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We tagged individuals of the sea urchin Diadema antillarum (Philippi) around the island of Gran Canaria (The Canary Islands) during winter 2001-2002 using a new technique, consisting of the insertion of a hook fastened to a fishing line into the aboral pole (periproctal membrane). This allowed individual identification of tagged sea urchins. The goals were: (1) to quantify nocturnal movements and the homing behaviour of this echinoid on shallow rocky bottoms, and (2) to assess short term spatial and temporal variability of these movements. Tagged sea urchins displayed clear homing behaviour. The mean distance travelled at night was 3.7€1.2 m (range 1.0-5.1 m). Mean speed of nocturnal movement was 33€26 cm h 1 (range 5-110 cm h 1 ). We observed greater movement at midnight than at the beginning and the end of the night.

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Preferences of the sea urchin Diadema antillarum (Echinodermata: Echinoidea) feeding on five species of brown macroalgae (Padina pavonica, Dyctiota dychotoma, Cystoseira abies-marina, Lobophora variegata and Halopteris filicina) have been studied using caging field experiments on Gran Canaria Island during August to October 2000. Results of three assays of both single and multiple diet experiments rejected the null hypothesis that Diadema does not feed selectively on the five selected algal species. In the multiple diet assays, Diadema consumed an average of 68–98 mg algae urchin−1 h−1 and 4–120 mg algae urchin−1 h−1 in the single diet experiments. On the basis of consumption, the five species of algae eaten can be divided into three groups. Thus Halopteris, Lobophora and Dyctiota were considered preferred algae, while Padina was considered an intermediate alga. Finally Cystoseira was significantly the less preferred and consumed seaweed in all sets of assays.

Interannual changes in the shoot density, biomass and leaf length of the seagrass Cymodocea nodosa and two descriptors of the associated fish communities (total fish abundance and species richness) were evaluated at Playa Blanca (Lanzarote, Canary Islands) from March 2001 to April 2002 to provide evidence for responses to the impact of a marina construction. The seagrass bed under influence, as well as a control location, were sampled using an experimental design which consisted of three sites nested within each of these two locations. We tested the null hypothesis that these parameters would not change over time as a result of this construction.A three-way mixed and symmetrical ANOVA detected significant 'Time of sampling ¥ Control vs Impact' interactions for the shoot density, biomass and leaf length. The same interactions were observed for the descriptors of the fish communities using a two way orthogonal ANOVA. We detected an initial fast increase in the mean leaf length as a possible morphological response to a reduction in the amount of light at the bottom. Furthermore, a significant decrease was obtained for the penetration of light into the water column in the perturbed bed. Therefore, we obtained significant evidence to reject the null hypothesis offered by our study.