Aposematism in pink warty sea cucumbers: independent effects of chromatic and achromatic cues

The importance of chromatic and achromatic cues in aposematism is well established, but in situ experiments investigating their comparative roles within a single warning signal remain rare. In a predation experiment, we examined their relative importance in pink warty sea cucumbers Cercodemas anceps Selenka, 1867, by asking (1) Do chromatic and achromatic cues have significant effects on attack rate? (2) Are their effects independent? (3) Does one have more influence on attack rate than the other? Using a multispectral imaging-based technique, we fabricated 4 types of clay models based on the hues, chroma and luminance levels of the animals and the background. Models were deployed in sets of 4 at 2 sites (n = 65 sets) for 3 days, and the number of imprints from predation attempts was recorded. A generalised linear mixed model analysis showed that both types of cues had significant but independent effects on attack rates and that chromatic cues had a greater effect compared to achromatic ones. This study, the first manipulative investigation into holothurian aposematism, demonstrates the potential for chromatic and achromatic cues to play distinct roles in animal signalling, and highlights the importance of accurate experimental models.