Essay About Study Of Crabs And Hemigrapsus Sanguineus

Study of Crabs to Intertidal Stimuli
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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 Celsius
, with the air temperature slightly above
freezing (0-1 degree Celsius
). 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 Celsius
, while the
remaining four were placed in the deli case at a temperature of 5 degrees Celsius
. 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