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The effect of grouping of Strongylocentrotus Franciscanus, the giant red sea urchin, on its population biology Low, Charles J.

Abstract

Strongylocentrotus franciscanus, the giant red sea urchins of the Pacific coast of North America, are customarily clumped. I set out to find out why, to identify the mechanisms which cause clumping, and to determine the benefits the urchins receive from clumping. The mechanisms which produce clumping include a preference for areas with current or surge and a hard bottom. Within suitable areas, density is low where the bottom is smooth and/or flat, and in deeper water. Where the bottom is rough and broken, and in shallow water below algae beds, urchins are more abundant. Within these constraints, urchins are aggregated. This can be most clearly seen where urchins are sparse and the groups are more distinctly separated. Behavioral mechanisms produce clumping. When an urchin is disturbed, it moves down slope stopping on an encounter with a stationary urchin(s). The presence of nearby urchins inhibits movement, implying that once a clump forms, it should be stationary. Above a critical size, roughly estimated as more than 32 urchins, groups were stationary for at least three years. The benefits of clumping for urchins may include efficient fertilization of eggs. Clumping may also act to reduce predation. Invertebrate predators were found to be ineffective unless urchins were injured. Even slight injuries attracted Pycnopodia helianthoides and Cancer products, but uninjured urchins elicited no response. Clumping seems to reduce predation further, as predators were much less common within clumps of urchins than in other locations. Another benefit of clumping concerns competition for space with macro-algae. On the south coast of British Columbia, algal beds exclusively occupy the shallowest water in the immediate subtidal, and are not invaded by the urchins. Where S. franciscanus is common, the bottom is grazed bare to the depth where algal beds begin. Where urchins are rare, algae occupy the bottom to considerable depths, with groups of urchins maintaining small clearings, or intruding into the lower edge of the algae beds. S. franciscanus does not move alone into algal beds because the closely spaced stipes of the large algae make it difficult for large urchins to move. Also, the presence of small foliose algae, which usually carpet the bottom in such beds, may make attachment difficult. Movement of the stipes in surge would also tend to abrade urchins. Clumping also provides protection for recruits. Urchins under 2cm in diameter were found almost exclusively under adults. No recruitment was observed in areas where adults were naturally absent, nor did any occur where adults had been removed. . This study of clumping is relevant to harvesting of urchins. They are at present exported on a small scale to Japan. The present harvesting technique is to clear a bed of urchins, leaving only a few scattered individuals. Algae will colonize the cleared areas, and since young recruit only to groups of adults, there will be no recruitment. If the harvest or urchins expands greatly, as can be expected, recovery may present problems. The immediate requirements are for research in areas which have been or will be harvested; and, as an interim conservation measure, the requirement that clumps of fifty or more urchins be left in harvested areas.

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