Push or Pull? The light-weight architecture of the Daphnia pulex carapace is adapted to withstand tension, not compression.

Daphnia (Crustacea, Cladocera) are well known for their ability to form morphological adaptations to defend against predators. In addition to spines and helmets, the carapace itself is a protective structure encapsulating the main body, but not the head. It is formed by a double layer of the integument interconnected by small pillars and hemolymphatic space in between. A second function of the carapace is respiration, which is performed through its proximal integument. The interconnecting pillars were previously described as providing higher mechanical stability against compressive forces. Following this hypothesis, we analyzed the carapace structure of D. pulex using histochemistry in combination with light and electron microscopy. We found the distal integument of the carapace to be significantly thicker than the proximal. The pillars appear fibrous with slim waists and broad, sometimes branched bases where they meet the integument layers. The fibrous structure and the slim-waisted shape of the pillars indicate a high capacity for withstanding tensile rather than compressive forces. In conclusion they are more ligaments than pillars. Therefore, we measured the hemolymphatic gauge pressure in D. longicephala and indeed found the hemocoel to have a pressure above ambient. Our results offer a new mechanistic explanation of the high rigidity of the daphniid carapace, which is probably the result of a light-weight construction consisting of two integuments bound together by ligaments and inflated by a hydrostatic hyper-pressure in the hemocoel. J. Morphol. 277:1320-1328, 2016.