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The gastropod Arion fuscus prefers cyanobacterial to green algal parts of the tripartite lichen Nephroma arcticum due to low chemical defence

Published online by Cambridge University Press:  26 November 2009

Johan ASPLUND  and
Yngvar GAUSLAA
Johan ASPLUND
Affiliation:
Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432 Ås, Norway. Email: johan.asplund@umb.no
Yngvar GAUSLAA
Affiliation:
Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432 Ås, Norway. Email: johan.asplund@umb.no
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Abstract

Although the tripartite terricolous lichen Nephroma arcticum is easily accessible to lichen-feeding gastropods, grazing marks are mainly restricted to localized cephalodia with N-fixing Nostoc. We tested if this gastropod preference for cephalodia can be explained by differences in carbon based secondary compounds (CBSCs) in cyanobacterial versus green-algal tissues. CBSCs were non-destructively removed from air-dry thalli by 100% acetone. Compound deficient and control thallus parts were offered to the slug Arion fuscus and grazing preferences were quantified by area measurements in ArcGIS™. The concentrations of CBSCs (phenarctin, usnic acid, nephroarctin and methyl gyrophorate) in thallus parts with and without cephalodia were quantified with HPLC. Compared to purely green-algal parts, cephalodial parts with adjoining fungal tissues contained less defensive compounds, and were preferred by A. fuscus. The cephalodia themselves do not contain any CBSCs. After acetone rinsing, A. fuscus did not discriminate between green-algal and cyanobacterial parts. The results were consistent with the hypothesis that CBSCs in green-algal parts of N. arcticum play a herbivore-defensive role. It is further hypothesized that grazing of cephalodia may lead to N-starvation and reduced growth of N. arcticum thalli in southern portions of its range where lichenivorous gastropods are more abundant. This may play a role in shaping the southern distribution limit of this arctic-boreal lichen species.

Keywords

acetone rinsingcephalodiumdepsidesherbivorymolluscs
Type
Research Article
Information
The Lichenologist , Volume 42 , Issue 1 , January 2010 , pp. 113 - 117
Copyright
Copyright © British Lichen Society 2009

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