Sea Urchins: Subtidal Grazers
(Published in the Quoddy Tides, April 28, 2000)
The shores and waters of Cobscook Bay harbor a variety of creatures important to the area's economy. The green sea urchin, with its prickly shell of mobile spines and tube-like feet, is one of the more intriguing ones. At first glance, it is hard to believe such an odd-looking creature is one of the major movers and shakers of the subtidal marine community.
The green urchin has drawn attention in recent years because of its new-found economic value. Their reproductive organs, commonly called roe, are highly prized as a delicacy in Japan and other Asian markets. Commercial harvesting in Cobscook Bay and elsewhere along the Maine coast increased rapidly in the late 1980's as the supply of urchin roe from other places in the world decreased. Yet in becoming one of Maine's most important fisheries, the urchin harvest has shown a classic boom and bust cycle, with management struggling to keep pace with harvesting activities. As a result, sea urchin populations in many parts of the coast, particularly in southern Maine, have gone into what some researchers feel is a steep decline.
Understanding of the ecology of the sea urchin is limited, which in turn has limited the ability to better manage this resource to ensure sustainable commercial harvests. Professor Bob Vadas of the University of Maine has been part of recent efforts in Maine to improve our understanding of the role sea urchins play in the natural system. Here, we will focus on the general ecology of sea urchins based on information provided by Vadas and his students.
As the primary grazers of kelp and other seaweed in our coastal waters, sea urchins play an important role in shaping the marine communities where they live. The sea urchin has a voracious appetite, and will eat most types of seaweed, including the tiniest of plants, the diatoms, which are deposited on rock surfaces. In the absence of seaweed, they will scavenge on whatever food is available, whether that is mussels, leftover lobster bait, or other decaying material. However, reports Vadas, "their preferred food is the kelp found in shallow, subtidal waters." And for good reason - when compared with urchins which relied on other sources of food, Vadas has found that kelp which mostly ate kelp grew larger and faster.
Sea urchins reproduce, or spawn, by gathering together in large numbers and then releasing hundreds of millions of eggs and sperm into the ocean water. Conditions need to be just right - typically only one percent of all the eggs released are actually fertilized.
"Urchins need to be pretty close to each other for successful fertilization to occur," says Vadas. "They need to get as many eggs and sperm into the water column as they can because the water is diluted quickly, sperm do not last very long, and the eggs may settle to the bottom without being fertilized if there is not enough water movement."
Thus, when populations are low, reproductive success decreases.
"We believe that urchins are triggered to spawn by temperature changes and by the spring bloom of microscopic plants called phytoplankton," Vadas states. "During this time, the fertilized eggs, called larvae, can take advantage of the abundance of food."
The timing of the spawning, typically in early spring, is important to management of the harvest. Vadas explains that the roe begins "melting" when spawning begins, and the quality of the roe declines rapidly.
"It is also important to restrict harvesting during the spawning season if we want to increase the chances for reproduction to occur, so the population can continue to grow," he adds.
In Cobscook Bay, spawning usually occurs four to eight weeks later than southern Maine. Urchin larvae float around in the water for six to eight weeks before settling to the sea floor and transforming into the adult form. Urchins typically reach sexual maturity at four to five years of age. The size of these adults has significance for reproductive success. Information gathered by Vadas and his students show that sea urchin size is closely related to the number of eggs and sperm they can produce.
"The current legal size limit of two inches for harvesting allows some fully mature urchins to become part of the breeding stock," Vadas reports. "However, the larger they grow the more eggs they are able to produce. For example, a legal size urchin might produce around 1 million eggs, while a larger urchin of 3 to 3 and a half inches can produce 10 million eggs."
Populations of sea urchins have fluctuated off the coast of Maine over the last thirty to forty years. A rapid rise in sea urchin numbers was observed during the 1970's and 80's. Researchers believe this increase was due to overharvesting of fish and other marine animals who preyed on sea urchins.
"Around offshore islands where fish are still present, we see very low numbers of urchins, crabs, and lobsters, or none at all," Vadas explains. "Our research has shown that wolffish, cod, and other large fish species, along with many seabirds, are important predators of urchins. Crabs and lobster are known to feed on smaller urchins closer to shore."
Researchers believe the current coastwide decline in urchin numbers is caused by overharvesting. At least in some areas, disease may have added to this decline during the past year. Over twenty study sites off the Maine coast have reported diseased urchins. The cause of the disease is still under investigation.
The next article in Cobscook Bay Soundings will explore how the rise and fall of sea urchin populations has changed the structure of subtidal communities.
This column was prepared by Cheryl Daigle and Jim Dow. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.
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