Respiratory Functions of Motile Tracheal Gills in Ephemeroptera Nymphs, As Exemplified by Siphlonurus Occidentals Eaton

Abstract

Four questions about the roles of nymphal tracheal gills were posed: do lamellae meet all structural criteria for being gills? How important are they as oxygen intake surfaces? Are lamellae ventilated and, if so, how? Do lamellae aid or enable a nymph to maintain respiratory regulation in the face of dwindling oxygen supplies? Siphlonurus occidentalis possesses 7 pairs of extremely large but variously sized, plate-like tracheal gills. Their respiratory importance was assessed by determining oxygen consumption of gilled and gill-less individuals, by measuring beating rate and percentage of time spent beating with changes in dissolved oxygen (DO) and temperature, and through measuring the volume of water passed over the body surfaces by fully-gilled and partially-gilled nymphs. Data substantiate 7 conclusions: (1) gills are important sites of oxygen intake, accounting for about 67% of the total consumed at 12–13 °C, and (2) gills take in oxygen proportionate to their fraction of total respiratory surface area. Maximum diffusion gradients are maintained in the face of rising temperature and decreasing DO by increasing (3) gill beating frequency and (4) the percentage of time spent beating. (5) At any particular combination of DO and temperature, beating frequency is fixed; that is, nymphs from whom gills 3–7 have been removed do not beat their remaining gills any faster. Rather, (6) such nymphs further increase the percentage of time spent beating. (7) Even so, individuals with all their paddles (and therefore all respiratory surfaces) force approximately 10% more water over their body than do those with about half such surfaces removed. Thus, by adjusting both beating rate and the amount of time spent beating nymphs control the water volumes moved and in so doing appear to regulate oxygen consumption.