The snail that listens for the war party

Pūpū whakarongotau (Placostylus ambagiosus) is a large leaf-eating land snail that has declined to fewer than 2000 individuals scattered over 19 populations. These snails are highly valued by tangata whenua of far north Aotearoa (Te Aupōuri me Ngāti Kurī), because in the past the snail was both kai and made alarm calls at night warning of approaching invaders. The sounds these pūpū (snails) make as they hastily retreat into their shells when disturbed at night once alerted the people to approaching invaders and so saved their lives. So, the snails are known as pūpū whakarongotaua -the snail that listens for the war party. Oral histories tell us that snails were moved to propagate new P. ambagiosus populations along with harakeke and karaka.

We know that individuals of this species seldom move more than a few metres from where they hatch, are long-lived (10–22 years), and show strong site fidelity – with individual snails being able to crawl home over at least 60 metres (Parrish et al. 2014; Stringer et al. 2017). The tough shell protects adult snails from native predators and the climate, and preserves evidence from the past.

Visible differences in the size and shape of snail shells of this species (Placostylus ambagiosus) led to numerous distinct isolated populations being given their own subspecies name. Using museum material collected 70 years ago we studied shell shape variation to determine whether it is the result of genetic differences or environmental differences. On a headland, previously the site of a pā (fortified settlement), one population that resulted from prehistoric cultivation of snails showed that shell shape differences are maintained when the snails are living and growing in the same environment.

A “common garden” experiment. Snails moved from different populations to the same area continue to show their distinct shell shape characteristics over generations, revealing their genetic differences.

Using geometric morphometrics of shell shape we could discriminate pūpū shells without reference to where they had been collected. Our genetic data confirmed that some human movement of snails had occurred but that this has not resulted in a loss of genetic differentiation from east to west. We recommend that the shell shape (not size) of these species can be used to infer genetic differences that might be important for the survival of the species as climate changes. All shape variation should be conserved by protecting all living populations from predators and competitors. The subspecies names are a good way to refer to this diversity and protect the evolutionary potential and historic record held in these populations.

Daly EE, Trewick SA, Dowle EJ, Crampton JS, Morgan-Richards M. 2020. Conservation of pūpū whakarongotaua – the snail that listens for the war partyEthnobiology and Conservation, 9:13(13 May 2020)doi:10.15451/ec2020-05-9.13-1-27

Notes from a small Island – in the Pacific

View from Mount Pitt to the Norfolk Island airstrip and Phillip Island beyond.

In the Pacific Ocean, east of Australia and about halfway between New Zealand and New Caledonia is Norfolk Island. This tiny island (~30 square kilometres) has a colourful history and enough endemic species to make it very valuable to biologists. Not as famous as the Galapagos Islands or the Hawaiian archipelago, Norfolk Island has its own endemic biodiversity including dozens of species of tiny snails in the forest leaf litter.

During WWII an aeroplane runway was constructed for refuelling Royal New Zealand Air Force (RNZAF) bomber patrols and for a transport service to Bougainville. Construction required relocating families and cutting down Norfolk pines (Araucaria heterophylla), and the runway takes up a huge portion of the land surface of Norfolk Island . The giant ‘T’ of 2.22km and 1.8km imprinted across an island that is only a little over 9km at its widest point. Access by commercial airlines now brings tourists and invasive species to the island on a regular basis. It only takes a couple of hours flying from Auckland to reach Norfolk Island, and there are flights in and out a couple of times each week. Before the runway and the ships that brought people, new species to the island arrived by long-distance dispersal – flying, swimming, ballooning, rafting, and hitch-hitching (Jordano 2016). Consider the many birds that fly to Norfolk Island, some are capable of bringing live snails in their guts. For example, some snails can survive being swallowed by silver-eyes and be found alive in their droppings. These little birds are found on many oceanic islands so can aid dispersal of snails (Zosterops sp. Wada et al. 2012). Ducks (also seen on Norfolk Island) can transport some snail species on the inside (Van Leeuwan et al. 2012). Seabirds, of which Norfolk Island has eight breeding species, are also potential source of long-distant dispersal (Viana 2016).

