Beyond morphology: Material alterations as inducible defenses in Daphnia longicephala

The freshwater crustacean Daphnia is well known to adapt to the presence of predators by forming inducible defence structures. Large-scale morphological defensive traits are usually easily recognizable in most species. However, hidden mechanical or microstructural defences in the integument, which can strongly impact defensive success through altered structural stability, remain largely unknown. We therefore investigated the inducible mechanical defenses of Daphnia longicephala against the heteropteran backswimmer Notonecta sp. In its presence, Daphnia individuals develop an elongated, widened head capsule and a more pointed tail spine. Nevertheless, the actual function of these traits is unknown, as Notonecta pierces its prey with a proboscis and is therefore not gape-limited. The increased body dimensions accordingly seem ineffective. In particular, the function of the very pronounced crest on the heads of defended specimens remains unclear. In this study, we documented significant alterations in Young's modulus and hardness of the integument of the crest between undefended and defended D. longicephala. At the same time, elemental composition in the cuticle between animals from different treatments did not differ in any of the investigated crest regions, except for calcium, which was elevated in the defended specimens and correlated with enhanced mechanical properties. This indicates alterations in the degree of mineralization in certain crest regions, which could be responsible for significant changes in hardness and stiffness. It is likely that hidden mechanical defences of D. longicephala's crest impair the predators' handling and hinder or complicate a piercing of the integument in the central head region, protecting the vital organ systems.