A plastron acts as a physical gill and enables permanent underwater dwellers, e.g. creeping water bug (A, Aphelocheirus aestivalis) to breathe in water (B). Some wetland plants also retain a thin gas film on their superhydrophobic leaf surfaces when submerged in water (C, Phragmites australis), and this feature also acts as a physical gill to facilitate O2 uptake from the surrounding water (D, Phalaris arundinacea). The gas film is visible as a silvery/golden layer (insect) or silvery layer (plant). The gas film can be removed by a few gentle brushes with 0.05% Triton X, and then rinsing, prior to submergence (Colmer and Pedersen, 2008; Pedersen et al., 2009), and such manipulation enabled assessment of O2 uptake by the two organisms when the gas film was removed versus when intact. The experiments were conducted at 20°C in 5 ml glass cuvettes with continuous mixing (400 revs min–1); see Colmer and Pedersen for detailed experimental procedures (Colmer and Pedersen, 2008). Duplicate measurements confirmed the responses shown. The saturation curve for O2 uptake against external O2 concentration, of both the insect and plant with intact gas film, resembles that of respiration curves obtained for aquatic insect larvae with haemoglobin, e.g. Chironomus sp. (Brodersen et al., 2008). Photos by Ole Pedersen.
The raft spider (Dolomedes fimbriatus) hunting underwater in a shallow stream in Öland, Sweden. The abdomen of the raft spider is superhydrophobic and retains a thin gas film underwater (visible as a silvery layer). The gas film acts as a physical gill (see text). We have observed that the gas film allows the spider to hunt underwater for a minimum of 20 min. In situ photo by Ole Pedersen.
Damselfly (Erythromma najas) laying eggs in the petiole of the yellow water lily (Nuphar lutea) underwater. The petiole contains large gas-filled spaces so eggs as well as young larvae are guaranteed safe protection and ample gaseous O2 supplied by the plant. The damselfly typically lands on the floating leaf and then crawls over the edge of the leaf into the water and down the petiole. The entire body, including the wings, is superhydrophobic and retains a thin gas film underwater (visible as a silvery layer). The gas film acts as a physical gill (see text) and enables the damselfly to breathe underwater for as long as it takes to lay its eggs, which can be up to 45 min. Photo taken by Ole Pedersen in Lake Hampen, Denmark, at a depth of approximately 1.2 m.