Uncovering Ultrastructural Defences in Daphnia magna – An Interdisciplinary Approach to Assess the Predator-Induced Fortification of the Carapace

Abstract

The development of structural defences, such as the fortification of shells or exoskeletons, is a widespread strategy to reduce predator attack efficiency. In unpredictable environments these defences may be more pronounced in the presence of a predator. The cladoceran Daphnia magna (Crustacea: Branchiopoda: Cladocera) has been shown to develop a bulky morphotype as an effective inducible morphological defence against the predatory tadpole shrimp Triopscancriformis (Crustacea: Branchiopoda: Notostraca). Mediated by kairomones, the daphnids express an increased body length, width and an elongated tail spine. Here we examined whether these large scale morphological defences are accompanied by additional ultrastructural defences, i.e. a fortification of the exoskeleton. We employed atomic force microscopy (AFM) based nanoindentation experiments to assess the cuticle hardness along with tapping mode AFM imaging to visualise the surface morphology for predator exposed and non-predator exposed daphnids. We used semi-thin sections of the carapace to measure the cuticle thickness, and finally, we used fluorescence microscopy to analyse the diameter of the pillars connecting the two carapace layers. We found that D . magna indeed expresses ultrastructural defences against Triops predation. The cuticle in predator exposed individuals is approximately five times harder and two times thicker than in control daphnids. Moreover, the pillar diameter is significantly increased in predator exposed daphnids. These predator-cue induced changes in the carapace architecture should provide effective protection against being crushed by the predator’s mouthparts and may add to the protective effect of bulkiness. This study highlights the potential of interdisciplinary studies to uncover new and relevant aspects even in extensively studied fields of research.

Effects of predation by Hydra (Cnidaria) on cladocerans (Crustacea: Cladocera)

“…The allelochemicals released by Hydra may be exerting a toxic effect on daphniids which explains the decrease in the fitness of the population. The allelochemicals released by Hydra may also bring certain morphological changes in the littoral prey such as thickening of the carapace, which has already been reported as a successful strategy against certain invertebrate predators (Laforsch et al, 2004;Rabus et al, 2013).…”

Carapace epithelia are rich in large filamentous actin bundles in Daphnia magna, Daphnia pulex, and Sida crystallina (Crustacea: Cladocera)

“…If the stress fibers in the pillars of D. pulex are primarily contractile, ACT2A and ACT2B may be the predominant actins incorporated into their arrays (Fig. In two separate studies in D. magna the abundance of an actin transcript identified as act3, as well as pillar diameter, increased in response to predator kairomones (Rabus et al 2013;Effertz & von Elert 2014). In two separate studies in D. magna the abundance of an actin transcript identified as act3, as well as pillar diameter, increased in response to predator kairomones (Rabus et al 2013;Effertz & von Elert 2014).…”

Interclonal proteomic responses to predator exposure in Daphnia magna may depend on predator composition of habitats

“…Morphological changes are known to be established in all animals of the Triops ‐exposed group after this duration (Rabus & Laforsch ; Rabus et al . ).…”

“…In addition, the ultrastructure of the carapace is altered, resulting in a thicker and harder cuticle (Rabus et al . ). Furthermore, the morphological defence of D. magna changes in a genotype‐dependent manner (Rabus et al .…”