Specificity in Mesograzer-Induced Defences in Seagrasses

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.

A 15-Month Survey of Dimethylsulfoniopropionate and Dimethylsulfoxide Content in Posidonia oceanica

“…Predator deterrence by DMSP and its metabolites was indicated by grazing observations by rats on inner tissues of S. alterniflora stems depleted in DMSP after nitrogen fertilization (Morris et al, 2002;Sundareshwar et al, 2003;Otte et al, 2004), difficulties to experimentally feed rats at high concentrations of DMSP (Nakajima, 1989) and DMSP cleavage to DMS and acrylic acid which deter feeding on algae by some herbivores (Van Alstyne et al, 2001;Van Alstyne and Houser, 2003). Changes in tolerance to grazing and defense allocation against herbivores in aging tissues (Schultz et al, 2013;Martínez-Crego et al, 2015) could then lead to the decrease of the DMSP concentration in P. oceanica leaves over time. Second, the protection against ROS as suggested above.…”

Trophic plasticity in the sea urchin Paracentrotus lividus, as a function of resource availability and habitat features

“…Stomach contents analyses. Differences in the abundance of food items (C. nodosa, C. prolifera, and A. verticillata) in stomach contents of individuals from seagrass and sand habitats was investigated with a one-way repeated measures ANOVA, using food item as the within-subject variable (see Martínez-Crego et al 2015 for a similar approach) and the habitat (sand vs. seagrass) as a between-subject variable.…”

The Role of Seagrass Traits in Mediating Zostera noltei Vulnerability to Mesograzers

“…insensibilis . Nonetheless, we are aware that the role of nutritional, chemical and structural traits in determining mesograzer feeding behaviour may differ between seagrass species, as has been pointed out by previous studies [ 55 ].…”

“…This tolerance has led to the classic hypothesis in which Hay et al [ 53 , 54 ] proposed that mesograzers selectively choose chemically defended seaweeds as habitat and food in order to lower predation risk to omnivorous fishes. More recently, strong interspecific differences have been reported between mesograzer species in their ability to induce seagrass chemical defences that deter further consumption [ 55 ]. At the same time, fine scale structural aspects of food such as toughness and thickness are expected to have a strong influence on small animals that need to scrape or bite through the full thickness of the leaf [ 56 ].…”