Fossil snails might tell us of the frequency of heavy rainfall in the past
Anew study comparing the stable oxygen and carbon isotope ratios of giant land snails in New Zealand and New Caledonia found a surprising result. New Zealand snails had, on average, higher oxygen isotope ratios values than their counterparts in New Caledonia, counter to the relative isotopic composition of rainwater between these two regions. This research just published in the Journal of Quaternary Science provides baseline data for using the shells of Placostylus snails as environmental proxies – allowing us to use fossil shells to estimate the temperature and rainfall when the snails were alive.
Most interesting are the dramatic drops in oxygen isotope ratios that seem to correspond to heavy rainfall – suggesting an opportunity to dig into the past to compare past precipitation with current frequency of wet weather events in New Zealand. There is also the potential to study the frequency of droughts from the pattern of snail shell growth.
Scientists at Massey University and NIWA sliced up shells of three species of giant land snail (pūpū whakarongotaua; Placostylus). The recent samples from New Caledonia showed drops in isotopic values in their high‐resolution profiles probably linked to periods of intense rainfall.
Very heavy rainfall events produce lower stable oxygen isotope ratios incorporated into the shells of the living-growing land snails. In contrast, the snails from New Zealand varied very little, suggesting that when they were alive, 74 years ago, there were few heavy rain events in the Far North of New Zealand.
The snails (pūpū whakarongotaua; Placostylus) are taonga of Ngāti Kurī who value them as security alarms (the snail that listens for war parties). Ngāti Kurī are working to save the local species from extinction but they are also kaitiaki (guardians) of fossil shells buried in the sand dunes and stored in museums. These fossil deposits could provide information about the past climate through high‐resolution stable oxygen isotope profiles.
“it is exciting to think of all the information locked up in snail shells – the shape of the shell, the DNA and the isotopes can all tell a story about the past” said Mary Morgan-Richards. “As Placostylus snails are slow growing, taking 10 years to reach maturity, and live for a long time, they can each tell their own story. There is much to be learnt by digging into old shells to reveal the frequency of heavy rainfall events in the past.”
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.
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.
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).
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).
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.
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