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    Genome Evolution and Introgression in the New Zealand mud Snails Potamopyrgus estuarinus and Potamopyrgus kaitunuparaoa
    (Oxford University Press on behalf of Society for Molecular Biology and Evolution, 2024-05-22) Fields PD; Jalinsky JR; Bankers L; McElroy KE; Sharbrough J; Higgins C; Morgan-Richards M; Boore JL; Neiman M; Logsdon JM
    We have sequenced, assembled, and analyzed the nuclear and mitochondrial genomes and transcriptomes of Potamopyrgus estuarinus and Potamopyrgus kaitunuparaoa, two prosobranch snail species native to New Zealand that together span the continuum from estuary to freshwater. These two species are the closest known relatives of the freshwater species Potamopyrgus antipodarum-a model for studying the evolution of sex, host-parasite coevolution, and biological invasiveness-and thus provide key evolutionary context for understanding its unusual biology. The P. estuarinus and P. kaitunuparaoa genomes are very similar in size and overall gene content. Comparative analyses of genome content indicate that these two species harbor a near-identical set of genes involved in meiosis and sperm functions, including seven genes with meiosis-specific functions. These results are consistent with obligate sexual reproduction in these two species and provide a framework for future analyses of P. antipodarum-a species comprising both obligately sexual and obligately asexual lineages, each separately derived from a sexual ancestor. Genome-wide multigene phylogenetic analyses indicate that P. kaitunuparaoa is likely the closest relative to P. antipodarum. We nevertheless show that there has been considerable introgression between P. estuarinus and P. kaitunuparaoa. That introgression does not extend to the mitochondrial genome, which appears to serve as a barrier to hybridization between P. estuarinus and P. kaitunuparaoa. Nuclear-encoded genes whose products function in joint mitochondrial-nuclear enzyme complexes exhibit similar patterns of nonintrogression, indicating that incompatibilities between the mitochondrial and the nuclear genome may have prevented more extensive gene flow between these two species.
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    Systematics and phylogeography of the large land snail Powelliphanta : a thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology, Massey University, New Zealand
    (Massey University, 2018) Walker, Kathleen Joy
    Without a generally accepted name, a species is less likely to receive conservation effort. For the large, colourful and carnivorous land snails in the New Zealand genus Powelliphanta, doubts about the validity of the existing morphologically-based nomenclature, and the absence of taxonomic assessment for many late-discovered taxa, have been hindering conservation efforts. Powelliphanta have been in decline due to continuing loss of habitat and a suite of predators recently introduced to New Zealand, but scarce conservation resources are targeted to formally described taxa. The aim of this study was to review and if appropriate revise the taxonomy of the genus to remove any taxonomic impediment to conservation action. Like other Pacific Ocean archipelagos, the fauna and flora of New Zealand is characterised by a small number of families with extensive radiations. Understanding of relationships is often hindered by the recency of lineage separations and requires multiple lines of evidence. Allozymes and mitochondrial sequences were used to identify genotypic clusters in Powelliphanta and to assess the relationship with presumed sister taxa Victaphanta. A matrix of morphological characters was assembled for multivariate analysis. These characters included for the first time in Powelliphanta, features of the soft body and Fourier coefficients describing subtle shape differences. Intraspecific variation within one of the species, P. lignaria, received detailed distributional, morphological and genetic evaluation to investigate the use of subspecies in the genus. Powelliphanta fiordlandica was found to be a sister taxon to all other Powelliphanta, with a deep genetic divergence and differences in morphology indicating it should be placed within its own monotypic genus. Allozymes, mtDNA and morphology all identified similar clusters, many of which equated to the already described taxa and to taxa which had been tag-named but remained undescribed. Shell colour, pattern and size, which show environmental plasticity in some molluscs, were found in Powelliphanta to be genetically-based and shape also proved informative. The described subspecies of Powelliphanta lignaria were found to be morphologically diagnosable, largely allopatric, and with gene flow limited to the extent that the integrity of each subspecies has been maintained. A recently discovered and critically threatened species was described as Powelliphanta augusta, and a new classification for the genus comprising 20 species and 59 subspecies (including the existing P. lignaria subspecies) was proposed.
