No abstract

Factors controlling herbivory pressure are of central importance in shaping the seascape. In the Mediterranean, the sea urchin Paracentrotus lividus is considered as a keystone herbivore in seagrass meadows and macroalgal communities. Here we explored the trophic behavior of this sea urchin in a shallow seagrass habitat of Cymodocea nodosa mixed with Caulerpa prolifera and interspersed with sandy areas in Alfacs Bay, Ebro Delta (NW Mediterranean). The seasonal pseudo-indigenous bryozoan Amathia verticillata is locally very abundant, and there is also an important population of pen shells Pinna nobilis, providing hard substrate and cover, thus being a unique environment for assessing sea urchin trophic behavior. To this end, we conducted an ensemble of food preference and foraging experiments and stomach content and stable isotope analyses. Our results showed that sea urchins strongly prefer A. verticillata over other local resources, and there was also an important presence of the bryozoan in stomach contents (ca. 44%), coupled with green and decayed seagrass leaves. Stable isotope analyses revealed that over the long term, ca. 65% of the diet of P. lividus was based on decayed seagrass leaves, followed by the bryozoan and green seagrass leaves (21.7 and 13.3%, respectively). The local availability of P. nobilis provides a preferred substrate for sea urchins, which showed limited foraging movements into the surrounding seagrass beds, particularly when A. verticillata was attached to the pen shells. The apparently high contribution of animal and detrital food to P. lividus diet is unprecedented, and suggests an opportunistic feeding behavior in sea urchins in those habitats.

Gorgonian gardens of a new species of the genera Leptogorgia sp. are found in Central Chile. Little is known about these populations including depth distribution, colony abundance, structural attributes, and associated macrofaunal assemblages. Hierarchical sampling was conducted at 20 m depth (maximum colony abundances) in order to assess spatial variability in abundance and colony attributes at two spatial scales (among sites and rocky walls). The abundance and composition of the associated vagile and sessile macrofauna were also examined using univariant (Taxa richness, and Shannon index (H’e)) and multivariant approaches, and were compared with adjacent bare rocky habitats. Our results showed a high abundance of gorgonians (ca. 28.9 to 36.5 colonies m-2) compared to other gorgonian gardens in the world. For structural attributes, our results showed smaller colonies with thicker holdfasts in more exposed sites, suggesting the influence of hydrodynamic forces on the colony morphology. Taxa richness and H’e of vagile fauna showed 3-fold and 2-fold, respectively, higher values in gorgonian gardens compared to bare walls, but no differences were observed for sessile fauna. In addition, PCoA and PERMANOVA evidenced a distinctive assemblages’ composition between habitats for both vagile and sessile fauna. Correlation analyses and dbRDA showed, however, little association between structural attributes and associated faunal assemblages (R2=0.06, and ca. 3 to 9.4 % of the total variation explained, respectively). Our results constitute the first assessment of structural habitat complexity and accompanying fauna in these gorgonian gardens and establish the baseline for understanding possible future changes associated to human activities.