Juvenile Blue Crab Cannibalism

Part of the Young Naturalist Awards Curriculum Collection.

by Lauren, Grade 11, Maryland - 2008 YNA Winner
Figure 1: Highly aggressive juvenile blue crabs utilized in experiment.

In recent years, the Chesapeake Bay's vital blue crab population has plummeted due to habitat destruction and overfishing. This has prompted extensive aquaculture efforts to increase the blue crab population, especially in the juvenile stages. However, a major problem in crab aquaculture is cannibalism, especially in the absence of wild bay grass habitats. In captive habitats, 75% to 97% of juvenile crabs die from cannibalism. This is a very difficult yet important factor to limit when the goal is to increase the species' population.
My project looks at how juvenile blue crab population densities in captive habitats influence cannibalism, with the hypothesis that cannibalism increases with higher density and declines with lower density. The 85 Year 0 (less than three inches long) juvenile crabs used in my experiment were placed in three separate populations of 10 crabs (10%), 25 crabs (30%), and 50 crabs (60%). These populations were divided into separate containers so the crabs could not move between them, yet they shared a communal sump and biofilter, resulting in an even water flow. After 18 days and 10 data collections, I measured the results by counting mortality and limb loss, and then analyzed these results on graphs. The final measure of cannibalism in my study was the "cannibalism index," which was found by adding mean daily mortality ½ mean daily limb loss. The results showed that cannibalism did indeed increase with population density, as the small population had an index of 26.7%, the medium an index of 32.9%, and the large an index of 33.4%.

A close-up of the open palm of a human hand holding a juvenile blue crab. The animal is about two inches in width.
Figure 2: Year 0 juvenile crab used in experiment, approximately 2 inches in width.


To Marylanders, the blue crab is much more than a favorite dish; it is a symbol of our culture. While many Marylanders no longer work on the Chesapeake Bay, we still take pride in our heritage, and the heart of this heritage is the Chesapeake Bay waterman. While we enjoy the bay for swimming, boating, and sailing, we have great respect for our neighbors who get up early every morning to pursue the age-old, difficult, yet rewarding profession of being Chesapeake Bay watermen. However, in recent years, the health of the bay has been rapidly declining. On school field trips we no longer focus on the great aspects of the largest estuary in the world.
Instead, we learn that the bay is sick and dirty and nearly beyond help. As a consequence, schoolchildren no longer want to swim in the local state parks. As a Marylander who enjoys sailing and swimming in the bay, I understand how important it is to improve the health of the bay while still relishing the grandeur of our local treasure. And I have great respect for the watermen who get up early to go out on their workboats, which have names like Miss Suzie . I know the preservation of our bay is even more important for those whose lives and professions revolve around the bay. Therefore, I decided that I needed to give back to the Chesapeake Bay ecosystem. For my project in my science research class, I decided to aim at helping to rejuvenate the blue crab population. When I found out about the difficulties of raising juvenile blue crabs in aquaculture research facilities, I chose to address the problem of cannibalism among juvenile blue crabs in captive habitats.

Plastic bins with water and live crab specimens under a filtration system.
Figure 3: Commune Biofilter shared by three tank populations with even water distribution.


For my main project, I developed an experiment to test whether cannibalism in juvenile blue crabs ( Callinectes sapidus ) increases with population density. The blue crab is vital to the Chesapeake Bay ecosystem, serving as both prey and predator in a fragile food web. The blue crab also serves as the main livelihood for Chesapeake Bay watermen. But its population has declined more than 85% in the last 10 years due to habitat destruction and overfishing (Chesapeake Bay Program). Dr. Charles Epifanio, a marine biologist, explains the importance of this relatively mysterious species: "From an economic standpoint, the blue crab is the most valuable shellfish in the mid-Atlantic region, so it's critical that we understand the factors that control its population." Conservation aquaculture and scientific institutions have been working to increase this dwindling population.
A major problem in crab aquaculture is cannibalism, which makes their survival rates extremely low in captive habitats (NOAA). While the crabs' favorite food is thin-shelled bivalves, as much as 25% of an adult blue crab's diet is other crabs (Hines, Shirley, and Walcott). In the Year 0 class, cannibalism is the major cause of juvenile mortality and accounts for 75% to 97% of juvenile mortality (Speir). Crabs are highly aggressive, and it has been suggested that crab cannibalism peaks when densities exceed 15 crabs per cubic meter; it is a common fate for crabs that are in poor health, are missing important limbs, or that have recently completed a molt (Hines, Shirley, and Wolcott).

