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

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.