Distribution of the structural parameters of UDGs in the Hydra I cluster (filled blue histogram) compared to UDGs in other environments, as listed in the legend of each panel. For each sample, we restricted the analysis to those UDGs that strictly respect the van Dokkum et al. (2015) definition. Optical g − r color (upper-left panel), effective radius R_e (upper-right), central surface brightness μ₀ (bottom-right), and Sérsic index n (bottom-right) are compared. References are given in the legend of each panel.