(A) Chthamalus malayensis belongs to the malayensis subgroup of Chthamalus and is the major occupier of space on the IWP high shore. The key diagnostic characters of the species in the malayensis subgroup include conical spines on the dorsal surface of the anterior ramus of cirri I (B and C) and II and the presence of multicuspidate setae which have basal guards (bg) on cirrus II (D). (E) Length and angle parameters measured in tergum (left) and scutum (right) of C. malayensis for comparison of opercular geometry among different clades. a occludent margin, b length of scutum, c basal margin of scutum, d angle between occludent and basal margin, f angle 1 of the tergal margin, g angle 2 of the tergal margin, h tergum height, i tergum width, j basal margin of tergum, k angle between occludent margin and scutal margin, l basi-scutal angle, m spur angle. Scale bars in μm, except A, E which are in mm.