Horseshoe Crab Density

Abstract

I chose to study Horseshoe Crabs in May of 2001 after listening to a scientist from DNR talk about the subject of raising Horseshoe Crabs. Then I talked to my science teacher and he said he would allow me to set up a few tanks in his classroom. From there, I was privileged enough to go on a spawning survey. My science teacher, my mother, and I went with a young DNR employee to count the number of Horseshoe Crabs spawning on a beach. After that, I was ready to start cultivating. I set-up seven tanks that would hold my Horseshoe Crabs. I received at least a thousand Horseshoe Crabs from the DNR scientist. I was ready to begin!

In the beginning, this project concerned only one problem. That was, what population density would maximize the growth of juvenile Horseshoe Crabs. I set up seven tanks with different amounts of Horseshoe Crabs in each one; two had 50, two had 100, and two had 200. Then I had another tank that was the nursery, all the extra Horseshoe Crabs went in there. Once a week I took water quality tests on all seven tanks and twice a week I fed them.

I planned on measuring them for two weeks in November. The way I would do this is to blow up the sand with a turkey baster. Then suck up the Horseshoe Crab with the baster and put it in a petri dish. Using a millimeter caliber, measure the length and width of the Horseshoe Crab. Unfortunately, when I went to measure the Horseshoe Crabs, I found them all to be dead! The only crabs left were the ones in the nursery. Now I had three problems, the original problem that I couldn’t answer, why they died, and how much a Horseshoe Crab can grow in 5 months.

The first problem I tried to answer was, how much Horseshoe Crabs can grow in 5 months. I used the turkey baster method and found 34 live Horseshoe Crabs in the nursery. Each one I measured with the caliber then put in a tank that was empty because all its occupants had died. I found out that a Horseshoe Crab grows about 8.35 millimeters in length. So my hypothesis was very close to being correct, only about 1 millimeter off. The next problem was why did the Horseshoe Crabs in my experimental tanks die? I e-mailed a scientist from DNR and asked him for his opinion. I also asked my science teacher about it. I have concluded that the Horseshoe Crabs died a natural death. In the wild, 1 out of 100 Horseshoe Crabs live, and I only put 700 Horseshoe Crabs in my tanks. So, only about 7 would have lived but numbers don’t match up exactly. In my whole experiment there is always the factor of Human error. Most likely, when I went to make sure all my Horseshoe Crabs were dead, I missed one or two in a tank being alive. I could use all this data for next year’s experiment to better understand Horseshoe Crab life cycle.

Problem 

1. What population density maximizes the growth of juvenile limulus polyphemus?
2. Why did the limulus polyphemus die?
3. How much do limulus polyphemus grow in a five month period of time?

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