Crab Study Essays - Oceanography, Grapsoidea, Aquatic Ecology

Crab Study 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