Evolution of aquatic snails' defences resulted in clade-specific differences in egg toxicity, pigments and warning coloration.

Oviparous animals have evolved diverse strategies that deter egg predation. In terrestrial species, these often include noxious compounds and aposematic signalling, but little is known in freshwater environments. Here, we unravel the evolutionary and ecological strategies of Pomacea, aquatic snails that lay conspicuous masses of toxic orange-pink eggs to reduce predation risk. We reveal the interplay among warning coloration, toxicity and predator visual perception that enables the evolution of advanced chemical defences. We provide evidence that snails modify dietary carotenoids and that this controls egg coloration in a clade-specific manner. Snails from the canaliculata clade accumulate more and brighter-coloured egg carotenoid pigments than those from the bridgesii clade. The conspicuousness of colour signals was assessed using field data, spectral reflectance measurements and visual modelling. We show that aposematic signal variation among species is likely noticeable to putative waterbird predators. Feeding egg extracts to birds adversely affected their gut morphology. Comparative analysis revealed a correlation among pigment modification, conspicuousness and toxicity, demonstrating that colour acts as an honest aposematic signal in apple snail eggs. To our knowledge, our study provides the first example of an honest aposematic signal in warning coloration among freshwater invertebrates.