Figure 5. Respiration and circulation in Recent myodocopid ostracods. Simplified transverse section through anterior body and carapace: hemolymph route indicated by the red arrows. Hemolymph sinuses in pink (see Figure 6). Gaseous diffusion through the inner lamella of the carapace (see [51]) indicated by yellow arrows. The soft body, with or without gills or gas-exchange area, is bathing in the domiciliary cavity ventilated by the beating activity of two ventilatory plates. doi:10.1371/journal.pone.0028183.g005 Oxygen and Early Arthropod Micro-Benthos Evolution is less abundant. If PO2 was low in the water column at the beginning of the Palaeozoic Era (PO2 at between 8 and 15 kPa, [2]), tiny early crustaceans similar to ostracods in terms of anatomy and physiology may have been able to live at the water-sediment interface or even in the water column without using migration through the sediment to reach low oxygen levels. The oxygenation status at tissue and/or cellular level is not only dependant on the ambient PO2. It is also dependant on the cellular metabolism and the arterial vascularization and micro- vascularization. By definition, hypoxia occurs at the cellular level when oxygen input is not sufficient to fuel O2-consumption or when O2-consumption overreaches the oxygen supply. Thus, small planktonic ostracods (e.g. halocypridids) that live perma- nently within the water column, being active swimmers, possibly avoid high concentrations of O2 in their tissues not by hiding in low-oxygen niches but by ‘‘burning’’ O2 via a relatively high metabolism. In marine and non-marine benthic aquatic environments most podocopid ostracods are found at and above the sediment-water interface and living interstitially within the sediment as meiofauna (e.g. [36,37]). One specialised group of benthic podocope ostracods, the Platycopida, has developed a filter-feeding strategy (see [38] for a review). Corbari et al. [32] reassessed previous models of the oceanographic response of platycopids to wide- spread anoxia [38] in the light of experiments with Cytherella, a platycopid that seems to have developed various adaptations to resist hypoxia. Cytherella is quite different from other ostracods in that it has very thick valves and the ability to close them hermetically (using powerful muscles). Moreover, its activity level and ventilatory frequency is only half that of ostracods studied previously. When subjected to a decrease in oxygenation, it Figure 6. General morphology of Recent ostracods exemplified by myodocopids. A, B, Vargula hilgendorfii (Mu¨ ller), left lateral view of live female carrying embryos in her domiciliar cavity (carapace translucent) and lateral view of left valve in transmitted light showing the gas-exchange area, an integumental hemolymph network (yellow arrows indicate hemolymph circulation). C, Azygocypridina sp. from New Caledonia, France. Scanning electron micrograph showing appendages, left valve removed, including the ventilatory plates (courtesy of Vincent Perrier). D, E, transverse section through the carapace of Vargula hilgendorfii showing hemolymph sinuses, stained microtome serial section and scanning electron micrograph, respectively (see [51,76]). Scale bars: 1 mm in A–C, and 20 mm in D and E. ef, epipodial fan for ventilation; hs, hemolymph sinus; il, inner lamella; ol, outer lamella. doi:10.1371/journal.pone.0028183.g006 PLoS ONE | www.plosone.org 6 December 2011 | Volume 6 | Issue 12 | e28183