Ramshorn Snails (Planorbidae, Anisus sp.) and the non-marine molluscs of Skokholm

Island, PembrokeshireReport to Wildlife Trust of South and West Wales / Skokholm Bird

Observatory June 2015

Ben Rowson, Department of Natural SciencesAmgueddfa Cymru – National Museum Wales

(Photo: Anisus sp. in garden pond south of the Wheelhouse, Skokholm, 12 April 2015)

1.0. SummaryThere is taxonomic debate over whether both the White-lipped Ramshorn Anisus leucostoma and the Button Ramshorn Anisus spirorbis occur in Britain, and how they can be distinguished. Museum specimens collected by Charles Oldham on Skokholm in 1925 are unusually like the much rarer A. spirorbis based on shell measurements. A survey of Skokholm’s freshwater bodies in 2015 obtained adult specimens for dissection from two locally-seeded artifical ponds. Analysis suggests these are conspecific with those from 1925, and that Skokholm shells differ in both size and shape from mainland A. leucostoma. However, their anatomy is intermediate between A. spirorbis and A. leucostoma. It is suggested that the Skokholm populations are a dwarf form of A. leucostoma.The other non-marine mollusc fauna of Skokholm was surveyed and compared with records from the Observatory’s library and national networks. At least 7 freshwater and 21 terrestrial mollusc species have been reported, including 3 apparently newly-recorded from the islands in 2015, including the Nautilus Ramshorn Gyraulus crista. Not all freshwater species previously recorded from Skokholm were refound.

2.0. IntroductionThere has long been a debate on the taxonomy of certain medium-sized Ramshorn Snails (Anisus spp., Planorbidae) in Britain and Ireland. Recently it was suggested that both the Button Ramshorn Anisus spirorbis and the White-lipped Ramshorn Anisus leucostoma occur in the UK, and advice was given on separating the two (Anderson & Norris, 2014). This is a challenging taxonomic problem since the two species are very similar in shape and yet seem to differ largely in size, or in anatomical traits that could be expected to be correlated with it. However, a recent redescription and revision found that the number of follicles of the prostate in the genitalia could distinguish the species in Europe (Gloer & Meier-Brook, 2008). The two species could easily have been present in Britain for thousands of years (rather than being recently introduced), thus both collections of shells and live-collected animals need careful examination.Upon examing specimens from across Britain and Ireland in the collections at Amgueddfa Cymru - National Museum Wales (NMW) it was found that the specimens most similar to A. spirorbis were seven shells collected from Skokholm (no more precise locality was given) by Charles Oldham on 7 July 1925 (NMW.1925.472.00005). The name “Planorbis spirorbis”

2

appears on the label, but also does on many other samples collected around this time while the species were still considered distinct. Oldham (1868-1942) was “a leading authority on British malacology and ornithology and a notable field naturalist” (Tomlin et al., 1943). NMW also has specimens of the New Zealand Mudsnail Potamopyrgus antipodarum (as P. jenkinsi; NMW.1925.472.00006) collected from Skokholm on the same day; both were probably personally collected by Oldham as his initials “C.O” appear on the labels. No other molluscs from Skokholm have been traced in the collections (apart from those resulting from the present study).As live-collected adults are needed to examine the genitalia, and adults are commonest in the early part of the year (Anderson & Norris, 2014) a visit to Skokholm was arranged for early 2015. This had to coincide with boat timetables and access to water bodies before the gull breeding season. The opportunity was also taken to survey the non-marine molluscs of Skokholm, in particular for comparison to Skomer which I had previously surveyed for slugs with S. Allen in 2011 (Allen, unpubl.).Skokholm is a plateau of Old Red Sandstone (in contrast to Skomer) supporting a variety of freshwater habitats (Fig. 1). The island freshwater habitats and invertebrates, including Mollusca, of Skokholm were dealt with by Davis (1956). Her information is here supplemented with some recent updates from Thompson (2007) and the current Wardens (pers. comm.).The island’s three large water bodies (North Pond, South Pond, and Winter Pond; Fig. 2) are shallow, seasonal and dry out completely in most years. They are fed wholly by rainwater, salinated by salt spray, and enriched by bird guano and carcasses. The substrate is silty and richly organic. Permanent water occurs at The Well and at Orchid Bog, and at a number of small seepages around the coast of the island. The Well is a permanent spring which Davis noted had a relatively constant temperature around 10oC. Both The Well and Orchid Bog have small, permanent outflowing streams and are relatively more calcareous than the other water bodies. Like the other seepages, these streams have gravelly, sandy or muddy bottoms in places. Near the outflow from the Well are a water-filled pit once used for sheep dipping and a marsh (Heligoland Marsh) into which reeds Phragmites and Yellow Flag Iris pseudacorus were deliberately introduced in past decades. The open water of the East Pond of Davis (1956) has now become smaller and shallower, and the area is known as the East Bog (Fig. 3). Very recently (2011) two artificially-lined ponds were established in the gardens north and south of the Wheelhouse (Fig. 4). The Wardens ensured that all biota introduced into these (e.g. Potamogeton spp.) was from water bodies on Skokholm and not elsewhere. The ponds are fed by rainwater from the gutters of the (whitewashed) roof of the Wheelhouse.

