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 in waters with salinities of 30 and 40 parts 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.