New Insights into the Functioning of Mediterranean Rocky Reef ecosystem When Algal Forests Are Lost

In the Mediterranean, overfishing of large predatory fish, particularly Diplodus spp., can cause severesea urchin outbreaks, leading to significant shifts in benthic communities. Variations in sea urchingrazing intensity may trigger transitions between complex algal-dominated states ("Forest") andsimpler, sea urchin-dominated states ("Barren"). Barren states, which are characterized by low diversityand productivity, can persist for long periods, even within marine protected areas (MPAs). The recoveryof key sea urchin predators in MPAs is slow due to initial overexploitation, resulting in persistent seaurchin population explosions.Forest states, with their rich biodiversity, transfer substantial quantities of carbon, nitrogen, andphosphorus to coastal food webs, whereas barren systems have lower diversity and productivity.However, recent studies suggest that barren systems can provide microhabitats for various cryptic andinvertebrate species, which may contribute to maintaining the barren state. The shift between forest andbarren states affects the entire food web, altering growth, feeding dynamics, and energy flow.This thesis aims to elucidate the functioning of forest and barren states in Mediterranean rocky reefsthrough a multidisciplinary approach, including descriptive, isotopic, mass balance, and molecularanalyses.1. Descriptive and Isotopic Approach: The study tested the hypothesis that corallinebarrens enhance benthic megafauna abundance and diversity. Isotopic analyses revealed a comparabletrophic structure between the two states, with higher isotopic uniqueness in barrens, primarilyinfluenced by sea urchins and carnivorous starfish. Results indicated that coralline barrens support adiverse benthic megafauna, challenging the notion of barrens as low-diversity habitats.2. Mass Balance Approach: Food-web models of algae forests and urchin barrens,representing pristine and collapsed states of rocky reefs, were developed. Both states showeddominance of low trophic level consumers and significant energy flow through detritus. Despitedifferences in primary production and energy utilization, both states exhibited similar complexity andstability. This highlights the importance of understanding ecosystem dynamics for effectivemanagement and conservation.3. Molecular Approach: Investigating the persistence of barren states despite predatorrecovery, this study explored the role of micropredation in controlling sea urchin populations. Specificprimers for detecting mtDNA of sea urchins Paracentrotus lividus and Arbacia lixula were designed.Testing invertebrates collected during urchin settling events identified potential micropredators,suggesting that micropredation may help maintain the forest state by controlling sea urchin populations.The findings provide insights into the structure and functioning of rocky reef ecosystems in differentstable states, underscoring the need for comprehensive management strategies to preserve these criticalhabitats

Functional traits of two co-occurring sea urchins across a barren/forest patch system

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

New body metrics to determine asteroid size and weight directly in the field

Asteroids are an important group of predators in many marine ecosystems. The measure of body size is crucial in studying asteroid biology as this variable affects both prey selection and predation impact. Current field techniques for estimating asteroid size (i.e. total length TL, total weight TW) include the measure of correlated variables, such us the central disk (CD) and arm length (AL). However, these variables are often time-consuming and require a direct handling of the organism. We tested the accuracy of new asteroid body metrics, the arm height (AH) or arm width (AW), to rapidly estimate body size in asteroids. AH and AW were measured in three of the most common Mediterranean asteroids (Marthasterias glacialis, Ophidiaster ophidianus and Coscinasterias tenuispina), sampled from April to August 2008, along the coasts of Ustica Island off the northern coast of Sicily. We used both linear and exponential regression analyses to compare the performance of AH, AL, AW and CD in estimat- ing size for the three species studied. Results suggest that, in M. glacialis and C. tenuispina, AH is strongly correlated (p<0.001) with both TL and TW, whereas in O. ophidianus it gives a good correlation (p<0.001) with TW only. AW was poorly correlated with both TL and TW in M. glacialis and C. tenuispina, but not in O. ophidianus, where it showed the highest correlation with TW. Thus, only the novel AH measure constitutes a convenient and reliable way of measuring asteroid body size in the field.peer-reviewe

Synergistic reduction of a native key herbivore performance by two non-indigenous invasive algae

none7siNative generalist grazers can control the populations of non-indigenous invasive algae (NIIA). Here, it was found that the simultaneous consumption of two co-occurring NIIA, Caulerpa cylindracea and C. taxifolia var. distichophylla, hinders the grazing ability of the main Mediterranean herbivorous, the native sea urchin Paracentrotus lividus. The ingestion of any of the two NIIA alone did not produce any difference in sea urchin righting time with respect to usual algal diet. In contrast, the simultaneous consumption of both NIIA, which grow intermingled in nature and are consumed by P. lividus, retarded its righting behavior. Such result reveals substantial physiological stress in the sea urchin, which resulted in reduced motility and coordination. The reported findings reveal the potential of NIIA co-occurrence to escape the supposed control exerted by the main native generalist grazer in Mediterranean sublittoral communities, which in turn can be locked in an “invaded” state.embargoed_20210301Vega Fernandez T.; Badalamenti F.; Bonaviri C.; Di Trapani F.; Gianguzza P.; Noe S.; Musco L.Vega Fernandez, T.; Badalamenti, F.; Bonaviri, C.; Di Trapani, F.; Gianguzza, P.; Noe, S.; Musco, L

Crushing predation of the spiny star Marthasterias glacialis upon the sea urchin Paracentrotus lividus.

Literature data report that only fish predators are able to crush sea urchin tests in Mediterranean rocky reefs. This experimental study showed that the spiny star Marthasterias glacialis is able to break Paracentrotus lividus tests and that the breaking event is more likely to occur for small-sized sea urchins than for big ones. Our results show that the role of M. glacialis in regulating P. lividus population density can be important in specific locations. They may have important implications, moreover, for the use of tethering techniques aimed at identifying predator types of sea urchin