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    Fine scale population structure through space and time : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology, Massey University, New Zealand
    (Massey University, 2016) Daly, Elizabeth Emma
    Terrestrial snails, with their diversity of interspecific forms, have provided biologists with fantastic material to study the evolution of ecotypes and the process of speciation. Snails have the advantage of shells that preserve well and exhibit trait variation readily perceived by taxonomists. Endemic to New Zealand is the genus of giant carnivorous Powelliphanta snails and three species of giant herbivorous Placostylus. Both genera display a range of phenotypic variation of shells within comparatively small geographic distances. The diversity within these snails has become a matter of high conservation interest, as many lineages occupy small or highly fragmented ranges that render them vulnerable to ongoing habitat loss, and predation by exotic pests. Combining mitochondrial sequence data and genotypes of microsatellite loci I documented the genetic structure within a species complex (Powelliphanta Kawatiri). Improved understanding of the distribution of this complex and the level of genetic diversity provided a picture of a naturally fragmented lineage, restricted to a particular ecological zone. To investigate the evolution of Placostylus ambagiosus its mitochondrial genome and that of its sister species P. hongii were assembled and annotated. Gene order was consistent between the two Placostylus species although it varies slightly within the wider Sigmurethra suborder due to minor tRNA rearrangements. To distinguish the shell shape of spatially distinct populations of Placostylus ambagiosus two-dimensional geometric morphometric methods were used. This tool was used to study shell shape evolution through time. Stasis was found to be the most common evolutionary mode, however shell size followed a different model, in one population, an observation which would not be expected if gene flow was preventing local divergence. Investigation into the genetic structure of Placostylus ambagiousus (using RADseq) revealed a single admixed population illustrating gene flow had occurred between populations in the recent past. The formation and maintenance of locally adapted populations (ecotypes) within Placostylus ambagiosus does not seem to be prevented by gene flow within species.
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    Ecology of the common snail, Helix aspersa Müller, in a disturbed dune environment : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University
    (Massey University, 1978) Millar, Ian Robert
    A study was made of the population ecology of Helix aspersa Muller in coastal dunes at Santoft forest, near Bulls. The environment was being altered by afforestation processes. The distribution of the animals is affected by the presence of tree lupin, Lupinus arboreus Sims, to which they are strongly attracted. Areas planted with lupin are capable of supporting a much greater density of snails than non-lupin areas, and this is thought to be due to the nutritive value of this species. Snail population densities appear to increase rapidly after lupin seeding in the dunes and this results in widespread lupin die-back after as little as three years from seeding. After lupin die-back the snail population decreases again. Some suggestions are made as to the origin of H. aspersa in the dune country and on the eventual fate of the populations under the maturing forest. Using shell characteristics, it was found that most juveniles in expanding populations reach maturity in little more than one year whereas those in high density, declining populations generally take two or three years. Individuals in expanding populations also attain a greater size on maturity than those in high density populations. The main factor affecting population density appears to be adult recruitment, which is considerably higher in expanding than in stable or decreasing populations. H. aspersa is found to be socially gregarious and this is particularly marked over the winter period when adults and large juveniles aggregate for hibernation. Hibernation begins in May and reaches a peak in July. Many animals are active again in mid-August. Predation by the song thrush Turdus philomelos was studied in one area. Predation occurs throughout winter and generally increases over late spring/early summer. Snails affect nitrogen fixation levels in lupins before lupin death occurs and it is considered that this is due primarily to disruption of the phloem tissue of the stems during snail feeding. It is suggested that this disruption of the translocation tissues is the ultimate cause of plant death. The possible economic significance of H. aspersa in dune forestry through its effects on lupin is discussed, and the need for further investigation indicated.
