Study of Shore Crab Response to Intertidal Stimuli

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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 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.

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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 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 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 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.

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 To conduct this experiment, a five gallon styrofoam box was used for the warm air experiments.

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.

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. 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.

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.

The differences between survival and mortality of Hemigrapsus sanguineus at various salinities was statistically signifigant (Contingency Hemigrapsus sanguineus showed lower rates of survival in waters with temperatures varying from room temperature (25 degrees Celcius), over a range of salinities.

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. The differences between survival and mortality of Hemigrapsus sanguineus in various water temperatures was statistically signifigant.

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.

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.

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