Horseshoe Crab Density

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Updated on Feb 11, 2009


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.


  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?


  1. I predict the tanks with 200 Horseshoe Crabs will maximize the growth rate. I think this because my background information has told me that I should put 100 Horseshoe Crabs in each 5-gallon tank. I am using a 20-gallon tank.
  2. I hypothesize the Horseshoe Crabs died for one of a few reasons. First of all, the food may have been wrong. Perhaps brine shrimp is not the correct thing to feed juvenile Horseshoe Crabs. The live ones in the Nursery could have fed off the dead in the tank. Also, the food may not have been enough and the Horseshoe Crabs may not have had enough food to live. Second, these Horseshoe Crabs may have died naturally. Usually, 1 out of 100 Horseshoe Crabs live in the wild, and my tanks held 700 Horseshoe Crabs, so 7 could have lived in all. This is only if the numbers lined up exactly and in nature that doesn’t happen.


To Set-up Tanks:

7- 10 Gallon Aquarium Tanks
Small Airstone
Air Tubing
Sponge Filter
Aquarium Tank Lid
Air Pump
3-way Valve
Marine Sand
Marine Salt
Floating Thermometer
Horseshoe Crabs

Water Monitoring:

Nitrate Test Kit
Ammonia Test Kit
Data Sheet


1 oz Packet of Sally’s Frozen Baby Brine Shrimp
1 mL Pipet
Horseshoe Crabs
Disposable Plastic Container with Lid


Turkey Baster
Petri Dish
Data Sheet
Horseshoe Crabs


To Set-Up Tanks:

  1. Find place for tank, must have easy access to electrical outlet and be away from windows.
  2. Clean tank.Let dry thoroughly.
  3. Place 1/4 inch layer of fine marine sand in the bottom of the tank.
  4. Fill tank with fresh non-chlorinated water.
  5. Add marine salt to tank until you get a salinity of between 15 and 18 ppt.
  6. Let tank sit for 24 hours
  7. Cut 1-foot length of air tubing. Connect one end to the air pump and the other to the 3-way valve.
  8. Cut another 1-foot length and 1.5 foot length of air tubing. Attach shorter length to air stone and the other end to the 3-way valve. Connect the 1.5 length to the sponge filter and the other end to the 3-way valve.
  9. Place air stone and filter in tank. Plug in air pump.
  10. Place Floating thermometer in tank.
  11. Put on tank lid.
  12. Repeat Steps for 7 tanks
  13. Put 50 Horseshoe Crabs in 2 tanks, 100 in 2 more and 200 in 2 tanks, this will be the experimental groups
  14. Put any extra Horseshoe Crabs in Nursery tank

Water Monitoring:

  1. Fill sample bottle to line with sample water.
  2. Add 10 drops of Nitrate Solution 1
  3. Cap.Invert Slowly 6 times
  4. Shake Nitrate Solution 2 for 30 seconds
  5. Add 10 drops of Nitrate Solution 2
  6. Cap. Invert slowly for 1 minute
  7. Wait for 5 minutes
  8. Compare sample color with color card
  1. Fill sample bottle to line with sample water.
  2. Add 8 drops of Ammonia Solution 1
  3. Add 8 drops of Ammonia Solution 2
  4. Cap.Invert slowly 10 times.
  5. Wait for 5 minutes
  6. Compare sample color with color card
  1. Fill hydrometer to line with sample water
  2. Let sit on flat surface for 1 minute
  3. Follow arrow to number of ppt


  1. Cut open package of frozen baby brine shrimp. Put in Disposable container.
  2. Let melt completely
  3. Use pipet to suck up set amount of brine shrimp.
  4. Squeeze into tank
  5. Repeat steps 3-4 for each tank stirring brine shrimp between each addition.
  6. Put Brine shrimp in freezer


  1. Blow water hard into the sand with turkey baster to move sand around until you find a Horseshoe Crab.
  2. Using turkey baster, suck up the Horseshoe Crab found, put in petri dish with small amount of water.
  3. Set Caliber to 0.
  4. Hold Caliber up to Horseshoe Crab while rolling out until the measurement is correct. Measure both length and width.
  5. Record on data sheet.
  6. Place Horseshoe Crab in empty tank so as not to measure it again.




What Population Density Maximizes The Growth of Juvenile Limulus Polyphemus:

In this experiment I was trying to see what population density maximized the growth of juvenile limulus polyphemus. I raised Horseshoe Crab under different population densities then measured and recorded their size over a 2-week period. I found that the Horseshoe Crabs were dead in my experimental groups so I could not measure them. I hypothesized that the Horseshoe Crabs would grow the most in the tanks with 200 Horseshoe Crabs in them. I believed this because in my background information it said to put about 100 Horseshoe Crabs in each 5-gallon tank. I was using a 20-gallon tank. So I am unable to answer this problem.

Why Did The limulus polyphemus Die:

In this experiment I was trying to see why all my limulus polyphemus died. I read up on raising Horseshoe Crabs and asked experienced people to find the answer. I hypothesized that the Horseshoe Crabs died because the brine shrimp was not the correct thing to feed them, the food may not have been enough, or, the Horseshoe Crabs died naturally. Usually, 1 out of 100 Horseshoe Crabs live in the wild my tanks held 700 Horseshoe Crabs, so 7 may have lived.But only if the numbers came out exactly and in nature that doesn’t happen. After all my research I have decided the natural death is the most reasonable. My Horseshoe Crabs were just a bad group and died off. It is possible that a couple errors may have occurred in my research. I may have asked the wrong people.I also could be wrong, maybe there is an answer I haven’t even thought about. So, my Horseshoe Crabs died naturally because only about 1 out of 100 Horseshoe Crabs live in the wild. I can use this knowledge to do a better project next year by understanding how to keep Horseshoe Crabs alive in captivity.

How Much Do limulus polyphemus Grow In A Five Month Period Of Time:

In this experiment I tried to see how much limulus polyphemus grow in five months. I raised Horseshoe Crabs, then measured and recorded their size over a 2-week period. I predicted Horseshoe Crabs grow about 7 millimeters long in 5 months. I think this because in 10 years, Horseshoe Crabs only grow about 1.5 feet. In less than half a year they won’t grow a whole lot.It turns out the mean length of a Horseshoe Crab is 8.35 millimeters. I took measurements of all the Horseshoe Crabs I could find and that was 34. The mean Horseshoe Crab length was 8.35 mm, the mode was 9.22 mm, and the median was 6.47. In my answer there is only a variation of 1.5 making the numbers all very similar. This small amount of variation in my data makes my answer accurate and my confidence level higher. In my experiment I could have measured wrong. I may have jerked the caliber too far in or out and gotten an incorrect measurement. Also I may have accidentally measured a dead Horseshoe Crab. Next year for my experiment I might try to compare different types of food such as clams versus brine shrimp with maximum growth rate. So, a Horseshoe Crab grows 8.35 millimeters in 5 months. We can use this experiment to better understand the growth and life cycle of Horseshoe Crabs.


  1. Life in the Chesapeake Bay By: Alice Jane Lippson & Robert L. Lippson
  3. Horseshoe Crabs in the Classroom Project Packet (provided by DNR)