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    Some aspects of behaviour and ecology of the land snail Powelliphanta traversi traversi Powell (Rhytididae: Rhytidinae) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University
    (Massey University, 1997) Devine, Christopher Denis
    Powelliphanta traversi traversi Powell was studied at two sites in the Horowhenua. The use of an harmonic radar allowed the snails to be relocated, and followed for 20 months in their natural habitat. Morphometrics, population sizes, diet, movement, dispersal, and predation were examined. Different formae (morphs) lived at each site but mean shell lengths (43.2 mm at Papaitonga, 42.41 mm at Makahika) did not significantly differ. Frequency histograms of shell length for live and dead P. t. traversi were of similar shapes and there were few small shells. This could suggest that mortality was constant regardless of age, that young grow rapidly, or they live for a long time once full sized. The mean growth of new shell to the lip of the shell was found to be 1.71 mm (range 0.11 - 6.82 mm).The densities were not significantly different at each study site at 282 snails ha-1 for Papaitonga and 300 snails ha-1 at Makahika. A significantly positive correlation was found between P. t. traversi presence and leaf litter depth. Karaka (Corynocarpus laevigatus) was the only plant that was consistently found in quadrats with high snail numbers. The number of empty shells in a quadrat was a poor predictor of the number of live snails present. P. t. traversi were nocturnal and moved slowly in comparison to the garden snail H. aspersa. P. t. traversi were not active continuously though the night, and moisture related factors were the only significant predictors of movement. The most active snail moved 152 m in 107 days. Maximum displacement from point of origin averaged 49.8% of total movement. I suggest movement could be random, but appeared to adhere to a home-range. Limited dispersal suggested that fragmented P. t. traversi colonies should be considered discrete populations. The primary predator of P. t. traversi was the ship rat Rattus rattus. There was no evidence of predation from the brushtail possum, Trichosurus vulpecula, an important Powelliphanta predator in other localities. Diet and water uptake of P. t. traversi was examined in the laboratory. P. t. traversi appeared not to drink, but rather obtained water via integumentary absorption. Full hydration was reached in around three hours. Earthworms were the only food items consumed in this study. The snails did not forage in dry conditions.
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    The effects of introduced predators and the invasive weed Tradescantia fluminensis ( Vell.) (Commelinaceae) on the land snail Powelliphanta traversi traversi (Powell) (Gastropoda: Pulmonata: Rhytididae : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Ecology at Massey University
    (Massey University, 2001) Bennett, Shaun James
    Powelliphanta traversi traversi (Powell) was studied at two forest remnants in the Horowhenua District. The effects of introduced predators, predator control, the invasive weed Tradescantia fluminensis (Vell.), and Grazon® herbicide on these snails were investigated. Brodifacoum poison was used in two areas of Lake Papaitonga Scenic Reserve to determine the effect of rodent control on P. t. traversi. Mouse abundance (inferred from tracking tunnel indices) was reduced in both poisoned areas below levels observed in two other areas that were not poisoned. Rat abundance was reduced below pre-poisoning levels but only to levels below one of the non-poison areas. In each poison and non-poison area, four 100m2 quadrats were searched for P. t. traversi snails immediately before poisoning, and 12 and 19 months after poisoning commenced. After 19 months, only one poisoned area showed an overall increase in the number of snails, with significantly more live snails found (45) than at either of the two previous searches (22 before poisoning and 28 after 12 months of poisoning) (P<0.05). Numbers of empty P. t. traversi shells found in each area decreased at each search suggesting that shell accumulation is constant rather than seasonal. Rats were the greatest identified predator of P. t. traversi at Lake Papaitonga (17.87% of all empty shells), but the proportion of shells damaged by blackbirds and song thrushes was also high (11.91% of all empty shells) and increased from pre-poisoning numbers in three of the areas. Overall, there was no conclusive evidence to suggest that the numbers of live P. t. traversi increased as a result of rodent poisoning during the time period of this study. The effect of T. fluminensis on the movements of P. t. traversi at Prouse Bush was determined using harmonic radar. There was large variation in the movements and a highly significant difference between individual snails (P<0.01), with some snails regularly moving between areas of T. fluminensis and leaf litter. There was no significant difference in the mean daily displacement of movements by snails in leaf litter and T. fluminensis, but T. fluminensis did appear to affect home range size. Snails always found under T. fluminensis had significantly smaller mean 90% home range estimates (43.91 m2) than snails that were only ever found in leaf litter or those that moved between litter and T. fluminensis (171.35 m2 and 610.14 m2 respectively) (P<0.05). Snails in T. fluminensis had a significantly wider size-frequency distribution than those in leaf litter (P<0.05) and no live snails <35mm were found in leaf litter. There was no significant difference between the size-frequency distributions of empty shells found in both habitats, but their density was significantly greater in leaf litter (P<0.05). Powelliphanta traversi traversi regularly use T. fluminensis as a habitat and any control measures affecting this weed in native bush remnants need to be considered with regard to their possible effects on these snails. The toxicity of a 1.4% Grazon® solution (active ingredient triclopyr) to P. t. traversi was investigated by first using three life history stages of the brown garden snail (Cantareus aspersus Müller). After 149 days, there was significantly greater mean mortality of C. aspersus exposed to a direct spray and to a sprayed environment (82.34% and 78.40% respectively) than in a control treatment (36.95%) (P<0.05). Cantareus aspersus egg mortality (86.00%) was significantly greater than adult and juvenile snail mortality (66.86 and 62.20% respectively) (P<0.05). Five P. t. traversi snails were also exposed to a single environmental spray of a 1.4% Grazon solution but no mortality or detrimental effects were observed after 149 days. A 1.4% Grazon solution does not appear to be toxic to P. t. traversi snails when sprayed on leaf litter where the snails live so Grazon appears to be a suitable herbicide for controlling T. fluminensis in forest remnants containing P. t. traversi.
