The Gyraulus subgenus Torquis (Mollusca:Gastropoda:Planorbidae) in North America.

Abstract

A conchological study shows that there are three valid nominal taxa in the Gyraulus subgenus Torquis: Gyraulus parvus, G. circumstriatus and G. huronensis. Gyraulus parvus is the most common and widespread species, but G. circumstriatus is also widely distributed in North America. Gyraulus huronensis is limited to the northern Michigan shores of lakes Huron and Michigan. Diagnostic anatomical characters of the subgenus Torquis are the sigmoidly folded kidney with undulating margins, the prostate diverticula arranged in a single row, and the straight, uncoiled intestine. The anatomy of G. parvus and G. circumstriatus is very similar; G. huronensis differs in external pigmentation, in the direct connection of the prostate gland diverticula to the sperm duct, in the overall shape of the prostate gland, and in the number of prostate diverticula. The three species exhibit several differences in radular teeth. The chromosome numbers of Gyraulus parvus, G. circumstriatus and G. huronensis are n = 36, 2n = ca. 72. These are tetraploid numbers. The tetraploid condition was probably the result of one single event. Once the tetraploid condition was reached, the stability of this number has been maintained in each of the three species. The habitats of the three species are strikingly different; the three species do not compete with each other for the same resources. Such habitat differences between the three species help prevent gene flow between them. Allozyme analyses show that there is a genetic basis for the taxonomic separation of the three species of Torquis, i.e., the morphological differences are not due merely to ecological influences. The phenogram generated for the genetic identities by the UPGMA cluster analysis showed a close relationship between G. huronensis and G. parvus; G. circumstriatus is more distantly related. A phylogenetic analysis using PAUP places Gyraulus on a major planorbid evolutionary branch which includes Helisoma, Planorbella, Planorbula and Promenetus. Gyraulus constitutes a clade separate from the line leading to the latter four genera. Biomphalaria and Physastra are on a second major branch. An unexpected finding from the allozyme study was that the populations of Torquis are reproducing by automixis, not by panmixis as has been traditionally assumed.