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The thermophilic Atlanto-Mediterranean purple starfish, Ophidiaster ophidianus, is a species protected under the EU’s Habitats Directive which is expanding its distribution in the northern Mediterranean areas as a consequence of global worming. The ecology of the species is largely unknown, specially for the diet. To cover this gap, the feeding habits of O. ophidianus have been studied in two different Mediterranean rocky reef areas: the southern Tyrrhenian Sea (Ustica Island, sampled in 2009) and the eastern Adriatic Sea (Molunat, sampled in 2014) by both field observations and ?13C and ?15N stable isotope analysis (SIA). SCUBA divers collected data by turning adult starfish downside-up and collecting photographs of the prey. Prey items of fifty individuals were identified to the lowest possible taxonomic level in each location by analysing the images taken. No significant differences were detected in the field, O. ophidianus fed mainly on encrusting coralline algae (Lithophyllum spp.), keratose sponges (Ircinia sp.), other encrusting organisms and sedimentary organic matter (SOM). Isotopic signatures significantly differed between the two areas: ?15N was significantly higher in O. ophidianus collected at Molunat than at Ustica. Mixing models showed that O. ophidianus collected at Ustica fed on Lithophyllum spp. (68.1%), Ircinia sp. (15%), SOM (16.3%) and bryozoans (Schizoporellidae, 3.7%), whereas at Molunat the starfish fed on Lithophyllum spp. (79.9%), Peyssonnelia spp. (7.9%), keratose sponges (Scalarispongia scalaris, 7.4%) and SOM (4.8%). Coupling field survey with SIA we argued that O. ophidianus feeds as a grazers, selecting algae and few other animal organisms in Mediterranean rocky reefs.

The purple starfish Ophidiaster ophidianus is an Atlanto-Mediterranean starfish protected under the EU’s Habitats Directive. Despite the wide distribution and the current range expansion of this warm affinity species in the northern Mediterranean areas, nothing is known about its diet. Using field observations and carbon (δ13C) and nitrogen (δ15N) stable isotopes, we explored possible changes in density and diet of O. ophidianus in relation to two different Mediterranean habitats: the coralline barren and the macro algae forest. Data were collected at the Marine Protected Area of Ustica Island (Southern Tyrrhenian Sea) from June to September 2009. Starfish density was not affected by the habitat, while on the contrary the diet of O. ophidianus was significantly different between barren and macro algae forest habitat. O. ophidianus fed mainly on crustose algae and bryozoans in barren, whereas it preferred the sponge Ircinia spp. in the forested habitat. Other food sources such as small gastropods and sedimentary organic matter were selected by this starfish. In general O. ophidianus showed a short niche breadth and omnivory habits with tendency to carnivory.

Ecosystem resistance to species invasion is considered to be related to the abundance and diversity of native species (i.e. diversity-resistance hypothesis). In particular, the integrity of predator assemblages may enhance the control of prey populations through direct and indirect interactions (e.g.: different predation strategies by different predators, facilitative interactions among predators). Depletion of predators due to overfishing is therefore expected to enhance the abundance and ultimately the spread of alien prey species. Manipulative field experiments were performed to evaluate the effects of different abundance and diversity of predator assemblages on the invasion success of the alien crab Percnon gibbesi. The experiments were conducted in northwestern Sicily (Italy, Mediterranean Sea), in summer 2015, on bare rocky boulders, the habitat preferred by P. gibbesi. The study sites were characterized by significantly different species richness and abundance of the potential predators of P. gibbesi. Predation rate of different predator assemblages was assessed by means of tethering experiments on the alien crab. As predicted, predation rate on P. gibbesi resulted significantly higher in sites characterized by higher predator abundance and species richness than in sites with lower values. Our findings, although restricted to small spatial scales, support the diversity resistance hypothesis, revealing a negative relationship between the diversity of predator assemblages and the invasiveness (average abundance) of the alien species P. gibbesi.

