Sea urchins, prized for their edible gonads, resort to cannibalism when starved and forced into overcrowded tanks, according to new research.
There are no documented reports of cannibalism among wild sea urchin populations, according to the study, so little doubt exists that manmade conditions are driving this behavior. Aquaculture of sea urchins is particularly popular in Japan and China, where millions of tons are harvested each year.
Certain fish raised for human consumption, such as Cobia, trout and flounder, as well as many species of shrimp and lobsters, are also known to become cannibals when under stress. That is one reason the claws of lobsters are bound when these shellfish are confined in small fish market tanks.
If a sea urchin attacks another in a tight space, there isn't much the victim can do.
"A sea urchin cannot defend itself in the sense that it can't fight back," lead author Cristina Richardson told Discovery News. "Its only defense is to move away and, of course, the presence of spines is typically an effective mechanical defense."
But Richardson and her team observed cannibal sea urchins consuming these spines first. With the victim defenseless, the attacker then eats the other's vital, and nutrient-rich, organs. Slowly, over a period of about 24 hours, the cannibal consumes its neighbor until the victim becomes a lifeless blob.
Richardson conducted the research while at the University of Alabama at Birmingham's Department of Biology. For the study, she and colleagues John Lawrence and Stephen Watts collected 2,000 adult sea urchins from Port St. Joseph Peninsula State Park, Fla.
The researchers first placed the sea urchins in a recirculating synthetic seawater aquarium, where the animals were maintained for a week to adjust to the conditions. Then, mimicking common aquaculture practices, the researchers placed sea urchins in tanks. The size of the urchins and their density within the tanks varied.
Starvation and high density contributed to the greatest amount of cannibalism among small urchins, according to the study, which is published in the Journal of Experimental Marine Biology and Ecology. Fed, high-density conditions promoted cannibalism among large urchins. When up to 80 percent of the tank's surface area was taken up by large urchins, close to 20 percent of the animals became cannibals.
"Smaller sea urchins tend to cannibalize more under starvation conditions because they are actively growing, require more nutrients for somatic growth and have been shown to require higher levels of protein and carbohydrates in their diets than adults," Richardson explained.
She added that sea urchins are omnivores, and have even been described as the "goats of the sea" because they are driven to eat almost anything. The densities that humans subject them to do not exist in the wild, however, so urchins normally do not prey upon each other.
"This is similar to human behavior," she continued. "If a preferred food becomes unavailable, we eat foods we would not prefer."
Anthony Siccardi, an assistant research scientist at Texas A&M University, has conducted aquaculture studies over the past several years. He told Discovery News that he, too, has observed cannibalism, particularly in younger animals. He was surprised that, among larger sea urchins, it was the fed group that cannibalized more, putting greater emphasis on the effects of overcrowding.
Robert Fischer, a biology professor at the University of Alabama at Birmingham, informed Discovery News that he also agrees with the new study's conclusions.
"From the data collected, it indicates that people who are interested in creating a successful protocol for culturing intensive populations of sea urchins will have to deal with the obstacle of cannibalism, especially in small urchins," Fischer said. "Understanding these findings is an important first step in identifying the conditions for culturing sea urchins for commercial use.
Richardson is now at Texas A&M University, where she is focusing her research efforts on shrimp nutrition.