ABSTRACT1. Maerl beds occur worldwide and are formed by an accumulation of unattached calcareous red algae (Rhodophyta).2. Maerl-forming algae grow in a superficial living layer on sediments within the photic zone. 3. Maerl beds are spatially complex habitats with a high degree of species and trophic group diversity.4. The European Commission's 'Habitats Directive' mandates the conservation management of two of the main European maerl-forming species, Phymatolithon calcareum and Lithothamnion corallioides.5. Mediterranean maerl beds are to be considered for inclusion in national inventories of sites of conservation interest, as required by the SPABIM Protocol of the Barcelona Convention.6. In spite of their importance, and the requirement for their conservation management, European maerl grounds suffer a variety of anthropogenic perturbations including direct exploitation through extraction, fishing impacts and chemical pollution by organic matter and excess nutrients.7. The ecology of northeast Atlantic and Mediterranean maerl beds has received little attention, in contrast to other marine communities (e.g. kelp forests, sea-grass meadows). * Correspondence to: P.G. Moore, University Marine Biological Station Millport, Isle of Cumbrae, KA28 0EG, UK. E-mail: pmoore@udcf.gla.ac.uk y Authorship alphabetical: cite as BIOMAERL team z Coordinator 8. Key conservation and management measures proposed include: the recognition that maerl beds are non-renewable resources and cannot sustain direct exploitation; prohibitions on the use of towed gear on maerl grounds; moratoria on the issue of further permits for the siting of aquaculture units above maerl grounds; monitoring of existing exploited or impacted maerl beds; the designation of 'no-take' reserves; measures to limit the impacts that might affect water quality above maerl beds; a programme of monitoring of the 'health' of European maerl beds; an awareness campaign on the biological importance of maerl beds; a higher conservation status for maerl habitats and maerlforming species in European legislation; and further research on maerl ecosystems.

In situ visual surveys using a hierarchical sampling design were carried out at 36 sublittoral rocky locations along the central-east Atlantic Canarian Archipelago to find relationships among (1) benthic primary producers, (2) the demographic structure of the herbivorous sea urchin Diadema antillarum Phillipi and (3) the trophic structure of coastal fish communities. Our correlation approach displayed a relationship between the lack of large macroinvertebrate-eating predatory fish and the increase in density of sea urchins, in addition to a decrease in fish richness. In contrast, increases in fast-growing plankton-feeding fish species were detected. The size structure of D. antillarum is dominated by small-to-intermediate sized sea urchins in environments with a high density of individuals, whereas low sea urchin density locations are characterized by the dominance of large sized individuals. The physical complexity of the substrate seems to play an important role in determining the local patchiness of D. antillarum. Finally, a non-linear decrease in the percentage of fleshy macroalgal cover with increasing density of D. antillarum was observed. We therefore propose D. antillarum as a key herbivorous species, which plays an important role in determining the structure of shallow, hard-substratum, infralittoral benthic communities throughout the Canary Islands. KEY WORDS: Urchin-fish interactions · Trophic cascades · Diadema antillarum · Sea urchins · Fish assemblages · Macroalgae · Canary Islands Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 278: [157][158][159][160][161][162][163][164][165][166][167][168][169] 2004 tourist industry and the increased demand of fishery resources (Bortone et al. 1991, Falcón et al. 1996. However, no empirical evidence of the effect of these disturbances is available, as there are no data on the nearshore fish populations during the predevelopment era of the 1960s (Falcón et al. 1996). Personal observations and several local studies have suggested that the long-spined sea urchin Diadema antillarum Philippi has experienced a significant increase in abundance throughout the central-east Atlantic (FAO fisheries region #34) in the last decades (Casañas et al. 1998, Alves et al. 2001, Tuya et al. 2004b. Although D. antillarum has been extensively studied in the western Atlantic, where it has a great impact on benthic community structure (e.g. Sammarco et al. 1974, Carpenter 1981, Lessios 1988, little research has been undertaken in the eastern Atlantic to link the demographic structure of this echinoid species with the abundance and biomass of rocky-reef fish assemblages in the Canarian Archipelago. Moreover, no study has addressed the above along with the coastal trophic cascades within this area (Pinnegar et al. 2000).The general aim of this study was therefore to determine whether there is an association between the structure of Diadema antillarum populations and the community structure of macroalgae and fish. This potenti...

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Post-MME Recruitment Pinna nobilis were recorded in a few sites where the resident population had been decimated. This hints to the importance of unaffected populations as larval exporting sources and the role of oceanographic currents in larval transport in the area, representing a beacon of hope in the current extremely worrying scenario for this emblematic species.