The effects of alien macroalgal invasions on ecosystem functioning may depend on the feeding choice of native herbivores between native and non-native algae. The co-occurrence of two invasive Caulerpa species, C. taxifolia var. distichophylla and C. cylindracea in Southern Sicily, offered the possibility to investigate the feeding preferences of a generalist native grazer, the sea urchin Paracentrotus lividus, for native vs non-native food. A multiple choice feeding experiment was set to assess whether and to which extent P. lividus consumes the exotic macroalgae while in presence of the preferred native ones. Multiple choice feeding assays were run with the following food sources simultaneously offered to P. lividus: C. cylindracea, C. taxifolia var distichophylla, a mix of C. cylindracea and C. taxifolia var distichophylla, Dictyopteris membranacea, Cystoseira spp. and a mix of D. membranacea and Cystoseira spp.. Paracentrotus lividus exhibited a stronger feeding preference for the mix of the two exotic macroalgae and for C. cylindracea alone. On the other hand, C. taxifolia var distichophylla resulted barely consumed, although the amount ingested was greater when C. taxifolia var. distichophylla was mixed with the congeneric alga. Our findings confirmed the strong palatability of C. cylindracea and the potential role of P. lividus in controlling the spread of this exotic species. Given the low consumption of C. taxifolia var. distichophylla recorded in this study, it is unlikely that P. lividus may contribute to limit the spread of the new exotic Caulerpa species.

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, peer-reviewed

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 urchins.

Predation of young sea urchins settlers are often invoked as a key process in the control of sea urchins populations and therefore in the structure and stability of rocky infralittoral communities. In this study a number of decapod species were detected as predators of young settlers of the sea urchin Paracentrotus lividus and their predation rates estimated by laboratory experiments. Abundance of these predators also resulted significantly lower in barren than in macro algae forests, suggesting that lack of predation of juvenile sea urchins facilitates the stability of Mediterranean barren systems.

Shallow CO2 vents are used to test ecological hypotheses about the effects of ocean acidification (OA). Here, we studied fish assemblages associated with Cymodocea nodosa meadows exposed to high pCO2/low pH conditions at a natural CO2 vent in the Mediterranean Sea. Using underwater visual census, we assessed fish community structure and biodiversity in a low pH site (close to the CO2 vent), a close control site and a far control site, hypothesising a decline in biodiversity and a homogenization of fish assemblages under OA conditions. Our findings revealed that fish diversity did not show a unique spatial pattern, or even significant relationships with pH, but correlated with seagrass leaf canopy. Among-site similarity was found in the abundance of juveniles, contrary to the expected impacts of OA on early life stages. However, pH seems an important driver in structuring fish assemblage in the low pH site, despite its high similarity with the close control site. This unexpected pattern may represent a combined response of fish mobility, enhanced food resources in the acidified site, and a 'recovery area' effect of the adjacent control site.

Understanding which factors influence the invasion of alien seaweed has become a central concern in ecology. Increasing evidence suggests that the feeding preferences of native herbivores influence the success of alien seaweeds in the new community. We investigated food selection of a generalist native grazer Paracentrotus lividus, in the presence of two alien seaweeds (Caulerpa cylindracea and Caulerpa taxifolia var. distichophylla) and two native seaweeds (Dictyopteris membranacea and Cystoseira compressa). Sea urchins were fed with six experimental food items: C. cylindracea, C. taxifolia var. distichophylla, a mixture of C. cylindracea and C. taxifolia var. distichophylla, D. membranacea, C. compressa and a mixture of D. membranacea and C. compressa. P. lividus ingested all the combinations of food offered, though it preferentially consumed the alien mixture, C. cylindracea and D. membranacea. The alien C. taxifolia var. distichophylla was consumed significantly less than the other food items and, interestingly, it was ingested in a greater amount when mixed with C. cylindracea than when on its own. This finding suggests that C. taxifolia var. distichophylla may become vulnerable to sea urchin grazing when it grows intermingled with C. cylindracea, which does not gain immediate protection from the presence of the very low palatable congeneric seaweed. The present study highlights the potential role of native grazers to indirectly affect the interspecific competition between the two alien seaweeds in the Mediterranean Sea.

Predator–prey interactions may be strongly influenced by temperature variations in marine ecosystems. Consequently, climate change may alter the importance of predators with repercussions for ecosystem functioning and structure. In North-eastern Pacific kelp forests, the starfish Pycnopodia helianthoides is known to be an important predator of the purple sea urchin Strongylocentrotus purpuratus. Here we investigated the influence of water temperature on this predator–prey interaction by: (i) assessing the spatial distribution and temporal dynamics of both species across a temperature gradient in the northern Channel Islands, California, and (ii) investigating how the feeding rate of P.helianthoides on S.purpuratus is affected by temperature in laboratory tests. On average, at sites where mean annual temperatures were 16°C (equivalent to summer temperatures at sites where P.helianthoides were rare) reduced predation rates regardless of predator and prey sizes, although larger sea urchins were consumed only by large starfishes. These results clearly demonstrate that the effect of P.helianthoides on S.purpuratus is strongly mediated by temperature, and that the local abundance and predation rate of P.helianthoides on sea urchins will likely decrease with future warming. A reduction in top-down control on sea urchins, combined with other expected impacts of climate change on kelp, poses significant risks for the persistence of kelp forests in the future.

