Ongoing speciation in southern semislugs

Helicarion semislugs are abundant from Tasmania to mid-NSW, but how many species are there?

Semislugs represent intermediate stages in the evolutionary process one could casually call ‘sluggification’. Semislugs still have a shell, but it’s reduced in size and ear-shaped, and the animal can no longer fully retract into it. In Australia, most semislugs belong to the native land snail family Helicarionidae. The southern-most genus in this family is Helicarion, a group of semislugs with a golden, glossy shell and a cream, brown to orange or grey to black body. These semislugs can be over 3 cm long and are found in habitats ranging from rainforest to dry sclerophyll forest.

Ranging from Tasmania to mid NSW, Helicarion has long been thought to contain four species, whose distributions oddly coincided with state boundaries. An observation that made us wonder… So, we studied specimens from across a broad geographic range, using comparative morphology and genetics, and determined that in fact there are only two species, but that each of these contains within it several divergent lineages that are probably in the process of evolving into new species. Some of these lineages were geographically restricted populations, usually from high mountaintops, while others had broad interstate geographic ranges. Each subgroup could be defined as unique based on differences in both morphology and DNA – but the morphological differences were very slight. This led to our next question: should these subgroups be treated as separate species?

Evolution is an ongoing process, and any group of organisms that we study may be at a different stage of speciation. Species that diverged a long time ago will generally appear much more distinct because they have accumulated greater genetic and morphological differences than more recently diverged species. The speed at which species diverge can also vary depending on internal and external factors.

One usually considers two groups of organisms as distinct species if they are unable to reproduce with each other successfully. Hence, the best indicator that two such groups are indeed distinct species is the anatomy of their reproductive system, since two animals with very different genitalia cannot interbreed. Our two major groupings had significant differences in their genitalia, but the various subgroups only showed slight differences and therefore we could not confidently state that they could not interbreed.

Based on the anatomy of their reproductive organs, we have concluded that at this time, we can only recognise two species in the genus Helicarion. We hope that in the future, we can carry out more detailed population genetics studies that may reveal more about these subgroups and how they can be defined. Meanwhile, our findings highlight the complex nature of speciation and evolutionary change in these small endemic semislugs, which may represent a suitable model system to study how species come to be.

Isabel Hyman, Scientific Officer, AMRI
Frank Köhler, Senior Research Scientist, AMRI

More Information

• Hyman, I. T. & Köhler, F. 2018. Reconciling comparative anatomy and mitochondrial phylogenetics in revising species limits in the Australian semislug Helicarion Férussac, 1821 (Gastropoda: Stylommatophora). Zoological Journal of the Linnean Society.