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    Studies on the ecology of Lymnaea tomentosa Pfeiffer 1855 and L. columella Say 1817 (Mollusca : Gastropoda), intermediate hosts of the common liver fluke Fasciola hepatica Linnaeus 1758 in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University
    (Massey University, 1974) Harris, Robert Eric
    Studies on the ecology of Lymnaea tomentosa and L. columella were carried out to provide a basis for further studies on the ecology of Fasciola hepatica infections in New Zealand. Intermediate hosts of F. hepatica in New Zealand are the native Australasian species L. tomentosa, an introduced American snail L. columella, and the European host L. truncatula. The literature on the systematics, geographical distribution, general biology and ecology of these snails, together with their relationships with F. hepatica, is reviewed. Field observations on snail habitats indicated that L. columella occupied ponds and spring fed marshes which remained wet even in dry seasons. L. tomentosa was found in similar marsh habitats. Stability appeared to be an important quality of the habitats of both species; factors unfavourable to the snails included rapidly flowing water, marked seasonal fluctuations in water level, and shade from tall vegetation. The calcium content of water in habitats ranged from 4 to 70 ppm. Snails were more often found on flocculent than on firm mud and this preference was more marked in the case of L. tomentosa. Snail population dynamics were examined on one pond habitat and three marsh habitats of L. columella and a marsh habitat of L. tomentosa. Age structure and density of populations fluctuated widely between and within each of the five years of the study. These changes were greater in populations of L. columella and not directly related to rainfall or temperature variations. Field and laboratory evidence showed that both species could breed throughout the year even when temperatures were as low as 5°C but populations were much larger in summer and early autumn. Studies of the fecundity of the two snail species and the relationship between temperature and rate of development of eggs showed that whilst L. columella has a higher adult mortality rate it also has a much greater reproductive potential than L. tomentosa. Eggs of L. columella develop over a wider range of temperatures, hatching up to 34.5°C whereas temperatures above 30°C are lethal to eggs of L. tomentosa. Eggs of both species underwent some development at temperatures between 5 and 10°C but the proportion that hatched at low temperatures was very small, particularly in the case of L. tomentosa. Both species showed similar responses to desiccation on filter paper at 16.5°C and 80 to 90% relative humidity; survival time was closely related to shell length, with large snails considerably more resistant than smaller specimens. When snails were subjected to desiccation on mud results were much less predictable and the mean shell length of survivors was often less than that of snails which died. A biometric analysis of shell shape showed distinct differences between large specimens of L. tomentosa and L. columella but shell dimension ratios were not diagnostic in specimens under 4 mm in length. Examination of a pond and a marsh population of L. columella showed no differences in shell shape attributable to habitats. Over a 5 year period there was no detectable relationship between the intensity of F. hepatica transmission from a L. columella habitat and snail population density. The only correlation between F. hepatica transmission and rainfall was a possible inverse relationship between uptake in tracer sheep and December/January (mid-summer) rainfall. There was no evidence of any overwintering infection of F. hepatica infection in L. columella. Uptake of flukes by tracer sheep was almost totally confined to the period between mid-summer and mid-winter. Both experimental and circumstantial evidence indicated that pugging by cattle rendered marsh habitats more suitable for snails, although exclusion of cattle from the experimental area coincided with an increase in the uptake of F. hepatica by tracer sheep.