I’ll start with a little background for context and to set the scene for those not familiar with Echinolittorina interrupta and Echinolittorina placida.  E. placida was first named in 2009 by David Reid as a result of his extensive molecular phylogenic studies of Littorininae.  During these studies*, he found that the long-accepted species E. interrupta (C. B. Adams in Philippi, 1847) represented two species with separate geographic distributions; shells found in Florida and along the American Atlantic and Gulf coasts and shells found in the Caribbean Sea south of Cuba. He assigned the new name (E. placida) to the former and E. interrupta to the latter.  This situation has created a problem in that many historical publications, illustrations and photo repositories still present the shell in Florida as E. interrupta.  The problem stems from the case that these two species are very, very similar, both with a good degree of overlapping variability.  Separating them requires a close comparative examination of details often discernable only at correct viewing angles or in high-quality photos.  It’s unlikely photos of these species in the wild would reveal the detail necessary to establish an identification with reasonable certainty.  However, it’s not an issue as long as we accept the geographic separation as valid and continuing.  But Reid’s data is now over 20 years old, and it is certainly possible that E. interrupta has been extending its range northward. 

Collectors should be wary of identifying any material found in Florida as E. interrupta without close examination and use of the identification guidance in this presentation and the presentation of E. placida.