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Marine Ecology Progress Series

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MEPS 441:117-128 (2011)  -  DOI:

A wolf in sheep’s clothing: carnivory in dominant sea urchins in the Mediterranean

Owen S. Wangensteen1,*, Xavier Turon2, Alex García-Cisneros1, Mireia Recasens1, Javier Romero3, Creu Palacín1

1Department of Animal Biology, University of Barcelona, 08028 Barcelona, Spain
2Center for Advanced Studies of Blanes (CEAB, CSIC), 17300 Blanes (Girona), Spain
3Department of Ecology, University of Barcelona, 08028 Barcelona, Spain

ABSTRACT: Arbacia lixula and Paracentrotus lividus are the dominant sea urchins in the Mediterranean sublittoral, where they are key structuring species due to their grazing activity. It has been commonly accepted that competition between both species is minimized by specializing in different algal foods. A. lixula is considered to feed mainly on encrusting coralline algae, while P. lividus prefers fleshy macroalgae. We used stable isotope analysis to test if these species occupy different trophic positions at 3 locations in the western Mediterranean and one in Macaronesia. Our results show unambiguously that A. lixula always occupies a higher trophic level than P. lividus, with a δ15N comparable in some locations to strict carnivores such as Actinia schmidti or Marthasterias glacialis. A temporal monitoring at one locality showed that this signature of a higher trophic level is consistent throughout the year. These results are incompatible with the current belief of an herbivorous diet for A. lixula and suggest that it must be considered an omnivore tending to carnivory in Mediterranean ecosystems, feeding at least partially on sessile animals such as Cirripedia, Hydrozoa or Bryozoa. A parallel analysis of gut contents showed a predominance of vegetal items in both species, although A. lixula consistently had a higher abundance of animal components than P. lividus. Our results challenge the validity of using gut content observations alone for characterizing the trophic behaviour of omnivorous marine invertebrates that feed on a variety of food sources with different digestibility.

KEY WORDS: Arbacia lixula · Paracentrotus lividus · Trophic relationships · Benthic community · Stable isotope analysis.

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Cite this article as: Wangensteen OS, Turon X, García-Cisneros A, Recasens M, Romero J, Palacín C (2011) A wolf in sheep’s clothing: carnivory in dominant sea urchins in the Mediterranean. Mar Ecol Prog Ser 441:117-128.

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