_crabs _
By: mike haff
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 necessaryThe 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.
_Bibliography _
McDermott. Crab studys. Journal of Biology. 1998. 36:36-42
Word Count: 2507
| 
