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Grazing by Diadema antillarum sea urchins is well documented for coral reefs, although information is scarce for the eastern part of the Atlantic Ocean, where rocky substrate dominates the sea bottom. This study analysed grazing activity by D. antillarum upon the algal communities living on rock substrates, and its possible impact on the subtidal communities. Controlled feeding experiments using exclusion cages were performed between May and September 1998 at Madeira Island (NE Atlantic). Three experimental treatments were used: (1) closed cages to exclude sea urchins, (2) open cage controls, and (3) uncaged controls (nine replicates in each treatment). After four months, in September 1998, the percentage algal cover was quantified. One-way analysis of variance followed by Post Hoc Tukey (HSD) tests showed significant differences between algal abundance in the presence (uncaged and open cage controls) or absence (closed cages) of sea urchins. In the areas where D. antillarum was excluded (closed cages), algal abundance increased by about 10% in the four-month period. Diadema antillarum thus effectively reduces algal abundance, and this may have important consequences in determining the algal community structure of rocky substrate.

A study was undertaken to evaluate the existence of depth segregation between size classes of Donax trunculus populations that occur along the Algarve coast (southern Portugal). Five collecting areas were sampled in the Ria Formosa barrier islands, namely in the Culatra, Barreta and Armona Islands, between March and June 1998. The study was based on size frequency distribution analysis and shell biometric relationships. The size frequency distribution showed the predominance of smaller individuals (< 25 mm) in the renewal area (0.5-1.0 m depth) and the prevalence of larger individuals (? 25 mm) at greater depths (4.0-6.0 m depth). The biometric study revealed the existence of significant differences in the type of growth between size classes in relation to depth distribution and burrowing activity. On the basis of these results, some specific management measures were proposed for this important bivalve fishery, taking into account the distinct fishing strategies and operations (manual and boat-operated dredging) performed at different depths.

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 R2=75.6%; n = 429; p< 0.05). The results showed that water turbulence was the most important factor limiting the distribution of 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/m2 (??)= 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 R2 = 60.99%; n = 454; p< 0.05).