Native 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.

The occurrence of the invasive seaweed Caulerpa taxifolia in the Straits of Messina (Italy) provides an excellent opportunity to examine the spatial and temporal patterns of abundance of the endemic Mediterranean sacoglossan Oxynoe olivacea. Densities of this species were recorded in March, June and October 2000 on three different habitats: the underside of boulders (UNB), the upper side of boulders (UPB), and the surface of concrete blocks (CB). Results showed that in March, O. olivacea was consistently more abundant on UNB whereas in June it was present exclusively on CB. The density of O. olivacea was similar in the three habitats in October. The hiding behaviour of O. olivacea and its seeming disappearance in March did not coincide with a decline of abundance of phylloids of C. taxifolia. The results represent an important basis for further studies devoted to the investigation of abundance of these marine invertebrates, which has been traditionally thought of as unpredictable.

Asteroids are largely recognized as important predators in all of the world’s oceans and for this reason, they play a crucial role in shaping the structure and functioning of benthic ecosystems. The spiny starfish Marthasterias glacialis is generally considered a voracious predator of molluscs, in particular bivalves. Using field observations and carbon (δ13C) and nitrogen (δ15N) stable isotopes, we explored possible changes in diet in relation to size of M. glacialis. Data were collected at Ustica Island (Southern Tyrrhenian Sea) from June 2008 to June 2010. M. glacialis showed a size shift in feeding preferences due to different use of food items: bivalves, Columbella rustica, Euthria cornea and Trochoidea were strongly selected by small starfish whereas the larger ones preferred Arbacia lixula and Paracentrotus lividus. The clear picture that emerged is that large M. glacialis is able to control sea urchin populations and indirectly Mediterranean rocky reef ecosystems.

Sympatric sea urchin species are usually considered to belong to the same grazer guild. Nevertheless, their role in community dynamics may vary due to species-specific morphological traits, feeding preferences and foraging behavior. In the Mediterranean Sea, the two species Paracentrotus lividus and Arbacia lixula co-occur in barren areas. Whereas P. lividus is usually considered responsible for creating a barren ground, the roles of the two sea urchin species in its maintenance are currently unclear. The relative and combined effects of P. lividus and A. lixula on maintaining the benthic community in the barren state were tested experimentally, using orthogonal exclusion of the two species. Results show that exclusion of A. lixula, regardless of the presence of P. lividus, led to a significant decrease in the surface of bare rock and a significant change of the algal assemblages, thus demonstrating the major role of this species in maintaining the barren state.

Although protected areas can lead to recovery of overharvested species, it is much less clear whether the return of certain predator species or a diversity of predator species can lead to re-establishment of important top-down forces that regulate whole ecosystems. Here we report that the algal recovery in a Mediterranean Marine Protected Area did not derive from the increase in the traditional strong predators, but rather from the establishment of a previously unknown interaction between the thermophilic fish Thalassoma pavo and the seastar Marthasterias glacialis. The interaction resulted in elevated predation rates on sea urchins responsible for algal overgrazing. Manipulative experiments and field observations revealed that the proximity of the seastars triggered an escape response in sea urchins, extending their tube feet. Fishes exploited this behavior by feeding on the exposed tube feet, thus impairing urchin movement and making them vulnerable to predation by the seastars. These findings suggest that predator diversity generated by MPA establishment can activate positive interactions among predators, with subsequent restoration of the ecosystem structure and function through cascading consumer impacts.

In the Mediterranean Sea the co-occurring sea urchins Paracentrotus lividus and Arbacia lixula are usually considered to share the same ecological role in the formation of barren from Cystoseira canopy. However, their foraging ability may vary due to feeding behavior and species-specific morphological traits. The relative effects of P. lividus and A. lixula on Cystoseira canopy was tested experimentally both in the laboratory, at a density of about 20 ind./m 2 , and in the field by gut content analysis. Field and laboratory results show that A. lixula is unable to affect Cystoseira spp. Furthermore, these results confirmed the great ability of P. lividus to consume Cystoseira canopy, thus supporting the evidence of the major role of this species in the creation of a barren state.

