Thwarting predators? A three-dimensional perspective of morphological alterations in the freshwater crustacean Daphnia

Abstract

Predation is a major selective agent, so that many taxa evolved phenotypically plastic defensive mechanisms. Among them are many species of the microcrustacean genus Daphnia, which respond to an increased predation risk by developing inducible morphological alterations. Some of these features are obvious and easily recognized, e.g., crests in D. longicephala, while others are rather hidden, such as the bulkier shape of D. magna induced by the presence of the tadpole shrimp Triops. In this study we investigated the extraordinary diversity of morphological adaptations in the presence of predators with different foraging strategies in six predator-prey systems. For the first time we were able to analyze the unexposed and predator-exposed morphs comprehensively using three-dimensional scanning and reconstruction. We show that morphological changes are manifold in appearance between species and predators, and go beyond what has been known from previous 2D analyses. This further demonstrates the enormous trait flexibility of Daphnia. Interestingly, we found that among this variety some species share morphological strategies to counter a predator, while others use a different strategy against the same predator. Based on these intra- and interspecific comparisons, we discuss the mechanisms by which the respective defense might operate. These data therefore contribute to a deeper understanding of the inducible defenses’ morphology as well as their diversified modes of operation in Daphnia, being a cornerstone for subsequent investigations, including the determination of costs associated with morphological change.

Specific Turbulence- and Chaoborus-Induced Morphotypes Affect the Streamlining Properties of Daphnia cucullata

“…That way, D. cucullata may swim in a kind of 'stealth mode'. The head capsule's lateral width at the fornices is reduced and the carapace is laterally wider, as previously described as defense of D. pulex against Chaoborus (Horstmann et al, 2021).…”

“…Animals were fixed, stained and processed as described previously (Horstmann et al, 2018(Horstmann et al, , 2021. In short, we fixed 10 animals per treatment with 4% formaldehyde diluted in PBS (pH 7.4, 0.1 M), dissected the antennae as they can be problematic during the confocal scanning, stained them with Congo red (Carl Roth GmbH + Co., KG, Karlsruhe, Germany) and scanned them with a confocal microscope (Leica TCS SP5II (Leica Microsystems, Wetzlar, Germany).…”

“…Average Procrustes-distances give the sum of the distances of respective vertices after a Procrustes analysis normalized on the number of vertices. While partial Procrustes allows comparisons of form, including size effects and therefore allowing the alteration of form, full Procrustes compares shapes without influence by rotation, translation and scaling, enabling pure shape comparisons (Horstmann et al, 2021). These measures therefore enable to evaluate the degree of shape and form alterations between the morphs.…”

“…In order to reveal the underlying mechanisms and explain the function of the observed behavioral and morphological alterations in D. cucullata, we analyzed the shape of animals exposed to turbulence and Chaoborus predation and compared them using Procrustes distance (Horstmann et al, 2018(Horstmann et al, , 2021. We also examined their swimming velocities.…”

One Kairomone and Multiple Effects in Daphnia Species—5α-Cyprinol Sulfate Induces Morphological Defenses in the Invasive Species Daphnia lumholtzi

“…Under continued exposure to fish kairomones, this induction of morphological defenses in D. lumholtzi is transferred to the next generation resulting in an earlier ontogenetic onset and enhanced magnitude (Graeve et al, 2021). Furthermore, fish-induced lateral deformations at the fornices (appendages of the head capsule) were observed in D. lumholtzi, resulting in a significantly increased lateral width of the animals (Horstmann et al, 2021). D. lumholtzi has invaded large parts of the United States over more than 25 years (Sorensen and Sterner, 1992;Beyer and Hambright, 2019).…”