A Norfolk Island robin, part of a species complex on Pacific islands.

Norfolk Island was formed about 3.05–2.3 million years ago from several volcanic eruptions (Jones & McDougall, 1973). The terrestrial fauna of Norfolk Island must therefore have developed in just a few million years (<3) from the descendants of long‐distance dispersing ancestors (Holloway, 1977). Many plants and invertebrates endemic to Norfolk Island look similar to species elsewhere in the Pacific. Their ancestors must have dispersed to colonise this volcanic island. Isolated on Norfolk Island, populations have accumulated differences (allopatric speciation) and in many cases can be readily distinguished as similar but different from Australian or New Zealand species. For example, the endangered Norfolk Island coastal shrub Coprosma baueri looks very like New Zealand taupata Coprosma repens and the Norfolk Island Palm (Rhopalostylis baueri) looks like New Zealand nikau (Rhopalostylis sapida). When the Norfolk Island boobook owl (Ninox novaeseelandiae undulata) was down to a single female, it was genetically similar enough to successfully hybridise with a male ruru from New Zealand (Ninox n. novaeseelandiae; morepork) to save the population from extinction (Garnett et al. 2011). The cicada species found on Norfolk Island, Kikihia convicta, is morphologically and genetically sister to the New Zealand species K. cutora (Arensburger et al. 2004). And sister relationships between NZ and Norfolk Island taxa are also seen with the extinct kaka (parrot; Nestor productus) and extinct pigeon (Hemiphaga novaeseelandiae spadicea; Goldberg et al. 2011). These Norfolk Island species have New Zealand affinities, but many others have close relatives in Australia, New Caledonia and other Pacific Islands. For example, of the larger butterflies and moths native to Norfolk Island about 22% are endemic, of which only 10% have New Zealand origins (Holloway, 1977).

Species Radiations

There is considerable species diversity of terrestrial micro-snails in Norfolk Island, best estimates are that there are about 40 living species (Neuweger et al. 2001; Varman 2016). Most are known only from empty shells, and species descriptions of micro-snails usually rely on shells  (Stanisic et al. 2010). While on Norfolk Island we focused our effort of getting photographs of the live snails of the common species . We collected snails from the ground from forest leaf litter and from leaves during the day.

We photographed living specimens from as many common species as we could find, with the hope that this resource can be used to provide better tools for their identification in the future and help people conserved the current snail diversity. For example the Pinwheel snail Cryptocharopa exagitans (from the family: Charopidae) is recorded (from empty shells) as the most common micro-snail in mixed forest leaf litter near Duncombe Bay, Norfolk Island (Neuweger et al. 2001). The shell of this pinwheel snail (3.5mm) is recognised by its frill of dried mud around the edge but when alive the tiny snail shell is fantastically camouflaged as rock so very easily over-looked. Our photos show the tiny snail hauling what looks like a stone on its back.

Leaf Beetles
Two species of eumolpine leaf beetle are described from Norfolk Island (Dematochroma shuteae and Dematochroma norfolkiana Jolivet et al. 2007), although it is likely there are more to study. The adult beetles are small and usually brown, bronze or black. They feed at night on leaves but as adults the beetles are probably short lived and likely to be seasonal. Larvae of these beetles live underground feeding on roots. The  two known species are probably related to the eumolpine radiation of New Caledonian beetles (Gómez-Zurita 2011), also found in New Zealand and Australia. The leaf beetle species on Lord Howe, look  quite different from one another but represent an island radiation from a single recent ancestor. We hope to find out whether the Norfolk Island leaf beetle species also represent an endemic radiation.

By searching in the Norfolk Island leaf litter during the day and at night on foliage we saw numerous individuals of at least four different types/forms that might represent four species. Most beetles were active during the night when mating pairs were frequently observed. One species was predominantly observed on the foliage of Piper excelsum psittacorum, and another species was seen on leaves of Coprosma pilosa. Leaf beetles often have a short season as adults, so one week of observations is likely to have included just a fraction of all Norfolk Island Eumolpinae beetles. We are fairly confident that work on these insects will double the known diversity of leaf beetles from Norfolk Island.

