Role of Sea Urchins
SEA URCHINS: MOVERS AND SHAKERS OF THE SUBTIDAL COMMUNITY
(Published in The Quoddy Tides , May 12, 2000)
The green sea urchin plays an important role in shaping what the marine environment of Cobscook Bay looks like. Below the low tide line, the urchin is the major feeder on seaweed, particularly kelp. With its ravenous appetite, the urchin causes changes to subtidal areas that influence what other marine animals can thrive there.
Researcher Doug McNaught recently completed a study at the University of Maine that is important to urchin management efforts all along the coast of Maine. His research suggests that sharp declines in the urchin population can result in changes to the subtidal environment which may in fact make it difficult for the urchin population to recover.
When populations of sea urchins are high, they can move rapidly through an area, mowing down wide strips of kelp and other seaweed as they feed. The pink and rose colored crusts left behind on the rocky bottom are called coralline algae. These cannot be eaten by the sea urchin, and are of less use to other creatures.
"Coralline barrens provide little protection or cover for smaller marine animals," McNaught reports. "So the loss of the large, fleshy seaweeds not only affects the food supply of the urchin, it also has an immediate impact on the ecology of the subtidal area."
Urchins will remain on the barrens to feed on microscopic algae, seaweed that has drifted on to the barrens, or decayed plant materials that have settled on the bottom. If commercial harvesting drastically reduces sea urchin numbers, as has occurred along much of the Maine coast, kelp beds usually grow back on these barrens.
Although this change in the subtidal habitat from barrens to dense seaweed provides a more protected environment for crabs, fish and other small marine animals, it does not necessarily bode well for the sea urchin.
"We noticed that urchin populations were not recovering as the large seaweeds grew back in," McNaught says. "So we began looking at different sites along the coast to see if this was influenced by the way that urchin larvae settle, or if something happens after the urchins settle that keeps them from coming back to these communities."
Urchin larvae develop from fertilized eggs immediately after successful spawning. These larvae float in the water column for six to eight weeks before settling to the bottom. Where they settle is determined by a variety of factors, including current, tide and wind activity.
"We have observed that the highest settlement of larvae occurs in the western region of York County, while the Downeast regions have consistently lower settlement," says McNaught.
Looking at the age of urchins in any given area can provide clues as to the supply of larvae settling there. In Jonesport, for example, the eastern-most site that McNaught studied, one year old and juvenile urchins are conspicuously absent.
"Our study has shown that Jonesport lacks a good supply of urchin larvae," McNaught says. "For example, we would only find 25 larva per square meter at our Jonesport sites, but would find 16,000 larva per square meter in our sites down near York."
Another study of an Eastport site in 1995 shows that similar patterns of low larvae settlement may exist there as well.
"We have also seen a declining trend in the amount of larval supply coastwide," McNaught adds. "We are not sure what's behind this."
It became clear in McNaught's work at the University of Maine, however, that declines in the amount of larval supply were not the reason sea urchin numbers were failing to recover in the subtidal areas where the kelp beds had grown back. Crabs and other small crustaceans were thriving in these areas, most likely because of the abundant food and protective cover.
"What we discovered is that the rock crabs, who have found shelter in the new stands of kelp and other seaweed, are eating the sea urchins just after they settle," reports McNaught.
The sea urchins are not able to survive long enough to re-establish their place in the community. This new information increases the significance of coralline barrens as nursery grounds for the urchin.
"In the four years of our study, no sites in the study have shifted back from kelp beds to coralline barrens - all are moving in one direction," McNaught says. "This has significance for urchin management because it shows recovery of urchin populations is even more complicated than previously thought. We can't just assume that the populations will bounce back simply by limiting harvesting activity."
The great variation in larval supply reaching different sites along the coast is also significant.
"Although the reasons for larval settlement patterns in the Gulf of Maine are not clear, the patterns are," says McNaught. "This information about where larvae settle, the larval ?hot spots,? is important to urchin management decisions. In areas of low larval settlement, it may be more difficult for populations to recover naturally."
Any time that a dominant species is removed from a community, whether it be a seaweed grazer like the urchin or a predator like the wolffish, shifts in the structure of the community take place. In the case of the sea urchin, these changes take place rapidly. Two distinct community types are created depending on the numbers of sea urchins: either coralline barrens or communities rich in kelp and other large seaweed. When balance exists in the community, sea urchins eat enough kelp to keep some areas open, but are controlled by predators so that they do not eat all the large seaweed.
McNaught's research suggests that subtidal areas with a patchwork of coralline barrens and dense seaweed communities may be required for the successful recovery of sea urchin populations along our coast.
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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