Since its itroduction to the New Jersey shore in 1988, the western Pacific shorecrab Hemigrapsus sanguineus has spread to inhabit rocky intertidal locationsalong the Atlantic coast from Massachusetts to North Carolina (McDermott 1998). Many reasons have been proposed to explain the rapid spread of this non-indiginousspecies. For example, it has been shown that H. sanguineus has longer spawningperiods along the mid-Atlantic coast than it does in its natural habitat in thewestern 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 thanindigenous crabs, providing one possible hypothesis for their populationexplotion. For this species to expand its range along the Atlantic coast, itwill need to have wide tolerances to temperature and salinity.
In 1998 Epifaniofound that The purpose of this study is to show the tolerance and behavioralresponses of H. sanguineus to varying water and air temperatures, and watersalinity concentrations. It is believed that these crabs will be very tolerantto the various extreme conditions that they will be put through. It is theability of these crabs to survive in these unfavorable situations that is key totheir success.
This experiment was also designed to prove the hypothesis thatthe tolerance of H. sanguineus to various environmental factors increases withsize. METHODS AND MATERIALS- In February 2000, a field trip was taken to CraneNeck Point to collect live specimens for the experiment. The field trip wasconducted at low tide. The water temperature was approximately 3 degrees Celcius,with the air temperature slightly above freezing (0-1 degree Celcius). Livecrabs were obtained by overturning rocks in the intertidal zone.
Hemigrapsussanguineus was found at all levels of the intertidal zone, although theirnumbers increased as one moved toward the waterline. The crabs were collectedwith no distinction toward size. The sizes of the specimens collected were foundto range from 0. 5 to 4. 2 cm.
The crabs were collected in a plastic five gallonbucket. Water was added to the bucket to keep the crabs from dehydrating. Thecrabs were taken back to the lab, where they were kept in the plastic fivegallon buckets for a few weeks until the experiment began. Air hoses were addedto the buckets in order to oxygenate the water. The water was changed, asnecessary. The first experiment conducted was the experiment regarding watertemperature and salinity tolerances.
The objective was to conduct an experimentthat would provide measurable data on the tolerance of H. sanguineus in variouswater salinities, over a range of temperatures. To conduct this experiment, 8one gallon acrylic tanks were obtained. Four were used for the cold temperatureexperiment, and four were used for the room temperature experiment. Next, waterof varying salinities were produced. We started with seawater that had asalinity of 30 parts per thousand.
To obtain water with a salinity of 15 partsper thousand, the sea water was slowly diluted by adding tap water. The waterwas added slowly, and frequently checked with a salinity refractometer until thedesired salinity of 15 parts per thousand was obtained. The water was furtherdiluted, using the method above, to obtain the 5 parts per thousand water. Toobtain the water with a salinity of 40 parts per thousand, the 30 parts perthousand sea water was again used, but this time was left uncovered as to allowfor water evaporation. After several days, and frequent testing with thesalinity refractometer, the water had a salinity of 40 parts per thousand.
Thecontainers of water were covered with plastic wrap, as to prevent evaporation,and keep the salinities constant. In additional a layer of mesh was used tocover 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 theremaining four were placed in the deli case at a temperature of 5 degreesCelcius. An air hose was added to each of the containers, in order to oxygenatethe 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 15fish food pellets were added to each container in order to provide the crabswith food, and hopefully reduce cannibalism. This was repeated a second an thirdtime for both the 25 degree Celcius and 5 degree Celcius experiments, with theabsence of fish food pellets. The next experiment that was conducted was theair/water temperature experiment.
The objective was to conduct an experimentthat would provide measurable data on the preference of submergence of H. sanguineus when air and water temperatures differ. The experiment was alsodesigned to determine the preference and tolerance of the crabs, as a functionof size. To conduct this experiment, a five gallon styrofoam box was used forthe warm air experiments (Figure 2). The bottom of the container was coveredwith rocks. A one gallon acrylic container was placed in the center of the fivegallon container.
A plastic mesh was draped over the sides of the one galloncontainer. The one gallon container was filled with sea water having a salinityof 30 parts per thousand. Surgical tubing was coiled and placed at the base ofthe five gallon styrofoam container. The tubing was connected to a refrigeratedbath/circulator that was actually used to heat the air in the container to atemperature of 26 degrees Celcius. Surgical tubing was again coiled, but thistime placed in the water. The tubing was connected to a water pump in a fivegallon bucket of 25 degree Celcius water.
Five large and five small crabsranging in size from 0. 5-4. 2 cm. were added to the water of the one gallonacrylic tank.
Five large and five small crabs were also added to the styrofoamcontainer. A five gallon acrylic container was obtained for the cold air/warmwater experiment (Figure 3). The base was covered with rocks. A one gallonacrylic container was placed in the center of the five gallon container. Thecontainer was once again draped with plastic mesh.
The entire five galloncontainer 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 gallonacrylic container. The tubing was connected to a pump placed in a five gallonbucket of 25 degree Celcius water. Five large and five small crabs were added tothe water of the one gallon acrylic tank.
Five large and five small crabs werealso added to the five gallon acrylic container. Air tubes were placed in thewater of each one gallon container in order to oxygenated the water, and preventhypoxic conditions. The experiment was repeated four times. The nuissancevariable that most effected this experiment was the cleanliness of the water.
Being that small one gallon containers were used in this experiment, the waterbecame dirty quickly. The health of the crabs was undoubtedly effected. Thecrabs also maintained an incredible ability to escape. The crabs were able toclimb up the air tube and seek escape via any cracks or holes on the top of thecontainer. RESULTS- Hemigrapsus sanguineus displayed higher survival rates inwater with salinities lower than that of normal seawater (30 parts perthousand), over a range of temperatures (Figure 4).
