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

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 colouration,
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 colouration 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 colouration among freshwater invertebrates.