INTRODUCTION

Freshwater snails are continually being translocated around the world via the aquarium industry, particularly the trade in aquarium plants (Walker 1978; Madsen & Frandsen 1989; Cowie 1998; Letelier et al. 2007), and a proportion of the species introduced to any country is likely to become invasive. South Africa is no exception to this and 40% of the introduced species listed by Appleton (2003) have become invasive. Invasive snails may have other important consequences such as direct environmental and economic effects, and indirect effects that impact human and animal health. Recent research has shown how one such species, Tarebia granifera (Lamarck, 1822), can become extremely abundant, dominate benthic communities (Miranda et al.. 2011b ; Miranda et al. 2014a , b ) and alter the functioning of environments it has invaded (Miranda et al. 2011a ; Moslemi et al. 2012). Indigenous snail species in particular are being affected and displaced by T. granifera in South Africa (Miranda & Perissinotto 2012; Raw et al. 2013). There is evidence that another invasive, Physa acuta Draparnaud, 1805, can displace the snail host of urogenital schistosomiasis and interrupt transmission of the disease (Dobson 2004).

With the exception of an Australian and an East African species that have not survived, all freshwater snail species introduced to South Africa before the 1990s (Appleton 2003) originated in the Americas. Then in 1999 the southeast-Asian caenogastropod Tarebia granifera was discovered in a reservoir in northern KwaZulu-Natal province (Appleton & Nadasan 2002). This contribution records the introduction of two additional Asian species, Radix rubiginosa (Michelin, 1831) and Gyraulus chinensis (Dunker, 1848). We also analysed data on all alien freshwater gastropod species reported from South Africa in terms of their dates of first collection and regions of origin. No freshwater bivalves have been introduced into South Africa.

TABLE 1

Selected chemical and physical characteristics of the tank and plant-tray water in which R. rubiginosa and G. chinensis respectively were collected.

MATERIAL AND METHODS

Molecular analyses

Lymnaeid snails were collected from Amatikulu (n = 12) and from Durban (n = 13) (29°52′S 30°59′E) in February 2007. Specimens were preserved in ethanol prior to analyses. DNA was extracted and isolated from foot tissue with the DNeasy® extraction kit using the solid tissue protocol. Fragments of the cytochrome c oxidase subunit I (COI) mitochondrial DNA and the 16S mitochondrial ribosomal DNA were amplified by polymerase chain reaction (PCR) and sequenced using primers and protocols outlined by Folmer et al. (1994) and Remigio and Blair (1997) respectively. PCR conditions were: 1 min at 95°C. then 34 cycles at 95°C for 30 s, 43°C for 30 s and 72°C for 1 min, followed by 72°C for 5 min for COI; and 2 min at 95°C; then 35 cycles of 95°C for 30 s, 54°C for 30 s, and 72°C for 30 s, followed by 72°C for 10 min for 16S. Sequencing was done at Inqaba Biotechnical Industries (Pretoria, South Africa) with an ABI 3730 Capillary Sequencer using Big Dye technology. Sequences were edited and aligned using the programs BioEdit Sequence Alignment Editor (Version 7.0.5.3; Hall 1999) and Clustal X (version 1.81; Thompson et al. 1997). Bayesian phylogenetic analyses were performed using MrBayes (version 3.1.1; Ronquist & Huelsenbeck 2003). Neighbour-joining (NJ) and maximum parsimony (MP) phylogenetic analyses were performed using MEGA (version 2.1; Kumar et al. 1993). The program MrModelTest (version 1.0; Nylander 2004) was used to select the most appropriate nucleotide substitution model for use in NJ and Bayesian likelihood analyses. Complete homologous sequences for the COI and 16S genes were obtained from Genbank. Planorbis corneus was used to root the COI tree and Radix ovata, Lymnaea aulacospira and Lymnaea stagnalis were used as sister groups to the groups of interest. The 16S tree was rooted with Stagnicolaelodes, and Radix rubiginosa, Radix quadrasi, Radix sp. EEAR-Canada-2002, Radix sp. EEAR-Philippines-2002, Lynmaea sp. EEAR-Hawaii-2002, Lymnaea sp. EEAR-China-2002 and Austropeplea viridis were used as sister groups to the groups of interest.

