M_r − M_HI plane. The main figure shows galaxies detected in H I as blue-filled circles, and the H I cloud 5σ optical flux upper limits are shown with orange left-pointing arrows. The grey line and shaded region is the mean and scatter of the scaling relation of CVn galaxies (Kovač et al. 2009); the solid red line represents the lowest M_HI detection limit of the mosaic, and the dashed green line at M_r = −11.7 denotes the completeness limit of the FDS catalogue. Top panel: Absolute and apparent r-band histogram of all cluster galaxies (black), H I-detected galaxies (blue), and the optical flux 5σ upper limits of the H I clouds (orange). The H I-detected galaxies and clouds are clearly two distinct populations.