were produced. We started with seawater that
had a salinity of 30 parts per thousand. To obtain water with a salinity of 15 parts per
thousand, the sea water was slowly diluted by adding tap water. The water was added
slowly, and frequently checked with a salinity refractometer until the desired salinity of
15 parts per thousand was obtained. The water was further diluted, using the method
above, to obtain the 5 parts per thousand water. To obtain the water with a salinity of 40
parts per thousand, the 30 parts per thousand sea water was again used, but this time was
left uncovered as to allow for water evaporation. After several days, and frequent testing
with the salinity refractometer, the water had a salinity of 40 parts per thousand. The
containers of water were covered with plastic wrap, as to prevent evaporation, and keep
the salinities constant. In additional a layer of mesh was used to cover the top of each
container, to prevent the crabs from escaping (Figure 1).
Four of the containers were left to stand at 25 degrees Celcius, while the
remaining four were placed in the deli case at a temperature of 5 degrees Celcius. An air
hose was added to each of the containers, in order to oxygenate the water. Ten crabs, of
a varying range of sizes, were added to each container. The crabs, once again, ranged in
size from 0.5 – 4.2 cm. In the first trial 15 fish food pellets were added to each container
in order to provide the crabs with food, and hopefully reduce cannibalism. This was
repeated a second an third time for both the 25 degree Celcius and 5 degree Celcius
experiments, with the absence of fish food pellets.
The next experiment that was conducted was the air/water temperature
experiment. The objective was to conduct an experiment that would provide measurable
data on the preference of submergence of H. sanguineus when air and water temperatures
differ. The experiment was also designed to determine the preference and tolerance of
the crabs, as a function of size.
To conduct this experiment, a five gallon styrofoam box was used for the warm
air experiments (Figure 2). The bottom of the container was covered with rocks. A one
gallon acrylic container was placed in the center of the five gallon container. A plastic
mesh was draped over the sides of the one gallon container. The one gallon container
was filled with sea water having a salinity of 30 parts per thousand. Surgical tubing was
coiled and placed at the base of the five gallon styrofoam container. The tubing was
connected to a refrigerated bath/circulator that was actually used to heat the air in the
container to a temperature of 26 degrees Celcius. Surgical tubing was again coiled, but
this time placed in the water. The tubing was connected to a water pump in a five gallon
bucket of 25 degree Celcius water. Five large and five small crabs ranging in size from
0.5-4.2 cm. were added to the water of the one gallon acrylic tank. Five large and five
small crabs were also added to the styrofoam container.
A five gallon acrylic container was obtained for the cold air/warm water
experiment (Figure 3). The base was covered with rocks. A one gallon acrylic container
was placed in the center of the five gallon container. The container was once again
draped with plastic mesh. The entire five gallon container was placed in the deli case
with a temperature of 6 degrees Celcius. Plastic tubing was coiled and then placed into
the water of the one gallon acrylic container. The tubing was connected to a pump
placed in a five gallon bucket of 25 degree Celcius water. Five large and five small crabs
were added to the water of the one gallon acrylic tank. Five large and five small crabs
were also added to the five gallon acrylic container. Air tubes were placed in the water
of each one gallon container in order to oxygenated the water, and prevent hypoxic
conditions. The experiment was repeated four times.
The nuissance variable that most effected this experiment was the cleanliness of
the water. Being that small one gallon containers were used in this experiment, the water
became dirty quickly. The health of the crabs was undoubtedly effected. The crabs also
maintained an incredible ability to escape. The crabs were able to climb up the air tube
and seek escape via any cracks or holes on the top of the container.
RESULTS-
Hemigrapsus sanguineus displayed higher survival rates in water with salinities
lower than that of normal seawater (30 parts per thousand), over a range of temperatures
(Figure 4). While no crabs were found dead
per thousand, 7 of 30 (23%) crabs were found in the waters with salinities of 15 parts per
thousand, and 18 of 30 (60%) crabs were found dead in the waters with salinities of five
parts per thousand (Table 1). The differences between survival and mortality of
Hemigrapsus sanguineus at various salinities was statistically signifigant (Contingency
Table Analysis, G=47.840, p*0.05).
Hemigrapsus sanguineus showed lower rates of survival in waters with
temperatures varying from room temperature (25 degrees Celcius), over a range of
salinities (Figure 5). Twelve crabs died in the 35 degree Celcius water, 9 crabs died in
the 5 degree Celcius water, while only 4 crabs died in the 25 degree Celcius water; all
samples contained forty crabs (Table 2). The differences between survival and mortality
of Hemigrapsus sanguineus in various water temperatures was statistically signifigant
(Contingency Table Analysis, G=74.334, p*0.05).
Small Hemigrapsus sanguineus preferred to be submerged in water rather than
exposed to air, over a range of water and air temperatures. 47 of the 68 crabs were found
submerged in water, while only 21 crabs were found in the open air (Table 3). The
differences between the number of crabs that chose submersion versus emersion was
statistically signifigant (Chi-square test, chi-square=9.942, p*0.05). The survival rate of
large Hemigrapsus sanguineus is substantially greater in water than it is in open air
(Table 4).