While no crabs were founddead 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 18of 30 (60%) crabs were found dead in the waters with salinities of five partsper thousand (Table 1). The differences between survival and mortality ofHemigrapsus sanguineus at various salinities was statistically signifigant(Contingency Table Analysis, G=47. 840, p*0. 05).
Hemigrapsus sanguineus showedlower rates of survival in waters with temperatures varying from roomtemperature (25 degrees Celcius), over a range of salinities (Figure 5). Twelvecrabs died in the 35 degree Celcius water, 9 crabs died in the 5 degree Celciuswater, while only 4 crabs died in the 25 degree Celcius water; all samplescontained forty crabs (Table 2). The differences between survival and mortalityof Hemigrapsus sanguineus in various water temperatures was statisticallysignifigant (Contingency Table Analysis, G=74. 334, p*0. 05). Small Hemigrapsussanguineus preferred to be submerged in water rather than exposed to air, over arange of water and air temperatures.
47 of the 68 crabs were found submerged inwater, while only 21 crabs were found in the open air (Table 3). The differencesbetween the number of crabs that chose submersion versus emersion wasstatistically signifigant (Chi-square test, chi-square=9. 942, p*0. 05). Thesurvival rate of large Hemigrapsus sanguineus is substantially greater in waterthan it is in open air (Table 4). Large Hemigrapsus sanguineus preferred to beexposed to the air rather than submerged in water, over a range of water and airtemperatures.
In a sample sizeof 26 crabs, 19 crabs were found in the open airwhile only 7 crabs were found submerged in water (Table 5). The differencesbetween the number of crabs that chose submersion versus emersion wasstatistically signifigant (Chi-square test, chi-square=5. 538, p*0. 05).
However,the survival rate of large Hemigrapsus sanguineus is substantially greater inwater than it is in open air (Table 4). DISCUSSION- In this experiment,Hemigrapsus sanguineus demonstrated tolerances to a wide range of watersalinities at various temperatures. The ability of H. sanguineus to survive in arange of salinities may be a key factor in its rapid spread along themid-Atlantic Coast. The durability of the crab may give it an advantage overindigenous species, in extreme conditions. H.
sanguineus showed a survival rateof 100% in water with salinities of 30 and 40 parts per thousand. Survival ratedecreased 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 (Figure4). An important factor in the ability of H.
sanguineus to spread north to thecolder waters New England and south to the warmer waters of the southernAtlantic coast, is its ability to survive a range of temperatures. Over a rangeof 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 datasupports the above hypothesis. This data is also consistent with previousexperiments regarding tolerance of H.
sanguineus larvae in a range oftemperature/salinity combinations (Epifanio et al 1998). However, some degree ofexperimental error was present due to the fact that, by adding fish food pelletsto the 35 degree Celcius water with a salinity of 5 parts per thousand, itquickly became quite polluted and was not changed in time to save the submergedorganisms. The only survivor of the trial, was able to do so by climbing out ofthe water via an oxygen tube. Efforts were made in subsequent trials to changethe water more often, as well as to deprive the crabs of food. It has also beenfound that juvenile and adult H. sanguineus show high tolerance levels forconditions with varying water and air temperatures.
Greater survival ofjuveniles to reproductive maturity allows for the population of H. sanguineus togrow exponentially. The durability of the juvenile H. sanguineus may be animportant 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, underidentical 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 aresult of the experimental design favoring the smaller crabs. In the small onegallon tanks, the small crabs had more room to move, relative to size.
The smallcrabs could also hide under the rocks and climb the mesh more easily than thelarge crabs, which gave the smaller crabs greater freedom of movement. Theseresults support the above hypothesis. The high survival rates of the juvenilecrabs was consistent with the experiments conducted regarding tolerance of H. sanguineus larvae in a range of temperature/salinity combinations. In which, thezoeal larvae showed an increased capacity to survive in water temperatures below25 degrees Celcius and water salinity below 20 parts per thousand, relative tomegalopa (Epifanio et al 1998). One unforseen problem that was encountered inthe first trial of this experiment was the relentless ability of the crabs toescape from their tanks.
The number of crabs missing was the same for the adultand juvenile crab experiments. Being that the number of crabs missing was smalland consistent by size, the missing crabs did not effect the outcome of thiscomparison. To correct this problem in later trials, a layer of mesh was drapedover the top of the tank and then secured with a rubber band (Figure 1). Aproblem also arose in the cold air/warm water experiment when the heatingelement failed to turn itself off and heated the water to temperatures exceeding40 degrees Celcius. Due to this equiptment error, alternate materials were used. Surgical tubing was coiled and then placed in the water, then a pump wasattached to the tubing and placed in a five gallon bucket ; room temperturewater was pumped through tubing thus heating the cooler water contained withinthe tank.
To obtain more accurate results, the experiment above would need to bereplicated a number of times to assure consitency. The number of crabs usedwould also need to be increased in number for the same reason. The size of thecontainers used would also need to be increased, as to prevent problems withwater pollution. Larger containers would also create a more natural environment,and remove some volitility over the competition that arises in a one gallonspace. In addition, the experiments above could be repeated in conditions wherethe crabs are fed periodically.
This would indicate if the lack of food in theabove experiments in any way effected the behavior patterns and the ability ofH. sanguineus to survive in extreme environmental conditions. To obtain moreconclusive results, the experiment above should be reproduced using a variety ofspecies that are indiginous to the Atlantic coast of the United States. Thesurvival rate of those species should then be plotted against the survival rateof H. sanguineus to determine if any have a selective advantage over oneanother, in terms of durability to extreme conditions. This experiment couldpresent more evidence to further prove that tolerance to temperature andsalinity variations is an important factor in the spread of H.
sanguineus alongthe Atlantic Coast of the United States.