Special Perspectives

Ecological Role of Purple Sea Urchins

Science  10 Nov 2006:
Vol. 314, Issue 5801, pp. 940-941
DOI: 10.1126/science.1131888

Abstract

Sea urchins are major components of marine communities. Their grazing limits algal biomass, and they are preyed upon by many predators. Purple sea urchins (Strongylocentrotus purpuratus) are among the best studied species. They live in environments that alternate between two stable states: luxuriant, species-rich kelp forests and sea urchin–dominated “barrens.” The transition from one state to the other can be initiated by several factors, including the abundance of algal food, predators, storm intensities, and incidence of disease. Purple sea urchins compete with other grazers, some of which are important fishery resources (such as abalones and red sea urchins), and they are harvested for scientific research. Revelations from their genome will lead to a better understanding of how they maintain their ecological importance, and may in turn enhance their economic potential.

Sea urchins are major grazers in shallow seas worldwide (1). Purple sea urchins (S. purpuratus) and other strongylocentrotid sea urchins of the Northern Hemisphere are particularly important and are the most intensively studied (2). A delicate balance between sea urchin grazing and kelp forest productivity leads to stable states that alternate between luxuriant kelp forests and relatively species-depauperate sea urchin “barrens” (3). Curiously, the densities of sea urchins are often similar within kelp forests and sea urchin barrens. Within kelp forests, sea urchins are nearly stationary, feeding on captured pieces of kelp litter (“drift kelp”) that are produced and shed in high quantities from the kelp plants. However, when the kelps are removed by storms or El Niño events, the remaining sea urchins actively forage on young kelp recruits and on drift kelp brought in from elsewhere, preventing the reestablishment of the kelp forest. The sea urchins can be decimated by storms (4) or diseases (5), allowing the kelp forest to return to the area.

In addition to the alternation of kelp forests and sea urchin barrens, temporal and spatial variation in recruitment of sea urchins can greatly influence their abundance, and therefore their ecological role, within kelp forests. Infrequent peaks in recruitment can lead to spurts in sea urchin densities (6), sometimes triggering intense grazing that removes the kelp, transforming a kelp forest into a sea urchin barren (7) (Fig. 1). Recruitment intensity is determined mainly by the supply of sea urchin larvae, which in turn depends on the oceanographic conditions that bring the larvae to suitable areas to settle (8).

Fig. 1.

Purple sea urchins (S. purpuratus) grazing on the remains of a giant kelp hold fast after an unusually heavy recruitment in Carmel Bay, California (7). [Photo courtesy of J. M. Watanabe]

Predators almost certainly limited the abundance of sea urchins in the past, and the presence of sea urchin barrens around the world is due at least in part to decimation of predators by humans (9). In southern California, where sea urchin–dominated barrens are common, major predators such as spiny lobsters, sheep-head fish, and sea otters have been greatly reduced during the past century (10). Sea otters, in particular, are effective in keeping sea urchin densities low throughout their range (11), which often tips the balance toward kelp forests and enhances kelp forest productivity and biodiversity (12). Other factors also can favor kelp forests or barrens, even in the absence of major sea urchin predators (13). For example, sea urchins compete with other grazers. Purple sea urchins compete in particular with abalones and red sea urchins (14). However, because both abalones and red sea urchins have experienced intensive fishing pressure for the past several decades, competitive interactions among these species have probably decreased, perhaps relieving purple sea urchins from adverse competition. On the other hand, juvenile purple sea urchins and abalones often are sheltered from predators under the spines of red sea urchins (15), and both may be negatively affected by heavy fishing of red sea urchins. The strengths of these interactions remain poorly known and are complicated by variation in fishing pressures. Red sea urchins have been the basis of a major fishery in the eastern Pacific for the past several decades. However, the U.S. sea urchin fishery has been in decline, mainly because of a decline in demand by the Japanese market ($110 million worth of sea urchins were exported from the United States to Japan in 1993 compared with about $36 million in 2003) (16). The smaller, less valuable, purple sea urchins have been a minor component of the fishery but continue to play an extensive role in scientific research (2).

Sea urchins are central in structuring marine benthic communities, both as grazers and prey, and are economically valuable in fisheries. There are important differences among the different species: Some are more effective as grazers than others, and they vary in their diets, growth rates, longevities, and importance in fisheries. Some show no sign of senility and live for well over a century (17). Elucidation of their genomes will open new avenues of research into the underlying genetic and evolutionary bases of these variations.

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