3

Figure 1. Aerial photograph of Skokholm showing water bodies at near their maximum modern extent. Modified from Bing Maps (labels removed).

Figure 2. South Pond, an area of large, very shallow, connected seasonal pools.

4

Figure 3. East Bog, now almost choked but an open pond in the time of Davis (1956).

Figure 4. The garden pond south of the Wheelhouse, established in 2011.

5

3.0. Methods

3.1. FieldworkAll water bodies on Skokholm were investigated between 10-13 April 2015, some repeatedly. The perimeter and exposed beds of all the large water bodies and nearby pools were inspected by two people. The vegetation, sediment and water column were swept with an aquatic net whose contents were inspected under water on a white tray, working from the edge to the centre of the pools. Submerged or partially submerged stones and gravel were inspected. A tea strainer was used to inspect finer gravels, especially in the seepages. In the garden ponds, even small molluscs were obvious to the naked eye.A small number of living specimens of Anisus sp. and other voucher specimens from the survey were collected with permission. These are preserved at NMW in 96% ethanol for future DNA studies under the accession number NMW.Z.2015.014.Terrestrial and amphibious molluscs were searched for by hand, among emergent vegetation, Phragmites litter, the roots of plants, by turning debris, and by torchlight after dark. During the fieldwork period, the weather was fine and dry with variable winds and light overnight rain.Earlier records from the islands were extracted from some literature on Skokholm and Skomer available in the Observatory’s library (Davis, 1956; Williams & Williams, 1978; Poole, 1994; Thomson, 1997). A list of all non-marine mollusc species recorded from the 10 km OS grid square SM70, which includes Skokholm, Skomer, and the headland including Marloes, was downloaded from the National Biodiversity Network (NBN) Gateway on 10 April 2015. This dataset includes that of the national mollusc recording scheme run by the Conchological Society of Great Britain and Ireland. Note: neither dataset nor the list here (Appendix 1) is expected to be comprehensive.

3.2. Analysis of Anisus specimensAll Anisus specimens collected were photographed while intact and digitially measured using the method of Anderson & Norris (2014). These authors found that the ratio between the width of the last whorl and the width of the penultimate whorl (a measure of shape rather than size) was a useful discriminator between A. spirorbis and A. leucostoma.For comparison, the same measurements were also taken on selected museum specimens from NMW from lowland localities across South Wales.

6

A deliberate effort was made to choose specimens closest in form to A. spirorbis and very large or otherwise aberrant (e.g. heterostroph) individuals were excluded. Two postglacial subfossil shells from Monmouthshire, again resembling A. spirorbis, were included in the mainland series.Three of the live-collected Anisus specimens were dissected to investigate the key character of the number of diverticles of the prostate gland. This is considered the only reliable anatomical indicator of species identity, regardless of the stage of maturity, since the range for each species does not overlap (Glöer & Meier-Brook, 2008).

