Abstract
This study describes sea urchin spatial distribution in relation to environmental factors, and the relationship between Diadema antillarum density and algal abundance. Twenty-three transects around Madeira Island were surveyed by scuba divers, and sea urchin density and algal cover were determined in situ. Sampling sites along these transects were characterised in terms of distance from the tide line, water depth, substratum type, bottom declivity and water turbulence. Diadema antillarum was the dominant sea urchin species. Paracentrotus lividus and Arbacia lixula occurred at shallower depths (2-6 m), contrasting with the distribution of Sphaerechinus granularis , which occurs among D. antillarum (4-20 m). Surveys found two alternative types of communities on rocky shores: 1) a community with high algal cover and low numbers of sea urchins, along the north and south-west coasts and; 2) a community with little algal cover and high densities of sea urchins, along the south-east coast. Macroalgal cover and D. antillarum densities were inversely correlated (adjusted R 2 =75.6%; n = 429; p D. antillarum on rocky substrates. We propose a multiple non-linear regression model (using backward stepwise analysis) to explain D. antillarum abundance on the rocky shores: D. antillarum/m 2 (??)= 0.121 - 0.209 distance from shore (in m) (??) + 2.052 water depth (in m) (??) - 1.778 water turbulence level (??) - 0.007 water turbulence level4 (??); where ?? indicates data are square-root transformed (adjusted R 2 = 60.99%; n = 454; p< 0.05).
Highlights
Several studies of sea urchin distribution and the effects of this on algal populations have been reported world-wide (e.g. Valdez and Villalobos, 1978; Bauer, 1980; Weil et al, 1984; Benedetti-Cecchi and Cinelli, 1995; Turon et al, 1995; Falcon et al., 1996)
The results showed that water turbulence was the most important factor limiting the distribution of D. antillarum on rocky substrates
Sea urchin species known to occur on the rocky shores of Madeira are Paracentrotus lividus (Lamarck), Arbacia lixula (Lamarck), Sphaerechinus granularis (Lamarck), Diadema antillarum (Philippi), and the less common Arbaciella elegans (Mortensen) and Centrostephanus longispinus (Philippi) (Wirtz, 1995)
Summary
Several studies of sea urchin distribution and the effects of this on algal populations have been reported world-wide (e.g. Valdez and Villalobos, 1978; Bauer, 1980; Weil et al, 1984; Benedetti-Cecchi and Cinelli, 1995; Turon et al, 1995; Falcon et al., 1996). Valdez and Villalobos, 1978; Bauer, 1980; Weil et al, 1984; Benedetti-Cecchi and Cinelli, 1995; Turon et al, 1995; Falcon et al., 1996). Sea urchin species known to occur on the rocky shores of Madeira are Paracentrotus lividus (Lamarck), Arbacia lixula (Lamarck), Sphaerechinus granularis (Lamarck), Diadema antillarum (Philippi), and the less common Arbaciella elegans (Mortensen) and Centrostephanus longispinus (Philippi) (Wirtz, 1995). Paracentrotus lividus and A. lixula have been studied in the Mediterranean Sea, where they play a decisive role in the development of benthic macrophyte communities (Kempf, 1962; Bergin, 1987; Benedetti-Cecchi and Cinelli, 1995; Turon et al., 1995; Fernandez and Boudouresque, 1997). The distribution of S. granularis has previously been studied only at Faial (Azores), by Maciel and Gonçalves (1998)
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