It's all about foot colour

Cerithiopsidae is a family of marine gastropods, probably including hundreds of species, living mostly from the intertidal zone down to about 200 m. All of them are associated with, and feeding on sponges. The genus that gave the family its name, Cerithiopsis, comprises a group of small species usually less than 10 mm long. 

Traditionally, shell characters are primarily used to identify and delineate gastropods, yet the identification of Cerithiopsis species by shell morphology alone can be very difficult or impossible because of their small size, frequent encrustations and damage, the extreme variability within some species (phenotypic plasticity) or extreme similarity of shell features between different species (morphological stasis). 

In many instances genital morphology is the best resource for species identification. But have you ever dissected a 10 mm long snail in order to find some diagnostic characters associated with its reproductive organs? I have and it is indeed as difficult as it sounds. It also requires a considerable level of experience to make the right judgement which in some cases leaves us only with a handful of people capable to do species identification for particular gastropod families. In other cases the knowledge is already lost as experts literally went extinct.

Sounds like a case for DNA Barcoding and it seems that the community of mollusk researchers is moving ahead albeit in some instances at a snail's pace. The more I am happy to find examples where DNA helps to overcome obstacles of morphological identification or simply helps to find the right morphological character. A group of Italian researchers used DNA Barcoding to confirm that Mediterranean gastropods usually identified as Cerithiopsis tubercularis in fact represent a complex of cryptic species, which can also be reliably diagnosed by the colour pattern of their head-foot. The only issue with this method is the fact that the animal needs to be alive. Dead specimens loose the characteristic patterns very quickly. Well, there is always a DNA Barcode that can help with that.