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Heat Cheats

The smallest camaenid snail in the Pilbara

The smallest camaenid snail in the Pilbara, a newly discovered species of Strepsitaurus found only on the south-facing wall of a single, small gorge. Photos: Roy Teale

By Michael Johnson

The diverse snails of the hot, dry Pilbara region survive by selecting the best microhabitats and through adaptations of shell form, reproduction and behaviour.

The Pilbara region is not the obvious place to look for snails. This 500,000 km2 area in the north of Western Australia is more widely known for its rugged gorges, oppressively hot summers and huge deposits of iron ore. Rain is scarce and highly unpredictable, but the Pilbara is subject to occasional cyclonic or winter flooding.

Despite this inhospitable environment, a diverse fauna of land snails is endemic to the region. Most of these snails are members of the Camaenidae, the largest family of snails throughout northern and central Australia, with more than 30 species unique to the Pilbara representing six genera, three of which are also known only from this region.

Since 2003 my students and I have been studying the genetic relationships and variation of shells in the camaenid snails of the Pilbara, trying to understand the evolution of this diversity, test and refine the taxonomy, and gain a better idea of distributions. My interest is “blue sky”, learning for the sake of understanding, but the work also has important connections to conservation in the face of massive resource developments in the region. Having a sound taxonomy and knowing the geographic distributions and habitat requirements of the snails are essential for assessing the impacts of proposed developments and ensuring the conservation of this unique fauna. The rapid and large-scale development in the Pilbara has increased both the need for that knowledge and our ability to study these animals, because the requirements for impact assessments have led to much more extensive biological surveys than were previously possible in this remote area.

Basic Problems
To understand the diversity of the snails, we need to understand how they cope with the harsh conditions of the Pilbara. Most obviously, they need shelter from heat and long periods of drought.

When conditions are poor, all land snails become dormant, or aestivate. The snails from arid regions such as the Pilbara are champions at shutting down for long periods; many can readily survive 2–3 years of dormancy while they wait for the rains to come. Even when it rains, however, the snails have only a few days of activity after the storm has passed.

Not surprisingly, the pace of life is slow, as my colleague Robert Black and I found several years ago in a coastal population of Rhagada capensis. It takes 5 years for this 2 cm-wide snail to reach maturity, and the adults can expect to live another 5 years, so the average age of adults is about 10 years.

The Pilbara snails are specially adapted to maximise their chance for reproduction in the brief periods of activity. The two most widespread genera in the Pilbara, Rhagada and Quistrachia, also have species in the Kimberley region to the north, which has a monsoonal climate with reliable summer rain and a dry winter.

Typical of snails in such areas, the reproductive organs of Rhagada and Quistrachia species in the Kimberley regress during winter dormancy, saving maintenance costs. When the summer rains come, the snails cannot reproduce immediately but must feed for a couple of weeks to develop their gonads.

In contrast, the snails in the Pilbara cannot afford this luxury because their lower total rainfall is also highly sporadic and unpredictable. Instead, the Pilbara species maintain their reproductive organs throughout dormancy and are ready to reproduce immediately. Whether they eat or mate first when the rain comes probably depends on when they meet another snail.

To minimise desiccation during the long periods of aestivation, the snails seal the aperture of their shells, either by glueing themselves to rocks or other shells (rock-sealers) or by secreting a calcified plug, called an epiphragm, across the shell opening (free-sealers).

The rock-sealers shelter beneath large rocks or in deep crevices, especially in shady sections of ravines or gorges. Because they hide so deeply, they are the hardest species to find, and are accessible only when they are active during rain.

Typically the species in each genus of snails are either all rock-sealers or all free-sealers. The exception is the genus Quistrachia, in which two species seal to rocks and seven species are free-sealers.

The free-sealers are largely coastal, where they can dig easily in the sandy soil, whereas the rock-sealers extend well inland. Genetic studies by Honours student Caitlin O’Neill showed that the rock-sealing Quistrachia evolved from the free-sealers, an adaptation that allowed occupation of the inland boulder habitats.

Free-sealers escape the extreme heat by burying themselves a few centimetres into the soil in sheltered places. The most widespread species in the Pilbara is Rhagada convicta, whose favoured habitat is the spinifex plains. These snails bury themselves beneath large spinifex tussocks, which retain moisture longer and provide shade.

Spinifex is the most common habitat in the Pilbara, contributing to the wide distribution of this snail. Despite its original vastness, however, this habitat is vulnerable. Fire destroys the large spinifex plants that are needed for shelter. Frequent fires and the introduction of buffel grass have reduced the areas occupied by the snails because the buffel grass does not provide the necessary protection.

