Preprint volume - SIBM

Pre-print Volume - Oral presentations
Topic 2: MARINE ORGANISMS AND ECOSYSTEMS AS MODEL SYSTEMS
C. BONAVIRI, C. PIPITONE 1 , P. GIANGUZZA, B. HEREU 2
Dipartimento di Ecologia Università di Palermo, Via Archirafi, 18 - 90123 Palermo, Italia.
chiabon@libero.it
1 CNR-IAMC, Sede di Castellammare del Golfo, Trapani, Italia.
2 Department d’Ecologia, Universitat de Barcelona, Spain.
PREDATION ON YOUNG PARACENTROTUS LIVIDUS SETTLERS:
IMPLICATIONS FOR MEDITERRANEAN ROCKY
INFRALITTORAL STABILITY
PREDAZIONE SU GIOVANILI DI PARACENTROTUS LIVIDUS:
IMPLICAZIONI PER LA STABILITÀ DEI SISTEMI INFRALITORALI ROCCIOSI
MEDITERRANEI
Abstract – 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.
Key-words: ecosystem stability, predator-prey interactions, sea urchin, crustaceans, hard bottoms.
Introduction - In the last decades temperate infralittoral ecosystems have undergone a
loss of habitat-forming algae. Erect macroalgae canopy can be massively reduced by
perturbation (e.g., loss of top-down control of grazers, destructive harvesting) leading
to a shift towards an alternative phase dominated by sea urchins and encrusting
organisms named barren (Sala et al., 1998). Whether encrusting- and erect macroalgae
dominated communities represent alternate stable states of rocky systems has not been
proven (Knowlton, 2004). Sea urchins biomass and abundance maintain high in barren
areas where few-months old individuals can be more numerous than in macro algae
forests (Rowley, 1989). Echinoids are invertebrates with a planktonic larval phase and
their population structure strictly depends on larval supply, settlement process and
post-settlement mortality. Some authors suggest that mortality of young settlers could
represent the bottleneck for sea urchin populations in erect macroalgae forests
(Jennings & Hunt, 2010 and reference therein). Given that erect macroalgae are
habitat-formers, they add physical complexity to the substratum by increasing species
richness and functional diversity of mobile epifaunal organisms (Taylor, 1998).
Density of small invertebrates is positively correlated with structural complexity of the
habitat, being low in barrens and high in erect algae assemblages (Taylor, 1998). The
fact that young settled sea urchins are vulnerable to predation by various small
invertebrates (Scheibling & Robinson, 2008), may make erect algae systems less
suitable for the survival of recruits. Mediterranean rocky littorals are characterized by
either barren or macro algae forests (Sala et al., 1998). The sea urchin Paracentrotus
lividus resides in both systems but displays always higher biomass and abundance in
the former. Since the settlement of P. lividus is probably independent from the benthic
assemblage (Hereu, 2004), the difference of the sea urchin population structure may
depend on the post-settlement mortality between the two systems. Mortality of P.
lividus recruits has been estimated to reach 75% during the first six months in a macro
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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