Study Of Shore Crab Response To Intertidal

Stimuli Essay, Research Paper

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 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 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

the crabs, as a function of size.

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

air experiments (Figure 2). 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 (Figure 3). 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.

RESULTS-

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

(Figure 4). 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 (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).

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.