Study Of Shore Crab Response To Intertidal Stimuli

.. s 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).

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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. Science Essays.