4.0. Results

4.1. Non-marine mollusc fauna of SkokholmIn total, 28 non-marine mollusc species (7 freshwater and 21 terrestrial) were found on Skokholm during the survey or have already been recorded (Appendix 1). At least three of those found seem to be new to the 10 km square SM70 or to the islands.Notably, freshwater molluscs were found only in the permanent water bodies on the island, including the seepages and garden ponds. No trace of molluscs or their shells were found in or around the three large, temporary water bodies (North Pond, South Pond, and Winter Pond). Few species were found at East Bog (Fig. 3).Anisus sp. was found only in the two garden ponds. At least 17 individuals were seen in the pond on the south side of the Wheelhouse but only one in the pond on the north. A total of 9 individuals were collected, 8 of them from the south garden pond. In the pond on the north side, a second planorbid species was found living alongside Anisus sp. The Nautilus Ramshorn Gyraulus crista is a new species record for the islands and for 10 km square SM70.

4.2. Analysis of Anisus specimensAs observed by Gloer & Meier-Brook (2008) the logarithm of the maximum shell width (radius) was similarly related to the number of whorls in all three series and did not reveal differences in shape, only in size (Fig. 5). The mean size of both series of Skokholm shells is obviously less than that of mainland shells.To examine differences in shape, not in size, one-way analyses of covariance (ANCOVAs) were used to see whether significant differences in

7

shape (whorl width ratio) were found using each of these size measures (maximum shell width, and number of whorls) as covariates. These were implemented in PAST v3.01 (http://folk.uio.no/ohammer/past/).The mean whorl ratio of the two Skokholm series was higher than that of mainland populations (Figs 6-7), denoting a subtle difference in shape as well as size. This difference was statistically significant using either size measure as a covariate (number of whorls, F = 17.730, p <0.001; maximum shell width, F = 8.822, p = 0.001). The slopes for the three series in relation to either covariate were not significantly different (number of whorls, F = 0.246, p = 0.784; maximum shell width, F = 0.640, p = 0.548).

3.00 3.50 4.00 4.50 5.00 5.50 6.000.30

0.40

0.50

0.60

0.70

0.80

0.90

MainlandOldham2015

Number of whorls

ln m

axim

um sh

ell w

idth

Figure 5. Shell size of Anisus sp. series from Skokholm in 1925 (Oldham), in 2015, and the mainland.

8

Figure 6. Shell shape of Anisus sp. series from Skokholm in 1925 (red), in 2015 (blue), and the mainland (black). Slopes calculated by ANCOVA with number of whorls as a covariate.

Figure 7. Shell shape of Anisus sp. series from Skokholm in 1925 (red), in 2015 (blue), and the mainland (black). Slopes calculated by ANCOVA with maximum shell width as a covariate.

9

Three Anisus specimens were dissected and found to be sexually mature or nearly so. They had 17-20 diverticles of the prostate gland, putting them outside the range (10-15 diverticles) given for A. spirorbis and towards or into that for A. leucostoma (20-25 diverticles) (Glöer & Meier-Brook, 2008).

5.0 Discussion

5.1. Non-marine mollusc fauna of SkokholmThe non-marine molluscan fauna of Skokholm (at least 28 species) is similar to that of Skomer (also at least 27 species), each island hosting half the total species recorded for 10km square SM70 (55). There are only minor differences between the fauna of the two islands, perhaps surprisingly given the geological contrast.Though more must remain to be found, this generally low diversity probably reflects the limited range of habitats or exposure, but also the absence of widespread species that have never established. Two freshwater species present on Skokholm in the past (Davis, 1956), that are often abundant where they occur, were not found in the present survey even as empty shells. These are the Dwarf Pond Snail Galba truncatula and River Limpet Ancylus fluviatilis. Davis (1956) and Thompson (2007) suggested that a marsh or Amber Snail (probably Oxyloma elegans based on the shells collected) might have been introduced in the early 1950s and might spread on the island. However, it was only found at one site (The Well) near where it was first recorded.The absence of many non-native molluscs is welcome, but likely to change, as illustrated by three species newly recorded from SM70. Among freshwater molluscs the distinctive Gyraulus crista may be a recent arrival, since past authors (e.g. Poole, 1994; Davis, 1956) seem unlikely to have overlooked it despite its small size. Birds are plausible vectors for the adults or eggs of these smaller mollusc species.The Yellow Cellar Slug Limacus flavus is not considered native, but is uncommon and apparently declining. It was found inside one of the wooden hides where it may survive on mildew and food scraps. The Tramp Slug Deroceras invadens, a species that has spread to remote islands worldwide has evidently also reached Skokholm where it was found at The Well. The record of the Smooth Jet Slug Milax gagates, an uncommon slug, from grassland by the sea from Thompson (2007) could not be verified although the species was searched for.