Livestock can also damage the habitat through compaction of the soil and the reduction of vegetation. The rock-dwelling snails have greater protection from such threats, and have fared better, despite their more restricted distributions.

Local Adaptation
Rock-sealers are not the only snails that live in rocky habitats. Some free-sealing species of Rhagada live in rocky areas that have enough soil for them to bury themselves beneath the rocks. Compared with their relatives on sandy plains, these rock-dwellers often have flatter shells, helping them to take shelter in crevices and increasing their stability when crawling on vertical surfaces.

Flatter shells have evolved several times in the genus Rhagada in the Pilbara, and PhD student Sean Stankowski has studied in detail a remarkable example on Rosemary Island in the Dampier Archipelago. Spanning only 5.4 km, this island has five species of Rhagada that span the range of size and shape in the entire genus.

By examining the snails at 103 sites across this small island, Stankowski showed that the flattest shells, R. dampierana, are found on only two rocky hills, a total area of about 0.25 km2. His genetic comparisons confirmed that the diverse shell forms on the island are very closely related, and that all the forms evolved locally on the island, adapting to local habitat.

Indeed, in the approach to the rocky hills from the sandy plains, the flat snails and the more globular-shaped snails interbreed, producing the full range of shell types. Here, over distances of 200 metres, we can see speciation happening with adaptation to different habitats.

Stankowski’s preliminary studies of Rhagada throughout the Dampier Archipelago indicate that similar local adaptation to rocky habitats has occurred repeatedly on many islands.

Recognition of the importance of habitat helps us to understand the evolution of the snail fauna of the Pilbara. The greatest diversity of Rhagada is in the Dampier Archipelago, which comprises 42 islands within a radius of 45 km from the port of Dampier. Isolation on separate islands favours genetic divergence, and the mixture of rocky and sandy habitats favours local adaptation of shell form.

On the mainland, the scale is different, with large expanses of spinifex plains. In addition to Rhagada convicta, whose distribution spans 400 km, the other mainland species in the Pilbara also have relatively large distributions of 100–250 km. The exception is R. pilbarana, which lives in rocky habitat over just a few kilometres surrounding Mount Herbert.

The importance of scale and habitat is especially clear for the unusual genus Strepsitaurus, which is restricted to the Cape Range. These snails are rock-sealers. The two species with the widest distributions live among rocky slabs, which are the main habitat across the top and at the base of the Range.

On the western side of the Cape Range, however, there are deeply incised gorges with much more specialised species of Strepsitaurus. These species are small and very flat, and they live on the south-facing, vertical sides of the gorges, away from direct sunlight. They shelter in crevices, and feed on the wet rock face.

We have collected these specialists in three gorges, and genetic studies by Honours student James Taylor showed that populations of the species in the continuous habitat on top of the range are connected, but the snails in the gorges are highly divergent species, each known only from a single gorge.

Thus, specialisation to a particular habitat is important, and the snails in each local area will evolve in isolation if that habitat is fragmented.

Complicated Distributions
While habitat clearly affects the distribution of each species, the snails themselves may also restrict the distribution of closely related species. Despite the regional diversity of species in the Pilbara, local species diversity is low because members of the same genus do not occur together.

In his 1997 review of the taxonomy of these snails, the late Alan Solem of the Field Museum of Zoology in Chicago knew of no cases in the Pilbara where two species in the same genus occurred together. One explanation could be that each species evolved in isolation as distributions became fragmented over many thousands of years of increasing aridity, so they simply have not come into contact. However, our discovery of several examples where two species meet conflicts with this interpretation.

Genetic comparisons by PhD student Zoë Hamilton have shown that the different forms did indeed evolve in separate areas, and have subsequently come into contact. The different forms may interbreed where they meet, but the hybrids occur only in a narrow zone, beyond which the species remain distinct. We have found several such contacts, and they are all narrow, so that species meet but do not overlap geographically.

Because these species have not spread into each other’s ranges, despite the opportunity, we suspect that each species excludes its close relatives, so that competition may limit the distributions. Thus, the distribution of each species depends on its requirements, its history and the histories of other species.

While some interpretations are still speculative, detailed sampling and the search for associations of shell form with habitat are giving us a clearer picture of how these snails are adapted to habitats in the harsh Pilbara region, and hence what kinds of places are especially important for their survival – and hence for conservation.

The adaptation of shell form to local habitat can make shells unreliable indicators of taxonomy, however, so we need molecular genetic comparisons to reveal the evolutionary relationships of the snails, which provide the framework for understanding the history of adaptation, the taxonomy of the snails and the geographic distribution of each species.

Michael Johnson is Winthrop Professor in the University of Western Australia’s School of Animal Biology.