Relation between the HI gas depletion timescale and the morphological type for HI-normal (HI-def ≤0.4; filled dots) and HI-deficient (HI-def > 0.4; empty circles) galaxies. The large filled blue dots indicate the mean values for each morphological class for normal gas-rich systems, and the empty red dots show cluster HI-deficient galaxies. The error bar shows the standard deviation of the distribution for the large symbols. Upper limits are indicated by triangles and are treated as detections in the derivation of the mean values.