Large Hemigrapsus sanguineus preferred to be exposed to the air rather than
submerged in water, over a range of water and air temperatures. In a sample sizeof 26
crabs, 19 crabs were found in the open air while only 7 crabs were found submerged in
water (Table 5). The differences between the number of crabs that chose submersion
versus emersion was statistically signifigant (Chi-square test, chi-square=5.538, p*0.05).
However, the survival rate of large Hemigrapsus sanguineus is substantially greater in
water than it is in open air (Table 4).
DISCUSSION-
In this experiment, Hemigrapsus sanguineus demonstrated tolerances to a wide
range of water salinities at various temperatures. The ability of H. sanguineus to survive
in a range of salinities may be a key factor in its rapid spread along the mid-Atlantic
Coast. The durability of the crab may give it an advantage over indigenous species, in
extreme conditions. H. sanguineus showed a survival rate of 100% in water with
salinities of 30 and 40 parts per thousand. Survival rate decreased slightly to 76.667% in
water with a salinity of 15 parts per thousand, and then fell to 40% in water with a
salinity of 5 parts per thousand (Figure 4). An important factor in the ability of H.
sanguineus to spread north to the colder waters New England and south to the warmer
waters of the southern Atlantic coast, is its ability to survive a range of temperatures.
Over a range of salinities, H. sanguineus showed survival rates of 90% at 25 degrees
Celcius, 70% at 35 degrees Celcius, and 77.5% at 5 degrees Celcius (Figure 5). This data
supports the above hypothesis. This data is also consistent with previous experiments
regarding tolerance of H. sanguineus larvae in a range of temperature/salinity
combinations (Epifanio et al 1998). However, some degree of experimental error was
present due to the fact that, by adding fish food pellets to the 35 degree Celcius water
with a salinity of 5 parts per thousand, it quickly became quite polluted and was not
changed in time to save the submerged organisms. The only survivor of the trial, was
able to do so by climbing out of the water via an oxygen tube. Efforts were made in
subsequent trials to change the water more often, as well as to deprive the crabs of food.
It has also been found that juvenile and adult H. sanguineus show high tolerance
levels for conditions with varying water and air temperatures. Greater survival of
juveniles to reproductive maturity allows for the population of H. sanguineus to grow
exponentially. The durability of the juvenile H. sanguineus may be an important factor
in the expansion of the species range along the Atlantic Coast, where it outcompetes
indiginous species. In this experiment, the juvenile H. sanguineus actually showed a
higher rate of survival than the adults, under identical experimental conditions. The
juveniles had a suvival rate of 61.42% while the adults had a survival rate of only 37.5%
(Figure 6). This may be a result of the experimental design favoring the smaller crabs.
In the small one gallon tanks, the small crabs had more room to move, relative to size.
The small crabs could also hide under the rocks and climb the mesh more easily than the
large crabs, which gave the smaller crabs greater freedom of movement. These results
support the above hypothesis. The high survival rates of the juvenile crabs was
consistent with the experiments conducted regarding tolerance of H. sanguineus larvae in
a range of temperature/salinity combinations. In which, the zoeal larvae showed an
increased capacity to survive in water temperatures below 25 degrees Celcius and water
salinity below 20 parts per thousand, relative to megalopa (Epifanio et al 1998).
One unforseen problem that was encountered in the first trial of this experiment
was the relentless ability of the crabs to escape from their tanks. The number of crabs
missing was the same for the adult and juvenile crab experiments. Being that the number
of crabs missing was small and consistent by size, the missing crabs did not effect the
outcome of this comparison. To correct this problem in later trials, a layer of mesh was
draped over the top of the tank and then secured with a rubber band (Figure 1). A
problem also arose in the cold air/warm water experiment when the heating element
failed to turn itself off and heated the water to temperatures exceeding 40 degrees
Celcius. Due to this equiptment error, alternate materials were used. Surgical tubing
was coiled and then placed in the water, then a pump was attached to the tubing and
placed in a five gallon bucket ; room temperture water was pumped through tubing thus
heating the cooler water contained within the tank.
To obtain more accurate results, the experiment above would need to be
replicated a number of times to assure consitency. The number of crabs used would also
need to be increased in number for the same reason. The size of the containers used
would also need to be increased, as to prevent problems with water pollution. Larger
containers would also create a more natural environment, and remove some volitility
over the competition that arises in a one gallon space. In addition, the experiments above
could be repeated in conditions where the crabs are fed periodically. This would indicate
if the lack of food in the above experiments in any way effected the behavior patterns and
the ability of H. sanguineus to survive in extreme environmental conditions.
To obtain more conclusive results, the experiment above should be reproduced
using a variety of species that are indiginous to the Atlantic coast of the United States.
The survival rate of those species should then be plotted against the survival rate of H.
sanguineus to determine if any have a selective advantage over one another, in terms of
durability to extreme conditions. This experiment could present more evidence to further
prove that tolerance to temperature and salinity variations is an important factor in the
spread of H. sanguineus along the Atlantic Coast of the United States.