References

Arensburger P, Simon C, Holsinger K. 2014. Evolution and phylogeny of the New Zealand cicada genus Kikihia Dugdale (Homoptera: Auchenorrhyncha: Cicadidae) with special reference to the origin of the Kermadec and Norfolk Islands’ species. Journal of Biogeography 31: 1769-1783.
Goldberg et al. 2012. Population structure and biogeography of Hemiphaga pigeons (Aves: Columbidae) on islands in the New Zealand region. Journal of Biogeography 38: 285-298. https://doi.org/10.1111/j.1365-2699.2010.02414.x
Gómez-Zurita J. 2011. Rhyparida foaensis (Jolivet, Verma & Mille, 2007), comb. n. (Coleoptera, Chrysomelidae) and implications for the colonization of New Caledonia. ZooKeys 157: 33-44.
Holloway, J.D. 1977. The Lepidoptera of Norfolk Island. W. Junk, The Hague.
Jolivet, Verma & Mille 2007. New species of Dematochroma from Lord Howe and Norfolk Islands (Coleoptera, Chrysomelidae, Eumolpinae). Nouv. Revue Ent. 23; 327-332.
Jones, J.G. & McDougall, J. 1973. Geological history of Norfolk and Philip Islands, southwest Pacific Ocean. Journal of the Geological Society of Australia 20: 239–257.
Neuweger et al. 2001. Land Snails from Norfolk Island Sites. Records of the Australian Museum Supplement Nov. 2001. DOI: 10.3853/j.0812-7387.27.2001.1346
Reid C. 2003. Chrysomelidae of Lord Howe Island. Chrysomelidae, 42: 7.
Van Leeuwen et al. 2012. Experimental Quantification of Long Distance Dispersal Potential of Aquatic Snails in the Gut of Migratory Birds. PloS One. https://doi.org/10.1371/journal.pone.0032292
Varman R. 2016. Norfolk Island Snail Species Collections made between January and March 2016. Report to Australian National Parks.
Viana et al. 2016. Migratory Birds as Global Dispersal Vectors. Trends in Ecology and Evolution 31: 763-775.
Wada, et al. 2012 Snails can survive passage through a bird’s digestive system. Journal of Biogeography 39: 69-73

punctuated equilibrium in New Zealand

The theory of punctuated equilibrium has two important elements, one is that evolutionary lineages can remain morphologically unchanged for millions of years, the other is that rapid morphological change is associated with speciation.

One of the best examples of morphological stasis within biological species comes from the New Zealand Olive Shells (Amalda australis, A. depressa, A. mucronata).

New research on olive shells has just been published: “Phylogenetic topology and timing of New Zealand olive shells are consistent with punctuated equilibrium“.
In this paper we have shown that the three species of New Zealand marine snails (Olive shell Amalda spp) cited by Stephen Jay Gould as important examples of stasis are part of a monophyletic New Zealand clade. This suggests that their evolutionary history has unfolded on the continental shelf around New Zealand, with new species evolving from ancestors in the same region. Using DNA sequences and a molecular clock analysis we determined that lineage splits (speciation) occurred before the 2–3 million years of morphological stasis identified within each of these three species.

Here, we use entire mitogenome and long nuclear rDNA gene cassette data from 11 Amalda species, selected from New Zealand and around the world. Within our sampling, New Zealand Amalda are a natural monophyletic group and estimates of the timing of cladogenesis from the molecular data for the New Zealand group are compatible with the fossil record for extant species and consistent with expectations of punctuated equilibrium.

We have yet to confirm morphological change revealed in the fossil record with the origin of new species, but our study indicates that new taxa in the New Zealand fossil record is not likely to be the result of colonisation by long-distance dispersal of Amalda species from other parts of the world, but instead, represent local evolution.