Fig. 1.

Radix rubiginosa from Amatikulu (photos: H. Madsen).

RESULTS

Table 1 shows that the water in both tanks and trays at Amatikulu was well oxygenated and slightly alkaline. The water in the trays had a much higher conductivity than the tank water, probably due to fertiliser used on the plants. The pH was higher in the tank water.

Fig. 2.

Shells of Radix rubiginosa from Amatikulu (photos: N. Miranda). Scale bars = 5 mm.

Fig 3.

(A) Tree based on 533 nucleotides of the cytochrome c oxidase subunit I mtDNA of lymnaeid samples, with the outgroup Planorbis comeus. (B) Tree based on 360 nucleotides of the mt 16S rDNA of lymnaeid samples, with the outgroup Stagnicola elodes. Numbers on branches represent Bayesian posterior probabilities percentages, neighbour-joining and maximum parsimony bootstrap values, respectively. Only values greater than 50% are shown; ^* indicates support of < 50% for distance analysis. Figures adapted from Nadasan (2011). RRA — lymnaeid samples from Amatikulu, South Africa; LND — lymnaeid samples from Durban, South Africa.

Fig. 4.

Gyraulus chinensis from Amatikulu. Live specimens (4A–B, photos: H. Madsen) and specimen preserved in 70% ethanol (4C, photo: N. Miranda). The shell of the specimen in 4B shows patches of erosion. Note the pigmented mantle in both live and preserved specimens.

Fig. 5.

Shell of G. chinensis from Amatikulu (photo: N. Miranda). Scale bar = 1 mm.

DISCUSSION

Radix rubiginosa and Gyraulus chinensis are ‘hothouse’ or ‘greenhouse’ species, i.e. they originate in sub-tropical regions but have been introduced to and thrive in artificially heated habitats in cooler climates such as in Europe. In some situations they may spread from these artificial habitats.

Molecular analyses confirm the presence of Radix rubiginosa in Amatikulu (Fig. 3) and results are also in agreement with recent work by Correa et al. (2010). Radix rubiginosa was referred to as Radix auricularia rubiginosa by Brandt (1974) and was confirmed as a member of the R. auricularia (Linnaeus, 1758) complex by Remigio and Blair (1997). Correa et al. (2010) found that R. rubiginosa was most closely related to R. quadrasi (Möllendorff, 1898) from the Philippines. It is endemic to southeast Asia, viz. Malaysia, Thailand, Vietnam, Laos, Cambodia, Burma and Indonesia (Brandt 1974; Burch & Lohachit 1983).

Figs 6, 7.

(6) Regression of shell flatness index (a/b) on (a) in mm for G. chinensis from Amatikulu; (7) Regression of number of whorls over shell diameter for G. chinensis (n=53) from Amatikulu. (n=51).

Radix rubiginosa is tolerant of a wide range of temperature, conductivity and pH, which may explain its occurrence in a variety of habitats such as rivers, lakes, ponds, rice fields and irrigation canals in its native region (Chitramvong el al. 1981). The only reported introductions of R. rubiginosa are isolated occurrences in aquaria in Israel (Mienis 1986), Britain and Ireland (Anderson, 2005) and in Australia where it was intercepted at Sydney airport in imported aquarium plants from Hong Kong (Walker 1978). In the same year Boray (1978) commented that R. rubiginosa could spread in Australia and ‘have a wide distribution’. Dondero and Lim (1976), Mienis (2004) and Nadasan (2011) have all commented on how easy R. rubiginosa is to culture in aquaria and how quickly it becomes a pest. In such situations snails are likely to be discarded into nearby freshwater habitats. Since its discovery at Amatikulu in 2004, R. rubiginosa has been found in garden fish ponds in Durban 110 km to the south (C.C. Appleton, unpublished data). If R. rubiginosa spreads in KwaZulu-Natal, it could compete with the indigenous Lymnaea natalensis. Indeed, recent experimental work by Nadasan (2011) on the bionomics of R. rubiginosa suggests that its invasive potential is similar to that of Physa acuta, the most widely distributed invasive freshwater snail in the world.

