Recurrent evolution of breathing microtunnel system in terrestrial operculate snails (Gastropoda: Caenogastropoda: Cyclophoroidea)

Abstract

The Cyclophoroidea are a group of land snails possessing an operculum that seals the aperture when the snail withdraws its body into the shell. Several groups have developed snorkel-like tubes, which are open at their outer ends, for gas exchange when the operculum closes the aperture. The tube of the cyclophorid subfamily Alycaeinae is, however, closed at its outer end. Recent investigations revealed that the tube of Alycaeinae, which starts to develop in subadults, is connected to several long microtunnels that run perpendicular to the tube and open near the umbilicus. We examined snails similar to Alycaeinae and discovered three types of gas-exchange systems that differ from the system in Alycaeinae. In Laotia, the microtunnels run to separate chambers, whereas in Platyrhaphe the microtunnels run to an internal tube. In both genera, the breathing device is already present in juveniles. Lastly, in Boucardicus there is no tube, but the microtunnels reach the common opening individually. Our molecular phylogeny, based on sequence data from the 28S, H3 and COI genes indicates that the microtunnel systems of these four groups have evolved independently. Based on these findings, three new subfamilies are proposed: Boucardicinae Páll-Gergely subfam. nov. (Hainesiidae), Laotiinae Páll-Gergely subfam. nov. (Diplommatinidae), and Platyrhaphidinae Páll-Gergely subfam. nov. (Cyclophoridae).