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

Cladocerans (water fleas) are planktonic crustaceans that typically have a bivalved carapace. Each valve of the carapace consists of two cuticle-secreting epithelial layers that are separated by a hemolymphatic chamber and joined by pillar structures. Ultrastructural analyses in several species of Cladocera have shown that the carapace epithelia and pillars contain filamentous structures of unknown composition. In the present study we used a fluorescent phalloidin conjugate to show that the carapaces of three cladocerans, Daphnia magna, D. pulex, and Sida crystallina, are rich in large bundles of filamentous actin (F-actin). In D. magna we employed confocal microscopy and orthogonal views of three-dimensional reconstructions to show that these bundles extend radially from foci in the pillars towards the integument surfaces, and their structure is consistent with that of contractile stress fibers. Using a fluorescent lipophilic stain, DiOC₆(3), we show that the F-actin bundles are distributed in membrane-rich regions within the carapace epithelia, and that, in the superficial epithelium, these may be large membrane-bound organelles. In D. magna, the F-actin bundles are present in embryonic, juvenile instar, and adult, developmental stages, and through development the bundles become larger, contain more F-actin, and become more widely spaced. We present an alignment of the deduced amino acid sequences of six putative D. pulex actin genes, and discuss the implications that their respective sequences have on the likelihood of their inclusion into the F-actin bundles of the carapace. Our identification of these large F-actin bundles within the pillars of three cladocerans provides new insight into the role these structures play in influencing carapace dynamics within this order.