10

5.2. Analysis of Anisus specimensThe new Skokholm shells and those collected by Oldham in 1925 appear to be conspecific. Both differ in both size and shape from mainland shells, even though an attempt was made to choose mainland shells to which they were most similar. Mainland shells also reach a larger maximum size and greater maximum number of whorls, which were considered important differences between A. spirorbis and A. leucostoma (Gloer & Meier-Brook, 2008; Anderson & Norris, 2014). However, despite this the analysis remains vulnerable to the lack of overlap in size between Skokholm and mainland series. A much larger mainland sample size could address this problem. Anatomically, however, the Skokholm population is intermediate between A. spirorbis and A. leucostoma by the criteria of Glöer & Meier-Brook (2008). Anatomical features are very often given precedence over shell morphology in molluscan systematics, so it is difficult to support the attribution of the Skokholm snails to the rarer A. spirorbis.These results are interesting in terms of a population from Co. Wicklow, Ireland discussed by Anderson & Norris (2014) which appears intermediate in shell form between A. spirorbis and A. leucostoma. DNA analysis could prove crucial in resolving the status of such forms. Until then, it is suggested that the Skokholm population is considered a small or dwarf form of A. leucostoma, the common species on the mainland.Dwarfism is common on islands for both genetic and environmental reasons, in the case of this species on Skokholm, possibly exposure or water chemistry. Shells of the Common Chrysalis Snail Lauria cylindracea found on walls on the island were also unusually short and broad. Interestingly, Anisus was found living only in artificial ponds but is thought to have been introduced from a natural water body elsewhere on the island. The population may have inherited its form from its ‘wild’ ancestors, or the conditions limiting size may apply equally to the artificial ponds. Regrettably it is not known exactly where Oldham found his specimens.

6.0. AcknowledgementsI am grateful to the Wardens, Richard Brown & Giselle Eagle, and the Islands Conservation Advisory Committee (including the Wildlife Trust of South & West Wales and Skokholm Bird Observatory) for advice, permits and discounted accommodation. I thank Mal Rowson for assistance in the field and other workers at the Observatory for their company. Simon Allen

11

kindly allowed the mollusc records from his study to be included. I also wish to thank the boat crews who make access to the islands possible.

12

7.0. ReferencesAllen, S. (unpublished; data on terrestrial slugs and snails of Skomer collected in 2011 as part of a parasitology study).

Anderson, R. & Norris, A. 2014. Separation of Anisus spirorbis from Anisus leucostoma in Britain and Ireland. Mollusc World 35: 6-8.

Davis, A. 1956. The freshwater fauna of Skokholm. Skokholm Bird Observatory Report (1956): 25-33. Glöer, P. & Meier-Brook, C. 2008. Redescription of Anisus semptemgyratus (Rossmässler, 1835) and Anisus leucostoma (Millet, 1813) (Gastropoda: Planorbidae). Mollusca 26 (1): 89-94.

Poole, A. 1994 (unpublished). Skomer Invertebrate Records. “Compiled using “Biorecs” […] from the Inverbrate card file, and the butterfly transect form”.

Thompson, G. V. F. 2007. The Natural History of Skokholm Island. Trafford Publishing, Victoria, Canada. 381 pp.

Tomlin, J. R. le B. [as Editor], Salisbury, A. E., & Adams, L. E. 1943. Charles Oldham (1868-1942). Journal of Conchology 22 (1): 1-2, pl. 1.

Williams, D. D. & Williams, N. E. 1978. Some stream invertebrates from Skomer Island, Wales. Nature in Wales 16 (2): 132-133.

13