Diagnostic field characters Typically occupies fully exposed, prominent position, attached to solid object such as coral boulder. Cylindrical column of uniform bright colour (commonly blue, green, red, white, chestnut brown). Oral disc to 1 m diameter (although commonly 300-500 mm), flat to gently undulating, densely covered with finger-like tentacles (to 75 mm long) that hardly taper to blunt or slightly swollen end. May irritate human skin and raise welts.