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Proximate and ultimate causes for transatlantic variation in seaweed defenses

Journal article
Authors JD Long
Henrik Pavia
Gunilla B. Toth
Published in Marine Ecology Progress Series
Volume 493
Pages 83-89
ISSN 0171-8630
Publication year 2013
Published at Department of Biological and Environmental Sciences, Tjärnö Marine Biological Laboratory
Pages 83-89
Language en
Links dx.doi.org/10.3354/meps10511
Subject categories Biological Sciences

Abstract

Previous tests of theories predicting tradeoffs between constitutive and inducible defenses are inconclusive. This may result from focusing on individual traits or using cultivated plants subjected to artificial selection. We examined this tradeoff in the seaweed Ascophyllum nodosum (hereafter Ascophyllum)—one of the best-studied seaweeds with inducible defenses. Although several European populations respond to grazing with induced resistance, a US population failed to express inducible defenses. This variation may be related to corresponding variation in constitutive resistance and consumer pressure. We compared the constitutive resistance of Ascophyllum from an inducing and a non-inducing population (Sweden and USA, respectively). We quantified constitutive resistance by conducting choice feeding assays on seaweed from both populations after inducible defenses had relaxed, rather than focusing on single traits. We measured constitutive phlorotannins to confirm that defenses had relaxed and to compare between-population levels. Periwinkles consistently preferred relaxed Swedish Ascophyllum, suggesting that US Ascophyllum without inducible resistance had higher constitutive resistance. Neither snail source nor experimental arena influenced this result. Phlorotannins were also higher in relaxed US Ascophyllum. To test the hypothesis that defense type may be related to current consumer pressure, we compared per capita grazing rates and average snail densities from both locations. US snails grazed more and were more abundant in the USA, suggesting that current consumer pressure may be higher on US Ascophyllum. Consistent with theory, constitutive and inducible resistance were negatively related between our sites. Inducible resistance may be absent from the US population because these individuals have high constitutive resistance, perhaps because they encounter greater consumer pressure.

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