Danielle Sedoris
November 11, 2009
Ecology – Friday
Parasitic Crab Lab
Background:
Mellita quinquiesperforata, commonly known as the sand dollar, is a familiar urchin on Florida coastlines. The species is flat and disk shaped that nestles into shallow sandy waters to protect itself from waves and predators. Dissodactylus mellitae is a parasitic crab that feeds on the spines of the hosting sand dollar where it remains its entire life. This study was arranged to observe the distribution patterns of crabs on various sizes of sand dollars. We hypothesized that the larger sand dollars would host more crabs because of the extended surface area available. We also predicted that the presence of adult sized crabs would limit the number of total crabs on the hosting sand dollar because of the larger size and resource demand leading to intraspecific competition among the crabs. Intraspecific competition occurs when members of the same species compete over limited resources considered vital for survival. These resources can be food, space, light, mates, anything necessary for the survival and reproduction of the individual. In our study the factors of interest are space and food.

Methods:
We began our study by surveying Ft. DeSoto’s East beach for sand dollars and their parasites. In order to avoid duplicate data collection we began 100m offshore and worked our way bacl towards the shore placing all collected samples behind us after recording the data. For each sand dollar collected, we measured its diameter and counted the number of adult and juvenile crabs present on it. Adult crabs were considered to have a width of 2mm or more and juvenile crabs were considered anything below that. We surveyed the area until approximately 100 sand dollars were collected before returning to analyze the data.

Results:
Our results showed that there is no strong correlation between the size of sand dollars and the number of crabs present. When analyzing graph 1 I noticed that...

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a.
b.
c.
d.
e.
f.
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P (0< Z < 1.43)
P (0.11 < Z < 1.98)
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P (Z < 0.92)
P (Z > -1.78)
P (Z < -2.08)
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♦ z = 2.0
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