Ecological Niche of the Crab

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INTRODUCTION: The ecological niche of the crab Hemigrapsus edwardsi.

The crab is a member of the Crustacea phylum and is in the family Grapsidae. This crab species is found only in New Zealand on rocky shores. The rocky shore, where the crabs studied in this investigation were found is quite exposed. There is a large rock platform that provides small crevices and small rocks which help to protect them from wave action and predators. There are also sea lettuce, and other algae growing on parts of the rocks.

The crab has many adaptations that allow it to live on the rocky shore including: • grey/black colour for camouflage
• food detection structures (antennae on its head and hairs on the mouth parts to sense chemicals in the sea water) • 4 pairs of legs with muscles that allow it to move sideways, as well as forwards and backwards • behavioural adaptations such as scuttling under rocks when the tide goes out or to avoid predators • freezing when being attacked (we noticed this when we touched some of them on their backs). This might confuse predators. • gills for gas exchange.

The rock pools provide a micro-climate where the temperature and salinity of the water will change, depending on the weather. If it was a really hot day, the rock pools will get warmer, more water will evaporate and the salinity will increase. The crab would have to be adapted to cope with these changes in salinity, otherwise, as the concentration of salt in the water around it changes, it will gain or loose mass due to osmosis. Through the process of osmoregulation, the crab is able to maintain a constant water balance in its body, but to do so requires energy and this could be measured by an increase in the respiration rate. This is what I am going to investigate.

Aim: To determine whether the respiratory rate of the crab changes in different salinities.

Hypothesis: The respiratory rate of the crab will increase as the salinity changes away from “normal” salinity.


Thirty crabs of similar size, were collected from the rocky shore.

The salinity was varied by diluting the 200% conc. seawater provided in to five different concentrations. The volume of the solutions was 200mL each. The concentrations were 150%, 125%, 100%, 75%, 50% conc. Water with 100% concentration is equivalent to the concentration of normal seawater.

Sixty-five mL of the 150% solution was poured into a petri dish. The petri dish had a thin layer of stones in the base to recreate the natural environment of the crabs.

One crab was put into this petri dish and left for five minutes to allow them to adjust to the conditions.

The petri dishes were floated on a water bath which was set at 180C. The water bath was placed in the room where the light intensity was the same for all petri dishes.

An indirect method of estimating respiration was used. This was to count the number of currents seen on the surface of the water. I assumed that a higher number of currents indicated a higher respiration rate. A little bit of carmine red was sprinkled onto the solution in the petri dish in order to see the movement of water more clearly. The crabs were left in the petri dishes for five minutes before counting the respiratory currents. The respiratory currents were counted for 30 seconds. This amount was doubled to give the rate per minute.

5 more trials with different crabs were done at each concentration. Each time a fresh 65 mL of solution was added.

The results were recorded and the rates per minute were averaged. This process was...
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