Abstract
Grazing-induced plant defences that reduce palatability to herbivores are widespread in terrestrial plants and seaweeds, but they have not yet been reported in seagrasses. We investigated the ability of two seagrass species to induce defences in response to direct grazing by three associated mesograzers. Specifically, we conducted feeding-assayed induction experiments to examine how mesograzer-specific grazing impact affects seagrass induction of defences within the context of the optimal defence theory. We found that the amphipod Gammarus insensibilis and the isopod Idotea chelipes exerted a low-intensity grazing on older blades of the seagrass Cymodocea nodosa, which reflects a weak grazing impact that may explain the lack of inducible defences. The isopod Synischia hectica exerted the strongest grazing impact on C. nodosa via high-intensity feeding on young blades with a higher fitness value. This isopod grazing induced defences in C. nodosa as indicated by a consistently lower consumption of blades previously grazed for 5, 12 and 16 days. The lower consumption was maintained when offered tissues with no plant structure (agar-reconstituted food), but showing a reduced size of the previous grazing effect. This indicates that structural traits act in combination with chemical traits to reduce seagrass palatability to the isopod. Increase in total phenolics but not in C:N ratio and total nitrogen of grazed C. nodosa suggests chemical defences rather than a modified nutritional quality as primarily induced chemical traits. We detected no induction of defences in Zostera noltei, which showed the ability to replace moderate losses of young biomass to mesograzers via compensatory growth. Our study provides the first experimental evidence of induction of defences against meso-herbivory that reduce further consumption in seagrasses. It also emphasizes the relevance of grazer identity in determining the level of grazing impact triggering resistance and compensatory responses of different seagrass species.
Highlights
Plants and herbivores are involved in complex interactions, in which plants play an active role
The amphipod G. insensibilis and the isopods I. chelipes and C. truncata are associated with both seagrass species, while the isopod S. hectica is solely associated with larger seagrasses like C. nodosa
I. chelipes and G. insensibilis grazed more than 50% of Z. noltei blades after 12 days, but only grazing by I. chelipes showed an asymptotic attenuation that reflected a reduction in the grazing impact in the last time interval (Fig 2b and 2c)
Summary
Plants and herbivores are involved in complex interactions, in which plants play an active role. Terrestrial plants, freshwater macrophytes, and seaweeds may respond to herbivory by inducing plastic defences to prevent further attacks (reviewed in [1, 2, 3]). Inducible defences involve several structural and chemical traits that decrease plant palatability or attractiveness to herbivores with negative effects on their preference or fitness [4, 5, 6]. Major known inducible chemical defences include a wide range of defensive proteins and secondary metabolites that have toxic, deterrent, and/or digestion-reducing effects on herbivores [9, 10]. Induced defences may have large effects on herbivore populations and community structure Induced defences may have large effects on herbivore populations and community structure (e.g. [11, 12, 13])
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