APPLYING GEOMETRIC MORPHOMETRICS TO SIBLING SPECIES OF POLYSTIRA (GASTROPODA:TURRIDAE)

Molecular work carried out on various Recent marine invertebrate groups suggests that many traditional “species” actually consist of several semi-cryptic species. Though these are genetically distinct, they are morphologically extremely similar and have previously been thought to represent intra-specific variation. This obviously has implications for Recent and past biodiversity estimates, as well as evolutionary studies. If these semi-cryptic species become obvious only using genetic methods, it is extremely difficult to extend this knowledge back into the fossil record.

Morphometric methods have been developed to examine shape in a replicable mathematical manner, and are ideally suited for applying to the problem of distinguishing sibling species. Here, they were applied to Polystira, a hyper-diverse monophyletic clade of gastropods endemic to the tropical and subtropical Americas which molecular work has shown to contain sibling species. As Polystira has a character rich shell with a wide range of ornamentation, which can be homologised, it is ideal for carrying out morphological studies in conjunction with genetic data.

Two methods, relative warp analysis and extended eigenshape analysis, were applied to a traditional “species” in an attempt to distinguish and delimit its sibling members. The methods were able to distinguish three sibling species and produce easily interpretable and biologically meaningful results. Specimens representing “unknowns” could then be compared to the known species to aid identification.