Frontiers in Marine Science (Jun 2014)
Sea urchins, their predators and prey in SW Portugal
Abstract
Sea urchins play a key role structuring benthic communities of rocky shores through an intense herbivory. The most abundant sea urchin species on shallow rocky subtidal habitats of the SW coast of Portugal is Paracentrotus lividus (Echinodermata: Echinoidea). It is considered a key species in various locations throughout its geographical distribution by affecting the structure of macroalgae communities and may cause the abrupt transformation of habitats dominated by foliose algae to habitats dominated by encrusting algae - the urchin barrens. The removal of P. lividus predators by recreational and commercial fishing is considered a major cause of this phenomenon by affecting the trophic relationships between predators, sea urchins and algae communities. Marine protected areas (MPAs) usually lead to the recovery of important predator species that control sea urchin populations and restore habitats dominated by foliose macroalgae. Therefore, MPAs provide a good opportunity to test cascading effects and indirect impacts of fishing at the ecosystem level. The ecological role of P. lividus was studied on rocky subtidal habitats of the SW coast of Portugal (Alentejo) considering three trophic levels: population of P. lividus, their predators (fish and shellfish) and their prey (macroalgae communities). Several studies were conducted: (1) a non-destructive observational study on the abundance and distribution patterns of P. lividus, their predators and preys, comparing areas with different protection; (2) a manipulative in situ study with cages to assess the role of P. lividus as an herbivore and the influence of predation; (3) a descriptive study of P. lividus predators based on underwater filming; (4) and a study of human perception on these trophic relationships and other issues on sea urchin ecology and fishery, based on surveys made to fishermen and divers. Subtidal studies were performed with SCUBA diving at 3-12 m deep. Results indicate that in the Alentejo coast: (1) P. lividus appears to have a “sit-and-wait” behavior remaining in crevices or burrows for prolonged times, feeding on drift algae trapped there and, therefore, does not seem to have the ability to significantly influence the structure of the benthic macroalgae community; (2) the studied marine protected area does not appear, to date, to have had significant effects on the abundance and/or on the size of the various studied organisms; and (3) the fish species Diplodus vulgaris and Coris julis may be important predators of P. lividus. Lastly (4), results obtained in the surveys indicate that in this coast: the vast majority of fishermen found traces of sea urchins in the stomach contents of certain fish species (e.g. Diplodus sargus, D. vulgaris, Sparus aurata); the occurrence of urchin barrens is not known by most respondents; the recreational harvesting of sea urchins is a frequent activity on intertidal and shallow subtidal rocky habitats, during winter and spring, and mostly in Easter.
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