The taxonomy of the periwinkles found on rocky shores of the western Atlantic has been in a state of confusion for the past 50 years. Originally, the identification of molluscs was based mainly on the appearance of the shell which, in periwinkles, is a poor guide because it is often more variable within species than between them.
For the first half of the twentieth century all the black-and-white striped periwinkles in the region were referred to as Littorina ziczac. Since then, detailed anatomical studies (particularly of the egg capsule and penis) led to discrimination of 5 species in this group, and to their classification in the genus Nodilittorina.
DNA sequencing allows definitive testing of these morphospecies, and their grouping into genera. As a result, in 2003 these 5 species were transferred to the large genus Echinolittorina.
In 2009, scientists realised Echinolittorina interrupta consisting of 2 genetically distinct species, and Echinolittorinaplacida was named as a new species in the group.
This refinement of classification using newly developed techniques has been repeated in many molluscan groups.
DNA sequences may be the ultimate authority, but with skill and patience most of these confusing Echinolittorina species can be identified from their shells:
These 3 species are common throughout the Caribbean and on the southeastern coast of Florida, but are virtually absent from the Gulf of Mexico, where Echinolittorina placida occurs.
The details of the glands of the penis permit males of the three species to be identified unequivocally. Indeed, it is believed that the shape of the penis is such a good taxonomic character because it is used by female snails to distinguish males of their own species from others that may attempt copulation.
A molecular phylogeny has been constructed for all 60 known members of the genus Echinolittorina. The 2 sister species Echinolittorinaplacida and Echinolittorina interrupta belong to the subgenus Amerolittorina.
The most similar species to Echinolittorina placida is Echinolittorina interrupta - the species to which it is most closely related, and from which it separated an estimated 5 million years ago. They are genetically distinct, and their ranges do not overlap.
The shells are similar - that of Echinolittorina placida is more rounded in profile, but the strong spiral black band can be seen in most specimens of both species. The penes are the same - perhaps this is because the 2 species formed following geographical separation, and the females have not exerted selection for divergent penile shapes.
Echinolittorina interrupta is found on the continental margins and larger islands of the Caribbean Sea, from the eastern side of the Yucatan Peninsula to Central America, Venezuela, the Lesser and Greater Antilles (excluding Cuba).