Bulgy tadpoles: inducible defense morph

Abstract

Predator induced morphological defenses are marked morphological shifts induced directly by cues associated with a predator. Generally, remote cues, i.e., chemical substances emitted from predators or injured conspecifics, are considered to be ideal signals to induce morphological change in aquatic environments rather than close cues, i.e., close chemical or tactile cues, since chemical substances that can propagate over relatively long distances and persist for a long period may allow organisms to keep safe and to deliberately change their morph. In fact, most organisms adopting an inducible morphological defense utilize remote chemical cues to detect predation risk and to produce morphological defenses. In this paper, we report a unique and functionally well designed inducible morphological defense strategy where the induction process requires close cues from a predator. The tadpoles of Rana pirica exhibited a bulgy bodied morphology when threatened with predation by larval salamanders, Hynobius retardatus, in close proximity. Predation trials and a function experiment showed that the induced bulgy morph is an adaptive defense phenotype against the gape-limited predator larval H. retardatus. Furthermore, R. pirica tadpoles use two adaptive strategies in terms of cost saving, i.e., adjustment of the extent of bulginess according to predation risk and reversibility by actual shrink of bulgy body after removing the predation threat. In general, R. pirica hatch earlier than H. retardatus. In natural ponds, during the early developmental stage R. pirica tadpoles live in close proximity to young H. retardatus larvae. As they grow, the salamanders gradually become serious predators and the predator-prey interaction becomes intimate. After a while, predation, cannibalism and metamorphosis decrease the number of salamanders in the ponds, and the predator-prey interaction weakens. Such a phenology in the predator-prey interaction allows the evolution of a close-cue detection system and adaptive cost-saving strategies. Our results highlight that the characteristics of the inducible defense depend on the intensity and specificity of the predator-prey system.

Investment in defense and cost of predator-induced defense along a resource gradient

“…At low resource availability growth is reduced resulting in small individuals that are very vulnerable to predators and which spend more time in vulnerable stages (Arendt 1997). At high resource availability individuals can grow quickly, allowing individuals to escape predation risk by either early metamorphosis, or rapid attainment of a size beyond the reach of gape-limited predators (Kishida and Nishimura 2004). Therefore, defensive mechanisms are more important to slow growing individuals and investment in defense should be higher at low resource availability.…”

Inducible defenses in prey intensify predator cannibalism

“…Indeed, the time required for expression of the bulgy morph under similar experimental conditions is ;1 week (Kishida et al , 2007. Moreover, at the end of the experiment, surviving tadpoles from the undefended treatment did not have the thickened epithelial tissue that is a diagnostic feature of the defended morphology (Kishida and Nishimura 2004). In contrast, surviving tadpoles in the undefended treatment were significantly longer than those in the defended treatment, suggesting that shorter individuals are also more vulnerable to salamander predation.…”

“…1a, left, Fig. 1b; see also Plate 1) by thickening their epithelium tissues in response to larval salamander risk cues (Kishida and Nishimura 2004). Because salamanders are gape-limited predators that swallow their prey whole, the bulgy morph is highly effective in reducing tadpole vulnerability to salamander predation because it is more difficult to swallow than non-induced tadpole morphs.…”

Differential Jumping Performance in Newly Metamorphosed Blanchard´s Cricket Frogs, Acris Blanchardi (Anura: Hylidae), From Fish-and Invertebrate-Dominated Ponds

“…However, tadpoles may buffer the costs associated with morphological or behavioral changes by exhibiting morphological sensitivity to variations in predator type and density gradients (Laurila et al 2004). This sensitivity can maximize phenotypic efficiency while potentially reducing metabolic costs (Kishida and Nishimura 2004).…”