TABLE 2

Snail species collected at aquarium plant and fish farm at Amatikulu.

Gyraulus chinensis is endemic over a wide area of southern Asia from New Guinea, Indonesia and China in the east to Sri Lanka, India and Iran in the west (Meier-Brook 1983, 1984). Over this vast area it occurs as what Meier-Brook (1983) calls a ‘Rassenkreis’ which he defines as ‘a polytypic species with a number of identifiable races that are not reproductively isolated’.

Evidence for the invasive potential of G. chinensis is provided by the distribution records given by Brown et al. (1998). These show that it was introduced to and has become invasive in Western Europe. Although it was found on many occasions in aquaria and greenhouses, it has, more importantly, been found since 1973 in rice fields and natural waterbodies in five European countries. Brown et al. (1998) also recorded its presence in rice fields in Guinea-Bissau, West Africa, in 1989, and its interception in South Africa in January 1995 by staff of the National Department of Agriculture's Directorate of Plant and Quality Control in a consignment of aquarium plants from Singapore. These latter snails were identified by Dr C. Meier-Brook (Institut für Tropenmedizin der Universität Tübingen, Germany) ‘as a Rassenkreis of Gyraulus chinensis’ (K.N. de Kock pers. comm. 2005). Recently the species has been collected in different types of habitat in different hemispheres: a pond in South Australia (Brown 2001), greenhouses in the Czech Republic (Beran & Glöer 2006) and a swamp in Martinique, West Indies (Pointier 2008). The present report is, however, the first of G. chinensis from inside South Africa and suggests that it may have become established. In 2004 G. chinensis was found in an aquarium in the foyer of a building on Howard College Campus, University of KwaZulu-Natal, Durban, South Africa, and traced to a collection of aquatic plants provided by the same commercial aquarium fish and plant breeder referred to under R. rubiginosa, i.e. at Amatikulu 110 km north of Durban, KwaZulu-Natal. Additional specimens were collected on visits to this facility on 17 August and 5 October 2005, 9 July 2008 and 18 January 2013. They were plentiful in shallow trays containing clear water ±30 mm deep in which pots of aquatic plants are stored for sale, and on the submerged parts of the plants themselves. These plants are sold (airfreighted) to other provinces of South Africa and also to Europe and elsewhere. It should therefore be expected that G. chinensis and/or its eggs laid on leaves will be (or have been) translocated to other areas as well, as they were to the aquarium at the University of KwaZulu-Natal.

TABLE 3

Updated list on introduced freshwater snail species in South Africa arranged according to their date of first collection. The earliest published source for each species in South Africa is also given.

It is not known how or precisely when R. rubiginosa and G. chinensis were introduced but the fact that plants from the same source as that examined here, and on which G. chinensis was found, are airfreighted around the country and as far afield as Europe raises the possibility that the snails have been introduced to aquaria and ornamental ponds over a wide area. It should therefore be expected to spread to natural systems in South Africa as well. The discoveries of R. rubiginosa and G. chinensis focus attention on the aquarium trade, whose imports into South Africa are poorly regulated. Seven snail species from five families have been found in the various habitats at the fish farm sampled here (Table 2). All but one of these are alien to South Africa; two of them have become invasive and at least one is potentially invasive. The single non-alien is Melanoides tuberculata (O. F. Müller, 1774) (see below).