The punctuated equilibrium model predicts prolonged morphological stasis through time with abrupt change associated with speciation (left). To test this idea we need to show that species diversity is not the result of colonisation from elsewhere or morphological change that is independent of lineage formation (right).

Quote from SJ Gould (1991)
The best treatment of this objection [that fossil taxa are not biological species] must be sought in studies of living species with good fossil records—where direct surveys can be made for correspondence of a morphological package with a true biological species, and the origin and history of the same package can then be traced in the fossil record and assessed for punctuated equilibrium. I am delighted to report that two such pioneering studies have been published in the past few years, and both support punctuated equilibrium.
New Zealand biologist B. Michaux did a morphological and genetic survey of four species in the snail genus Amalda. He found no cryptic populations; each morphologically defined package corresponds perfectly with a biological species. Three of these species extend back in the New Zealand fossil record for several million years. In an elegant, multivariate study of morphological pattern, Michaux demonstrated stasis throughout the ranges of all species. He concludes (in the Biological Journal of the Linnaean Society of London, vol. 38, 1989): This study demonstrates that fossil members of three biologically distinct species fall within the range of variation that is exhibited by extant members of these species. The phenotypic trajectory of each species is shown to oscillate around the modern mean through the time period under consideration. This pattern demonstrates oscillatory change in phenotype [our jargon for overt morphological appearance as contrasted with underlying genetics, or genotype] within prescribed limits, that is, phenotypic stasis.

Hopped out

Stand at the side of a mainland island reserve and the impact of humans on New Zealand’s natural environment is obvious. From a landscape naturally dominated by tall forest, agricultural ‘improvement’ rapidly moved us to a uniform, virtual biological desert. Not only are the trees and birds missing, but the lichens, fungi, insects, worms and molluscs are gone. even the bacteria and other microbes of the soil are replaced. In response we resort to counting species and prioritising conservation efforts on the scarcest and restoration effort on the rarest habitats. But, wholesale environmental changes alter no just the abundance of native species but their ecology and interactions. Ultimately, by restructuring the landscape we alter evolutionary outcomes and this has become increasing apparent as research explores biological responses to human induced climate change.

Predator fence on boundary of native forest and exotic paddock. Bushy Park sanctuary near Wanganui.

An obvious difficulty with understanding environmental change is that it is much easier to say what is, compared to what was. We are readily inured to the situation and so are accepting of the status quo. One very powerful tool that has helped biologists understand how the geographic ranges of species and population change over time is phylogeography. Simply put, this approach combines information about where individuals and populations of a species are found with information about how those individuals are related to each other. DNA sequence data reveals how closely related individuals are (their genealogy), and how genetically diverse populations are. It is this type of data that shows, for instance, how our human ancestors left Africa and migrated into Europe, then Asia before eventually colonising islands in Oceania. We now know that New Zealand was probably the last major island to have be reached by people travelling by foot and finally boat.

Genetic data from living people has revealed how their ancestors migrated from Africa around the world (values are years before present).

Since the 1990’s phylogeographic studies have revealed the influence of many environmental factors on the distribution of biodiversity. In particular, natural, global climate cycling during the last few million years of Earth’s geophysical prehistory (the Pleistocene epoch) is known to have been influential. We now know for example that in the northern hemisphere repeated extension of the arctic ice cap during ‘glacial’ episodes extinguished populations of all species in northern Europe, Asia and America; remnant populations survived in warmer southern areas. As climate alternately warmed and cooled over 10–100 thousand year cycles, the ranges of animal and plant species expanded and retracted in response.

Estimated distribution of vegetation types in New Zealand during the Last Glacial Maximum. See Wild Life New Zealand.

 

In New Zealand a related pattern of species range change has been inferred. Pollen records show where plant species once lived and genetic data show that during cold phases of the Pleistocene, forest reduced and was replaced in many areas by scrub / grassland communities. Animal species are expected to have responded to these changes tracking their preferred habitat in space and time (or going extinct), and this has been found to be the case for some. North Island tree wētā, for instance, appear to have tracked climate niche.

