Nerites of Changi and Labrador

Over the last two days, we've been doing rocky shores at Changi and Labrador. Nerites (Family Neritidae) are very commonly encountered in this habitat.Nerites have a distinctive semi-spherical, globular shell. The shell is quite thick and heavy. When alarmed, these snails may just let go and they will drop off the rocks and bounce around like little marbles. This is probably how they may escape from crabs who try to eat them.

The hole in the shell is closed with a door called an operculum. The operculum has an internal peg to lock firmly in place against the teeth at the shell opening. This makes it difficult for a crab to stick a pincer in and dig out the snail.

A preliminary identification of the various species of Nerites can be made by looking at the underside of the shell and the teeth-like structures at the shell opening (these do not actually function as teeth to chew food), and the shell's general shape and texture. But very similar-looking Nerites can only be positively distinguished by looking at internal features of the shell and animal.

When looking at a nerite's underside, do it gently and remember to put it back where you first found it.
This is probably Nerita chamaeleon or the Chameleon nerite.

This is probably Nerita undata.

This is definitely Nerita lineata or the Lined nerite.

While these are probably Nerita squamulata.
Nerites of the same species may have very different patterns and colours on the shell.

Prof Leo Tan found this nerite on Labrador. I have no idea what it is and I've never seen this nerite before!
Unlike the other more commonly seen nerites, this one does not have 'pimples' on the operculum. Instead, the operculum is smooth. It also has a very smooth shell without the rough ribs seen on the other nerites we usually see.