Figure 4: Flake based food.

In wild habitats, the blue crab hatches as a 0.25 mm zoea larva, which lives on the surface of the Chesapeake Bay until it hatches. Then it moves out to the open sea and undergoes a series of molts until it reaches about 1 mm in size (Blue Crab Archives). After about 50 days, the zoea larva becomes a megalops larva, which appears more crablike and grows for another 20 days. During this time, it migrates back to the bay. The next stage, known as the Year 0 juvenile stage, begins when the crab is between 2.5 mm and 76.2 mm (3 inches) in length. This stage lasts about a year, as the juvenile undergoes 18 to 20 molts in the lower-salinity areas of the estuary or in a river (Blue Crab Archives).
The crabs utilized in my experiment were Year 0 juvenile crabs, which are all less than three inches in size (Figure 2). As adults, a blue crab can reach nine inches in width and half of that in length, and live up to three years in the mid-Atlantic region. A female will only mate once in her life, right after she molts, when she is called a "sook" (Chesapeake Bay Program). She stores the sperm in sacs to be fertilized at a later time, usually from May to August. She releases a few million eggs into the high-salinity waters of the bay; only a small percentage survive to adulthood (Epifanio and Garvine). Larvae and juvenile crabs are easy targets for a wide variety of predators, including fish, shrimp, birds, eels, and of course, other crabs. Juvenile crabs and adult crabs alike have a wide diet, ranging from detritus to mollusks to other crabs (Blue Crab Archives). In wild habitats, cannibalism is common and may even be necessary to regulate the population. However, in juvenile crabs, submerged aquatic vegetation (bay grasses) is the key habitat for reducing extreme cannibalism among the vulnerable juveniles in low-salinity areas of the estuary. In recent years, the health of the bay has declined, and the amount of bay grass has dropped more than 50 percent, to 75,000 acres from more than 200,000 acres in 2005 (Chesapeake Bay Program).

Figure 5: Ammonia test on day 9 (January 2) showing ammonia levels of between 0 and .25 ppm

Regardless of the dwindling amount of bay grasses in the wild, aquaculture facilities are unable to provide captive juvenile blue crabs with these nurturing habitats, which greatly increases juvenile blue crab cannibalism. Facilities such as the UMBI Center of Marine Biotechnology have experimented with alternative structures, such as mesh, to reduce cannibalism, but the incidence is still extremely high. Thus, the purpose of my experiment is to reduce cannibalism in captive habitats by understanding how population densities affect cannibalism. My experiment was set up with three different population densities, with the proposal that higher population densities will lead to increased cannibalism, and smaller population densities will lead to less cannibalism among juvenile blue crabs.


On December 6, I obtained around 150 Year 0 juvenile blue crabs, about a quarter-inch in size, from the Smithsonian Environmental Research Center (SERC) in Edgewater, MD. The tank provided for them had dimensions of 45 by 41 by 36 inches. Their food was flake-based and was provided by SERC (Figure 4). 

Figure 6: Performing one of the many time consuming water changes.

