Interplay of encrusting coralline algae and sea urchins in maintaining alternative habitats

Abstract

In proximity of shelters, grazing by sea urchins plays a fundamental role in establishing and maintaining areas dominated by encrusting corallines. Much attention has been given to the effects of urchins on algal assemblages in shallow subtidal reefs, but few studies attempted to clarify the role played by encrusting coralline algae in this system. It has been shown that encrusting corallines are able to reduce settlement of potential competitors, suggesting that they do not rely on grazing by herbivores to prevent swamping by erect algal species. In shallow subtidal reefs of the Mediterranean Sea, the sea urchins Paracentrotus lividus and Arbacia lixula are the main herbivores, whose grazing commonly leads to a mosaic of areas dominated alternatively by encrusting corallines and turf-forming algae. This study aims to separate the effects of urchins and encrusting corallines on the re-colonisation of encrusting coralline-dominated patches (barren habitat) by surrounding erect algal species (turf-forming algae). Different hypotheses were tested by means of multivariate and univariate analyses. The multivariate hypothesis was that the algal assemblage developing when encrusting corallines and urchins are simultaneously removed would be more similar to that establishing in stands of turf-forming algae, than that developing when none or only 1 of the 2 factors is manipulated. The univariate hypotheses tested were: (1) that there is a negative effect of sea urchins and encrusting corallines on the re-colonisation of barren areas by surrounding turf-forming algal species and (2) that the effects of encrusting corallines are weaker than those of urchins, but that they operate in the same direction. These hypotheses were tested by means of an orthogonal manipulation of urchins and encrusting corallines. At each of 3 study sites, 2 replicate barren patches were assigned to each of these treatments: (1) +corallines+urchins; (2) +corallines-urchins; (3) -corallines+urchins; (4) -corallines-urchins. The results suggest that the occurrence of areas dominated by encrusting corallines on shallow subtidal reefs in the northwest Mediterranean is not simply the result of grazing by sea urchins on turf-forming species. The removal of encrusting corallines also affected the abundance of dominant algal species and determined the development of an algal assemblage resembling those occurring within stands of algal turfs. The effects of the removal of urchins on turf-forming species were generally positive, while those of encrusting corallines varied from negative (Padina pavonica) to positive (Acetabularia acetabulum and filamentous algae). Therefore, the role played by encrusting corallines in maintaining alternative habitats on shallow subtidal reefs should be taken into account, thus avoiding the overestimation of the effects of grazing by sea urchins

Community dynamics and ecosystem simplification in a high-CO 2 ocean

“…Furthermore, our results suggest that ocean acidification could cause a change in the functional role of calcareous grazers. Changes in the role of large calcareous grazers, such as sea urchins and herbivorous snails that are known to influence community structure in many different marine ecosystems (18,(20)(21)(22)(23), could have widespread consequences for many ocean ecosystems (17,18).…”

“…The conditions in the low pH zones are most comparable to nearfuture (i.e., 2100) scenarios, whereas those in the extreme low pH zones correspond to more extreme scenarios based on high CO 2 emissions or the more distant future (e.g., 2500) (38) ( Table 1). The two most common sea urchins at this site, Arbacia lixula and Paracentrotus lividus, routinely graze fleshy algae (18,23,39).…”

“…Herbivory is a natural disturbance that can cause habitat patchiness (16)(17)(18) by limiting the abundance of dominant plant species and facilitating the persistence of other species that would be outcompeted and/or overgrown (18,19). In marine ecosystems, calcareous invertebrate herbivores, including sea urchins and herbivorous snails, play fundamental roles in structuring communities by creating disturbances (20)(21)(22)(23) and are thought to be some of the most vulnerable species to ocean acidification (10).…”

Role of two co-occurring Mediterranean sea urchins in the formation of barren from Cystoseira canopy

“…In the Mediterranean Sea Paracentrotus lividus and Arbacia lixula are commonly found in both barren grounds and erected macroalgae assemblages (Guidetti and Dul ci c, 2007;Agnetta et al, 2013) and traditionally they have been considered a guild of herbivores (Bulleri et al, 1999(Bulleri et al, , 2002. However recent investigations have distinguished the diets and trophic positions of the two species showing that A. lixula is an omnivore tending to carnivore, while P. lividus is basically a herbivore that can turn into an omnivore both on barren and vegetated rocky areas .…”

“…Canopy-forming brown algae are habitat formers able to locally modify physical and biological factors and understory assemblages (Ballesteros et al, 1998;Bulleri et al, 2002;Graham, 2004) leading to biological amelioration (sensu Moore et al, 2007). There is concern globally about their widespread loss across many rocky shores worldwide (e.g.…”