Induced defence to grazing by vertebrate herbivores: uncommon or under-investigated?

Abstract

There are many examples of macroalgae inducing defence in response to small invertebrate herbivores like amphipods, isopods, and gastropods but few cases of induction in response to vertebrate macrograzers like herbivorous fishes. This may be because larger grazers rapidly consume large quantities of seaweed before induction can occur, thus selecting for constitutive rather than induced defences. Alternatively, the pattern could occur because induction due to feeding by macrograzers is less commonly investigated. In Fiji, field assays with the brown macroalga Sargassum polycystum demonstrated that thalli growing in marine protected areas (MPAs) with abundant herbivorous fishes were significantly less palatable than those growing in adjacent fished areas (non-MPAs) with few herbivorous fishes. This significant preference occurred in 11 of 13 trials over 5 time periods and across 3 pairs of MPAs and spatially associated non-MPAs. This preference was not positively associated with algal nitrogen content or with the toughness of algal fronds. When S. polycystum ramets were taken from the non-MPA and half were partially grazed by fishes while the other half were protected from grazing, new growth from the controls was strongly preferred to new growth from the previously grazed ramets although these fronds originated from the same holdfast. This suggests that S. polycystum upregulates defences (probably chemical) in response to grazing by herbivorous fishes. This is one of the few published examples of induction of macroalgal defence in response to feeding by large, mobile grazers. It is unclear whether induced defences against fishes are rare or just under-investigated.

High metabolic variation for seaweeds in response to environmental changes: a case study of the brown algae Lobophora in coral reefs

“…Metabolic variations may arise from different nutrient concentrations and herbivory pressure between sites. Nevertheless, Canard islet and Larégnère are marine protected areas so herbivory pressure must be higher compared to other sites, potentially leading to an increase in algal chemical defense as suggested in other works 6 . To further investigate this hypothesis, it would be interesting to test the bioactivity of MeOH fractions among sites.…”

“…They are involved in chemical communication and play diverse ecological functions in macroalgae. Even if the best known and studied ecological role of these metabolites is the deterrence against competitors and herbivores 5,6 , they can also act as defense against pathogens 7,8 ( e . g .…”

Marine Autotroph-Herbivore Synergies: Unravelling the Roles of Macroalgae in Marine Ecosystem Dynamics

“…A multitude of scientific research indicated that ocean warming has caused ecological impacts on various marine flora and fauna species across the globe, with a range of species marching away from their native homes in search of cooler climes [ 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 , 131 , 132 , 133 ]. With heatwaves sweeping through oceans twice as much as they did in the early 1990s, many biodiversity hotspots around the world are on the verge of imminent collapse.…”

“…Tropical macroalgal taxa have been reported to produce a higher diversity of metabolites compared to their temperate counterparts, dominated by halogenated metabolites, terpenoids, acetogenins, and phenolics [ 127 , 128 ]. Mainly regulated by developmental, genetic, and environmental factors, these metabolites play diverse ecological functions in macroalgae, from being deterrent against herbivores to defenders to fight against specific pathogens and competitors for space with other marine organisms [ 127 , 129 , 130 ].…”