 

Phaulacridium marginale

A recent study examined the response of two related grasshopper species. These endemic Phaulacridium grasshoppers live in low elevation habitat, but as is typical of short-horn grasshoppers in temperate regions they require open habitat so they can gain heat by basking in the sun. That means Phaulacridium grasshoppers do not live in forest, and they do not survive above the treeline in the subalpine zone where cool temperatures prevent trees growing (other grasshoppers are adapted to those conditions). So space for Phaulacridium would have been restricted in prehuman New Zealand to scarce open areas such as coastal dunes, river flats, wetlands and semi-arid areas.  In fact, one species (Phaulacridium otagoense) occurs today only in the semi-arid McKenzie – Alexandra area of Central Canterbury and Otago. The other  species (Phaulacridium marginale) is today found in many places around the country.

Vegetation types across New Zealand before arrival of people (left) and in modern times (right).

Known occurrences of the two New Zealand Phaulacridium grasshoppers.

A small species range usually means a small population size, compared to a species with a big range; and small populations usually have a lower level of genetic variation. Low genetic diversity is documented in many endangered species such as the famous black robins of the Chatham Islands. Paradoxically, in Phaulacridium the opposite pattern exists; the species with the smallest range (pink in map) has much higher genetic diversity than the widespread more common species. The simplest explanation is that P. otagoense (pink),  had until recently a much larger range and so bigger population. Conversely, P. marginale (turquoise) appears to have expanded its range recently and has not yet had time to accumulate new genetic diversity.

Niche models for Phaulacridium otagoense indicating optimal habitat (red, orange) during the last glacial phase may have been similar to today.

It is known that global temperatures had recovered from the last cold phase of the Pleistocene by about 15,000 years ago. Perhaps P. otagoense had a much larger range in the period before that when cooler, drier conditions allowed scrub grassland to expand; similar to conditions where it occurs today? Niche modelling indicates that in current conditions the potential range of this species is bigger than the actual range in which it is found, and  taking into account estimated temperatures during the last glaciation suggests that the habitat preferred by this species had not been much more extensive.

So, probably the major change in fortunes for these Phaulacridium species relates mostly to the recent expansion of P. marginale. Climate modelling shows that the range of this species today is close to the potential occupiable range, but there is a problem. Although the climate across much of New Zealand suits this grasshopper, other factors in the environment do not. In particular, the presence of native forest excludes these little grasshoppers because they need to bask in the sun every day to warm up. How has P. marginale become so abundant and widespread?

The answer lies not in global climate change, but in recent anthropogenic changes to the environment much closer to home. By removing New Zealand native forest, humans created a landscape with the climatic conditions to allow P. marginale to increase in abundance and expand its range across the country. The addition of a mix of northern hemisphere grasses and herbs that thrive in this artificially open environment provided the nutrient-rich food for P. marginale. So that’s great! Well no.

Males and females of different species are capable of reproduction when they meet due to anthropogenic habitat change, resulting in loss of diversity. Male Phaulacridium otagoense with female P. marginale.

The increase in available habitat has meant that the spatial range of P. marginale now meets the range of P. otagoense. Where they meet, the grasshoppers makes mistakes when choosing mates resulting in gene flow. Genetic evidence shows that pure P. otagoense remain in only part of their natural ecological range. Genetic mixing is part of the natural evolutionary mill, but around the world human activity has accelerated the rate at which species meet and interact in new ways. This adds to the trends of biological homogenisation that characterises biodiversity loss in the Anthropocene.


Original science:

Bulgarella M, Trewick S, Minards NA, Jacobson MJ, Morgan-Richards M. 2014. Shifting ranges of two tree weta species (Hemideina spp.): competitive exclusion and changing climate. Journal of Bioleography 41: 524–535.

Goldberg J, Morgan-Richards M, Trewick SA. 2015. Intercontinental island hopping: colonization and speciation of the grasshopper genus Phaulacridium (Orthoptera: Acrididae) in Australasia. Zoologisher Anzeiger 255: 71–79.