Sea urchins of the genus Arbacia are widely distributed throughout the world occurring in tropical, temperate and sub-Antartic zones. In the past, Arbacia has been the object of intensive investigations in cell biology and biochemistry of fertilization and early development. Renewed interest in the genus Arbacia arose mainly due to its ecological role and its unusual geographic distribution. All the six species of Arbacia are omnivorous with a strong tendency to carnivory. Recent manipulative and morpho-functional studies demonstrated that Arbacia species play an important role in the ecology of rocky reefs. They maintain barren areas by scraping either fleshy and encrusting algae and animals from the substrata, thanks to their large Aristotle’s lantern, indicative of a durophagic habit. The genus Arbacia is distributed in both high and low latitudes, and its phylogeny does not confirm the tendency of marine species to arise in the tropics and then spread into the temperate zone. Arbacia is a gonochoric genus with a very early maturity. Generally Arbacia species show a reproductive cycle typically annual or semi-annual. Some species of this genus (i.e. A. lixula) seem to be favored by the increase of temperature. These findings point out the potential role of Arbacia in a scenario in which the mean global temperature of sea surface and the frequencies of temperature anomalies are increasing and the necessity to amply the study on biology and ecology of this genus.

Despite the large body of work published in the last two decades on the reproduction of the sea urchin Paracentrotus lividus, the reproductive aspects linked to hydrodynamic conditions and their influence on gonad production remain poorly understood. The present paper aims to evaluate the effect of hydrodynamism on the reproductive cycle of P. lividus. Variability in the gonadosomatic index (GSI) of P. lividus was estimated seasonally from 2007 to 2008 at two shallow sub-littoral flat basaltic areas at Ustica Island (Western Mediterranean). GSI was higher in the sites characterized by low hydrodynamism than in those with high hydrodynamism. Results also suggest a possible role for hydrodynamism in triggering processes of resource limitation (food shortage), probably by interfering with P. lividus feeding activity.

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.

Rocky reefs can shift from forest, a state dominated by erect algae with high biodiversity, to barren, an impoverished state dominated by encrusting algae. Sea urchins, abundant in barrens, are usually held responsible for the maintenance of this state. Predation by large fish can revert the barren state to forest by controlling sea urchin populations. However, the persistence of a community state sometimes seems to be independent from the presence of such large predators, suggesting the existence of other, unknown mechanisms ensuring their stability. Theoretical studies suggest that the settler stage of sea urchins is determinant for maintaining a given rocky reef state. In this study, we have identified several potential invertebrate micropredators of settlers of the sea urchin Paracentrotus lividus and measured their predation activity. Predation rates showed marked differences among species, possibly due to morphological and/or behavioral traits. Micropredators were more abundant in the forest than in barren, and their potential impact on the sea urchin community differed between the two states by two orders of magnitude. These findings suggest a novel self-perpetuating mechanism stabilizing rocky reef systems, where the abundance of micropredators may contribute to shape the sea urchin population, which in turn is responsible for the persistence of the state.

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.

In the Mediterranean, sea breams are the most effective Paracentrotus lividus and Arbacia lixula predators. Generally, seabreams dislodge adult urchins from the rocky substrate, turn them upside down and crush their tests. Sea urchins may respond to fish attacks clinging tenaciously to the substratum. This study is the first attempt to investigate sea urchin adhesion strength in two alternative algal assemblages of the rocky infralittoral and valuated its possible implication for fish predation. We hypothesized that (1) sea urchin adhesion strength is higher in rocky shores dominated by encrusting macro-algae (ECA) than in erected macro algae (EMA); (2) predation rates upon sea urchins are lower in ECA than in EMA; and (3) predation rate on A. lixula is lower than that on P. lividus. We observed that attachment tenacity of both sea urchins was higher in ECA than EMA and that A. lixula exhibited a stronger attachment tenacity than P. lividus in ECA. Results supported the importance of adhesion strength, as efficient defence against sea bream attacks, only for, P. lividus. A. lixula adhesion strength does not seem to be an important factor in avoiding fish predation, possibly because of the low palatability of the species.These patterns may deserve particular interest in understanding the processes responsible for the maintenance of sea urchin barrens that are dominated by ECA assemblage.