“Crown of thorns” of Daphnia

Abstract

DNA barcoding has emerged as valuable tool to document global biodiversity. Mitochondrial cytochrome oxidase I (COI) sequences serve as genetic markers to catalog species richness in the animal kingdom and to identify cryptic and polymorphic animal species. Furthermore, DNA barcoding data serve as a fuel for ecological studies, as they provide the opportunity to unravel species interactions among hosts and parasites, predators and prey, and among competitors in unprecedented detail. In a recent paper we described how DNA barcoding in combination with morphological and ecological data unraveled a striking predator-prey interaction of organisms from temporary aquatic habitats, the predatory notostracan Triops and its prey, cladocerans of the Daphnia atkinsoni complex.

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The Barcode of Life initiative attempts to inventory species richness on earth. Short DNA sequences such as the plastid trnH-psbA intergenic spacer1 or the plastid matK gene2 in flowering plants, and the cytochrome c oxidase subunit I (COI) in metazoans3–5 have recently been applied as molecular markers for sequence-based species identification. For the animal kingdom, genetic data (COI barcodes), as well as morphological, geographical and other relevant information are assembled in a large-scale public taxonomic library.6 The DNA barcoding approach has proven to be useful in various ways; for example, it provides the opportunity to discover cryptic3 or alien7 species, to catalogue ancient taxa8 or to identify unknown developmental stages.9,10 Although such single-locus approaches have a number of limitations,11–13 barcoding was successfully used to unravel several basic questions related to biodiversity, systematics and taxonomy.14 Furthermore, barcoding projects inspired a number of novel ecological and evolutionary approaches, such as community phylogenetics or ecological genomics.15 In a recent paper16 we demonstrated how a study integrating genetic data from an ongoing barcoding campaign with information on ecology and morphology of the studied animal species revealed the function of an inducible morphological structure in a cryptic Daphnia species complex. We discovered that two lineages of the D. atkinsoni complex changed substantially their morphology in response to infochemicals (kairomones) released by the tadpole shrimp Triops cancriformis. Mature daphnids developed longer tail spine and formed a distinct morphological structure, the so-called “crown of thorns”. This “crown”, a dorsal extension of the carapace, consists of a heavily spined cuticular shield at both sides of the head (Fig. 1). Our predation experiments subsequently proved the protective effect of those traits.

Our barcoding results per se were relevant for Daphnia taxonomy. On the one hand, we discovered yet unknown cryptic lineages within Western Palaeartic populations of the D. atkinsoni complex. On the other hand, we showed a discrepancy between molecular data and putative species-specific morphological characters, which questioned the validity of a previously recognized taxon. Such results were in accordance with other studies which emphasized the usefulness of the barcoding approach for identification and diversity assessment in other important zooplankton groups.17,18 Analysis of the barcoding data was therefore the first critical step stimulating the experimental work elucidating some aspects of the Daphnia-Triops interactions in temporary waters.

In addition to previously published results,16 we show here that both lineages of the Daphnia atkinsoni species complex, which we tested in our experiments, respond to Triops cancriformis during their entire life cycle. We exposed the daphnids to the predator as previously described,16 and measured traits affected by the kairomone in four different size cohorts according to their developmental stage. Both traits, the “crown of thorns” and the tail spine, were significantly pronounced in the animals exposed to predator kairomone compared to the control animals in all body-length size classes (Fig. 2, nested ANOVA, all p < 0.02). As the prey species (Daphnia) as well as the predator reproduce in the habitat, various size-classes of both species may occur simultaneously. Hence, daphnids of different developmental stages, ranging from neonates to large adult individuals, are under predation by Triops. It is therefore likely that the same defensive mechanisms are effective in both juvenile and adult prey animals. The spikes of the “crown of thorns” and of the tail spine, possibly in combination with a stronger cuticular armor,19 may interfere with the feeding apparatus of the omnivorous notostracan and thus may explain the protective effect of induced traits. However, as in a number of other Daphnia species which show inducible morphological defences,20 the exact defensive mechanism of the “crown” still remains unknown.

The discovery of the inducibility of the daphnid “crown of thorns”, and experimental proof of its protective mechanism,16 demonstrates the potential role of the DNA barcoding approach as an essential asset of integrative studies10,21 to reveal mechanisms with an impact transcending the borders of taxonomy and species identification. To reach such a goal, a balance between cataloguing biodiversity and careful analyses of the resulting patterns must be established. Molecular barcodes not only have proven to represent a helpful tool for taxonomy, but they are also bound to become an inspiring source of information for ecologists or evolutionary biologists.

Figures and Tables

Figure 1 (A) Daphnia atkinsoni sensu stricto (lineage 3; whole body SEM image) shows in the presence of notostracans a distinctly enlarged carapace extension into the head shield lined by spines, forming the “crown of thorns” (indicated by arrow). (B) Dorsal view on the “crown” (SEM image).

Figure 2 Predator cue induction experiments. Mean relative (%) lobe width (A and C) and tail spine length (B and D) in four different body length size classes (1: below 700 µm; 2: 701 to 1,200 µm; 3: 1,201 to 2,000 µm; 4: over 2,000 µm) of two clones representing different lineages of the D. atkinsoni complex (lineage 1, lineage 3) raised with (induction) and without (control) chemical cues released from Triops cancriformis. Asterisks indicate significant differences (nested ANOVA, ***p < 0.001; **p < 0.02), error bars indicate standard error of the mean.

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