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A mathematical analysis of gastropod shell geometry is employed as a tool for examining the taxonomy and phylogenetic structure of a group of cryptic, sibling littorinid species, herein termed the Nodilittorina ziczac species-complex. The constituent members of the complex are Nodilittorina ziczac, N. lineata and N. lineolata. The species-complex was previously described by Borkowski and Borkowski (1969), but these authors overlooked ontogenetic changes in shell geometry, and the possibility of geometric variation in the complex's geographic range, which is Florida to Brazil.
Populations were sampled at one site in Florida and two sites in St. Croix, U.S.V.I. Shells were examined using an application of Raup's (1966) description of shell geometry, which makes it possible to examine shell geometry in a combined ontogenetic and allometric approach. Of the three species, N. lineolata and N. lineata were found to differ in shell geometry between Florida and St. Croix, thus dispelling any hope of deriving a geometric signature for each species. The cause of this variation is believed to be morphological plasticity, rather than genetic variation or local selection for a particular genotype. There is, however, the possibility that N. lineolata is actually two separate sibling species, N. riisei and N. glaucocincta. This idea would then be supported by the geometric differences presented in this study.
The study concludes that shell geometry is a limited descriptor of taxonomic identity, and bears dubious phylogenetic information at the specific level, particularly when the subjects belong to widely dispersing, panmictic populations. Moreover, it is absolutely necessary to utilize several analytical methods in order to gain any understanding of phylogenetic relationships. Shell geometry is, however, an important indicator of the interaction of genotype and environment at the population level, and can thus be used locally as a taxonomic discriminator.
Peter D. Roopnarine. 1988. A Geometrical Analysis of Shell Morphology in the Nodilittorina ziczac species-complex (Mollusca: Gastropoda: Prosobranchia). Master's thesis. Nova Southeastern University. Retrieved from NSUWorks, Oceanographic Center. (368)