Recent ecological studies have shown a strong relation between temperature, echinoids and their grazing effects on macro-algal communities. In this study, we speculate that climate warming may result in an increasingly favourable environment for the reproduction and development of the sea urchin Arbacia lixula. The relationship between increased A. lixula density and the extent of barren grounds in the Mediterranean Sea is also discussed.Keywords: ocean warming; overgrazing; sea urchin fertilisation and development Factors influencing the functioning of ecosystems, such as climate change, inputs of nutrients and/or toxic chemicals, groundwater reduction, habitat fragmentation, harvest of biomasses or loss of biodiversity, are never constant. The state of some ecosystems may respond gradually to such changes, whereas others might remain unresponsive over time (or ranges of conditions), and then respond abruptly when conditions reach a critical level. This implies that, for certain environmental conditions, the ecosystem might have two or more alternative stable states, separated by an unstable equilibrium that marks the border between the 'basins of attraction' of the states (the theory of alternative stable states; ASS) [1,2]. This theory also hypothesised that natural systems are often in persistent, resilient, alternative states: alternative combinations of ecosystem states and environmental conditions that may persist at a particular spatial extent and temporal scale.These states have been shown to be maintained by intrinsic mechanisms involving biotic and abiotic interactions (grazing or predation intensity, storm frequency, pollution, local extinction, invasion, nutrient loading, etc.) which inhibit reversal to the previous community state [3]. In combination with other factors, the loss of a keystone species [4], which results from changes in top-down interactions between (1) predators and herbivores and (2) herbivores and macroalgae, may cause changes in grazing intensity and consequent switches between two or more alternative states in marine ecosystems.For example, in temperate seas, several species of echinoids play a crucial role, and are even more important than other herbivores (i.e. fishes) as controllers of benthic communities [5][6][7][8][9][10][11][12][13].

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Benthic—pelagic coupling plays a pivotal role in aquatic ecosystems but the effects of fishery driven interactions on its functioning has been largely overlooked. Disentangling the benthic—pelagic links including effects of mixed fisheries, however, needs sketching a whole description of ecosystem interactions using quantitative tools. A holistic food web model has been here developed in order to understand the interplay between the benthic-pelagic coupling and mixed fisheries in a Mediterranean system such as the Strait of Sicily. The reconstruction of the food web required review and integration of a vast set of local and regional biological information from bacteria to large pelagic species that were aggregated into 72 functional groups. Fisheries were described by 18 fleet segments resulting from combination of fishing gears and fishing vessel size. The input-output analysis on the food web of energy pathways allowed identifying effects of biological and fishery components. Results showed that the structure of the Strait of Sicily food web is complex. Similarly to other Mediterranean areas, the food web of the Strait of Sicily encompasses 4.5 trophic levels (TLs) with the highest TLs reached by bluefin tuna, swordfish and large hake and largely impacted by bottom trawling and large longline. Importantly, benthic-pelagic coupling is affected by direct and indirect impacts among groups of species, fleets and fleets-species through the whole trophic spectrum of the food web. Moreover, functional groups able to move on large spatial scales or life history of which is spent between shelf and slope domains play a key role in linking subsystems together and mediate interactions in the Mediterranean mixed fisheries.

Temperate rocky reefs may occur in two alternative states (coralline barrens and erect algal forests), whose formation and maintenance are often determined by sea urchin grazing. The two sea urchin species Paracentrotus lividus and Arbacia lixula are considered to play a similar ecological role despite their differing morphological traits and diets. The patchy mosaic areas of Ustica Island, Italy, offer an ideal environment in which to study differences in the performance of P. lividus and A. lixula in barren versus forest states. Results show that the two sea urchin species differ in diet, trophic position, grazing adaptation, movement ability and fitness in both barren and forest patches. We confirmed herbivory in P. lividus and omnivory with a strong tendency to carnivory in A. lixula. When the sea urchin escape response to a predator was triggered, P. lividus responded faster in barren and forest patches. Forest patch restricted movement, especially in A. lixula (velocity in barren ≈ 10-fold greater than in forest). A large Aristotle's lantern, indicative of durophagy, confirmed adaptation of A. lixula to barren state.

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