Study of Crabs to Intertidal Stimuli

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 Celsius

, with the air temperature slightly above

freezing (0-1 degree Celsius

). 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 Celsius

, while the

remaining four were placed in the deli case at a temperature of 5 degrees Celsius

. 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