Key role of scale morphology in flatfishes (Pleuronectiformes) in the ability to keep sand.

Flatfishes bury themselves for camouflage and protection. Whereas species-specific preferences for certain sediments were previously shown, the role of scales in interaction with sediment has not been investigated. Here, scale morphology and sediment friction were examined in four European pleuronectiforms: Limanda limanda, Platichthys flesus, Pleuronectes platessa, and Solea solea. All species had different scale types ranging from cycloid to ctenoid scales. On the blind side, the number of scales is higher and scales have less ctenial spines than on the eye side. The critical angle of sediment sliding (static friction) significantly depended on the grain size and was considerably higher on the eye side. The effect of mucus was excluded by repeated measurements on resin replicas of the skin. Our results demonstrate the impact of scale morphology on sediment interaction and give an insight about the ability of scales to keep sand. Exposed scales and a higher number of ctenial spines on the eye side lead to an increase of friction forces, especially for sediments with a smaller grain size. Our results suggest that the evolution of scales was at least partly driven by their interactions with sediment which confirms the relevance of sediment for the distribution and radiation of Pleuronectiformes.