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

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 δ 15 N 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. Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 441: [117][118][119][120][121][122][123][124][125][126][127][128] 2011 This herbivorous behaviour described in Arbacia lixula is, however, in sharp contrast with other species in the genus Arbacia, where omnivorous or unambiguously carnivorous diets have been reported. North American A. punctulata feeds on sessile invertebrate species, sand dollars and other Arbacia individuals, as well as some algae (Harvey 1956, Karlson 1978, Cobb & Lawrence 2005. The diet of South Atlantic A. dufresnei is mainly carnivorous (Penchaszadeh 1979, Penchaszadeh & Lawrence 1999. The Pacific A. spatuligera showed preference for animal food over common species of algae from its habitat (Silva et al. 2004). Moreover, some observations indicate omnivorous or carnivorous behaviour of A. lixula outside the Mediterranean (Marques 1984, Oliveira 1991, Tavares & Borzone 2005.The crucial importance of sea urchins in shaping benthic ecosystems (Lawrence 1975) has been demonstrated by many ecological experiments along the Mediterranean coasts (e.g. Benedetti-Cecchi & Cinelli 1995, Sala & Zabala 1996, Benedetti-Cecchi et al. 1998, Palacín et al. 1998a, Bulleri et al. 1999, Guidetti et al. 2004, Bonaviri et al. 2011. The underlying premise in these ...

Functional traits of two co-occurring sea urchins across a barren/forest patch system

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The reproductive cycle of the sea urchin Arbacia lixula in northwest Mediterranean: potential influence of temperature and photoperiod

“…This maturation heterogeneity may also be found in other sea 347 accumulated rather than the maturation stage, and therefore the GSI alone would not be a good 359 parameter to assess the gametogenic state of these species. However, A. lixula is an omnivore 360 tending to carnivory (Wangensteen et al 2011) whose densities are low to moderate in our sampling 361 area. We measured the density values at the beginning and at the end of our study, and did not find 362 any significant differences which could influence the gonad index.…”

Trophic plasticity of Antarctic echinoids under contrasted environmental conditions

“…Second, stable isotope ratios of C and N were analysed. These trophic markers have proven particularly useful to unravel trophic interactions in Antarctic benthic food webs (Nyssen et al 2002), but also to identify food sources of sympatric regular echinoids from other regions when gut contents look alike (Wangensteen et al 2011). …”