We extrapolated the hull of the gills by filling the small voids between the secondary filaments with a closing filter \cite{docs}. This is analogous to covering the gills with cling-film and gives us an approximation of the total volume which the gills occupy in the animal. Dividing this hull volume by the gill volume calculated above gave us an estimate of the filling factor of the gills, e.g. the space filling complexity. The gills of the swimmer group are filling significantly (p<0.01) less space in the total organ hull (35.9\(\pm\)2.0 % for the swimmer vs. 38.8\(\pm\)2.9 % for the control group, a decrease of 8.2 %). This confirms the qualitative finding from the SEM images and helps us to conclude that the gills of the swimmer group are less compact than the gills of the control group. We thus expect that the flow of oxygen-rich water is facilitated in the gills of the swimmers.