Sivyer L, Morgan-Richards M, Koot E, Trewick SA. 2018. Anthropogenic cause of range shifts and gene flow between two grasshopper species revealed by environmental modelling, geometric morphometrics and population genetics. Insect Conservation & Diversity 11: 425–434.

Media:
NZ Herald – hoppers
NZ Herald – Mackenzie biodiversity

Sticky sex

New Zealand stick insects have invaded the United Kingdom, but in the process they have lost the ability to reproduce sexually. This is odd because the vast majority  (more than 99%) of  multicellular creatures (primarily eukaryotes) engage in sex during reproduction.

Sex involves two individuals with different properties. Typically one sex (the male) produces abundant small and often motile gametes that carry genetic information to the larger egg produced by the other (female). Through this process, genetic information is passed from two parents to their offspring and results in shuffling of genetic variation. The results are readily evident in the variation seen among offspring that is prominent in human families.

Stick insects are (mostly) no exception even though scientist can show that reproduction without two sexes can have a numerical advantage over sexual reproduction. Simply, females that make only self-fertile daughters leave more of their genetics to future generations.  Theoretically it seems that clonal reproduction is advantageous, as long as the environment does not vary too much; producing offspring that are not the same as the parent could make some of them less successful. It is telling then, that despite the numerical advantage of clonal reproduction, that vast majority of large organisms do use sexual reproduction. Natural selection has made its choice.


One group of New Zealand stick insects includes individuals that differ in colour, size, and shape. In particular the number and size of spines they have varies among individuals. This group (genus Acanthoxyla) includes several described species, although in this case defining species is difficult. All are female, which means all come from self-fertile eggs produced by one parent (the mother). Hatchlings grow up to look like their mums, so are effectively clones.

Among the many individuals of common and widespread Acanthoxyla (literally: prickly stick) observed in New Zealand, no male has been encountered. Yet. But recently a male belonging to this genus turned up in England.

Rare males like this emerge among all-female stick insect populations, probably as a result of a random mutation deleting one of the XX sex chromosomes that denotes a female stick insect. XO individuals are male in appearance, but are usually not reproductive.


Research on the New Zealand genus Clitarchus has been revealing about the switching between sexual and asexual (all female) reproduction. As reported in Nature a population of Clitarchus hookeri accidentally introduced to the UK about 100 years ago has lost not only its homeland but also its sex life.

Analysis of genetic variation shows that the origin in New Zealand of the UK stick immigrants was most likely in Taranaki, North Island. This agrees with historical records indicating that native plants collected in this area were shipped to England and then the nearby Isles of Scilly. In particular the Abbey Gardens on Tresco are now home to a range of New Zealand plants, and it is likely that stick insect eggs in the soil around plant specimens were accidentally transported around the world. Hatchlings that grew into adult stick insects able to produce abundant self-fertile females were likely at an advantage. The potential of this species to switch to asexual reproduction has also resulted in a pattern of geographic parthenogenesis in New Zealand.

Genetic variation (mtDNA COI) in Clitarchus hookeri across New Zealand (A, B), highlighting the mainly parthenogenetic lineage in NZ (C), and the lineage associated with the one variant found in the UK population (D).

Closer examination of two New Zealand populations of the same species add to our understanding of the drivers and mechanisms of reproduction strategy switching. The UK population lost sexual reproduction and evolved a barrier to fertilisation, which has been demonstrated by providing captive female stick insects from UK with NZ males. Meanwhile two NZ populations recently gained sexuality and genotypic data indicate this happened via two different pathways.


Original Science:
Morgan-Richards M, Langton-Myers S, Trewick S. 2019. Loss and gain of sexual reproduction in the same stick insect. Molecular Ecology

Trewick SA, Morgan-Richards M. 2018. Missing New Zealand stickman found in UK. Antenna 42: 10–13.

Comment:
Nature

Media:
Guardian
Telegraph
STUFF/Dominion Post
BBC radio