N. histrio are described to have the shell width of up to 2.5cm (Wilson, 1993). This is relatively small compared to some of the larger species in the genus Nerita, which can have a shell size of up to 4cm, seen in Nerita balteata (Wilson, 1993).

There was some variation present in the size of the N. histrio collected from Wellington Point, Queensland (figure 1). This is most likely due to difference in the age of the individuals. From the 15 individuals collected, the length of the shell ranged from 1.3cm to 1.8cm, with the average being 1.64cm. The height of the shell was less variable than the length, ranging from 0.7cm to 1.1 cm tall, with an average of 0.93cm. Shell width was the least variable parameter measured, ranging from 1.2 cm to 1.5 cm, with the average shell width being 1.36cm. Figure 2 shows the definitions of length, width and height. 

It is well documented that N. histrio shows a great deal of variation in shell colours and patterns, and this was clearly seen in the individuals collected. Colours ranged from orange to dark brown, and the patterns varied from almost one solid colour, to alternating stripes of colours, to seemingly random speckles of two or more colours (figure 3). The shell for all individuals collected had a depressed spire (figure 3), and had some degree of flare at the shell opening (figure 4).