As the juveniles were growing, I was busy devising a three-in-one tank setup so the populations could be divided and the crabs couldn't move between tanks (Figure 3). Unfortunately, the ammonia levels in the original tank rose to dangerously high levels, up to 2 PPM, at the beginning of the experiment, resulting in a high mortality level among the juveniles. The high ammonia levels called for many time-consuming water changes (Figure 6). I finally designed a new setup for the tank in which three separate bins, each measuring 20 by 14 by 7 inches, were placed inside the larger container, with a communal "bucket" biofilter. The container itself served as a sump that was evenly shared by each three populations (Figure 7).

The three containers were free of any structure, such as mesh, that would limit cannibalism.
On December 17, the actual experiment began. The surviving 85 juvenile crabs were divided into populations of 50 crabs (60%), 25 crabs (30%), and 10 crabs (10%). Levels of food were as proportionate to population as possible, to limit any outside variables in the experiment. The ideal water temperature was near but not above 15ºC, which is when important growth can occur, since blue crab growth is regulated by water temperature. The salinity was around 15 to 17 PPT. The high ammonia levels had dropped to a near-perfect level of 0 PPM by the end of the experiment (Figure 5).

Figure 7: The "3 in 1" tank system comprised of three separate tanks in one recirculation filtration

Over a period of 18 days, or 10 data collections, I analyzed daily mortality, limb loss, and total mortality by simply counting dead crabs and limb loss in living crabs on each data collection. In analyzing the results, I used six different data sets, including median daily mortality, mean daily mortality, total mortality, median daily limb loss, mean daily limb loss, and the cannibalism index. The "cannibalism index" is an equation that places mortality and limb loss together when calculating overall cannibalism: mean mortality ½ mean limb loss = cannibalism index.

I analyzed my results by recording them in six separate graphs, which are summed up in one graph below. The clear trend is that cannibalism, both mortality rate and limb loss rate, increases with population density, which is the case with all the data sets except for median daily limb loss. I used the cannibalism index as the overall measurement of cannibalism in juvenile crabs, because it was the clearest way of analyzing limb loss and mortality as one data set. In the cannibalism index, cannibalism increased 18.4% between the small population (10 crabs) and the medium population (25 crabs), and 1.5% between the medium population (25 crabs) and the large population (50 crabs).

Graph 1: Complete data set for juvenile blue crab cannibalism.


I collected sufficient data over an acceptable time period to gather satisfactory results for my experiment. My hypothesis was correct, in that cannibalism increases with higher population densities and declines with lower population densities. There was a much greater increase between the lowest population density and the middle population density than between the middle population density and the highest population density, suggesting that cannibalism greatly increases when the density is altered from a relatively low number of crabs to a moderate number of crabs. My conclusions were in line with other research, although I could find no experiment that directly tested the impact of population densities on juvenile crab cannibalism.
My data matched the research that suggested that cannibalism is the cause of 75% to 97% of crab mortality, since the small population density had a total mortality rate of 80% and the medium and large populations each had a total mortality rate of 88% (Speir). The population densities were definitely greater than 15 crabs per square meter, which is where a previous study found that cannibalism increased (Hines, Shirley, and Wolcott). Although my data was sufficient, high ammonia levels in the tank were a huge problem in the beginning, and 65 crabs died before the experiment even started. This did not directly impact my experiment, but it would have been useful to have 150 crabs to start out with instead of 85 crabs. Also, the data was collected over Christmas break, so I could not get daily collections. The time period during which cannibalism increased the most, as shown on Graph 2, occurred during this time period, possibly because the crabs could not be fed every day or have their ammonia levels tested.

Graph 2: Daily crab mortality.

Variables were not a huge problem in the experiment. Since each separate container had the same communal water flow system, water quality remained constant between each container. Food was distributed in relation to the amount of crabs, though not by weight measurements but merely by guessing. If I did this project over again, I would use more crabs, fix the ammonia levels early on, and pursue more consistent data collections. However, my results still proved that cannibalism definitely increases with population density. I hope further research will suggest possibilities for how to decrease juvenile crab cannibalism in aquaculture habitats in order to help save the dwindling blue crab population.


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