The three non-Asian species in Table 2, Pomacea diffusa Blume, 1957 (recorded as P. bridgesi by Appleton 1996, 2003), Helisoma duryi (Wetherby, 1879) and Physa acuta, commonly occur in aquaria worldwide and are to be expected wherever the aquarium trade is practised. Hayes et al. (2008) concluded that the species of Pomacea commonly distributed via the aquarium trade is P. diffusa whereas the notoriously invasive P. canaliculata (Lamarck, 1819) has been distributed for food, largely in Asia. Ampullaria lineata, recorded by Appleton (1996), is a misidentification of the wild-type of P. diffusa. It is, however, noteworthy that the three most recent additions to the South African list of introduced freshwater snails, Tarebia granifera (1999), G. chinensis (2004) and R. rubiginosa (2006), all originated in southeast Asia. It is significant that while seven species occur at the fish farm, Pseudosuccinea columella, the most widely distributed invasive snail in South Africa (De Kock et al. 1989) does not. The fact that P. columella does not occur in southeast Asia either (Brandt 1974; Burch & Lohachit 1983) while the others do, supports an Asian origin via the aquarium trade for the species listed in Table 2.

Table 3 presents an updated list of 13 species known to have been introduced to South Africa with estimated dates of first collection and regions of origin. The approximate rate of introduction over the 72 years since the earliest reported introduction in 1942 is one new species every 5½ years—similar to the rate estimated for introduced terrestrial gastropods by Herbert (2010) though over a much shorter time period. We estimate that 76.9% (10/13) were introduced via the aquarium or ornamental plant trade. Of the remaining three (Biomphalaria angulosa Mandahl-Barth, 1957, B. glabrata (Say, 1818) and Marisa cornuarietis (Linnaeus, 1758)), two, B. glabrata and M. cornuarietis, were introduced for schistosomiasis research purposes, but no explanation can be offered for the entry of B. angulosa into South Africa. Biomphalaria glabrata (Fig. 8) was in fact introduced twice, first to a man-made canal in central Durban, probably during the 1960s, but the population had died out by the mid-1980s (J. Agnew pers. comm. July 1983; Bruton & Merron 1985; Ashton et al. 1986). Its shells measured up to 26.5 × 8.3 mm (n=16) with 6 to 7 whorls. It is thus much larger than any African Biomphalaria species including B. angulosa (Brown 1994) and also larger than most Neotropical species except B. glabrata and B. tenagophila (Orbigny, 1835), both of which commonly reach 30 mm and are hosts for Schistosoma mansoni Sambon, 1907 in Brazil (Malek 1985; Anon. 2008). Joubert et al. (1986) reported it as Biomphalaria cf. glabrata and it proved susceptible to an Egyptian strain of S. mansoni (K.N. de Kock pers. comm. 2013). After examining photographs of the Durban shells, Dr J-P. Pointier (France) (in litt., 17.07.2014) and Dra S. Thiengo (Brazil) (in litt., 31.07.2014) supported Joubert's 1986 provisional identification with near certainty. Unfortunately no soft parts are available so this identification cannot be confirmed.

Biomphalaria glabrata was reintroduced, but to laboratories for schistosomiasis research purposes in 1982, the same year that Pflüger warned of the dangers of doing so (Pflüger 1982). This species is an efficient host of S. mansoni in Brazil and M. cornuarietis, introduced in 1986, is a herbivore which destroys the egg masses of other snails, including species of Biomphalaria, as it feeds. It was thus tested as a competitor/ predator of the local schistosomiasis host snails. There is no evidence that either B. glabrata or M. cornuarietis have escaped or occur in natural systems in South Africa today.

Biomphalaria angulosa, an east African species that is susceptible to Schistosoma mansoni, was identified from ‘near’ Durban and in Oaklands, Johannesburg, by Mandahl-Barth (1957, 1958). Van Eeden et al. (1965) and Van Eeden (1966a , b ) noted that the Johannesburg population existed for two years but had died out by 1957 as had the ‘near’ Durban population. Brown (1994) considered both reports to be doubtful and that the ‘near’ Durban record may have been a case of incorrect locality data.

Fig. 8.

Shell of Biomphalaria glabrata collected in Durban in the 1980s (photos: C. Appleton).

Two species not included in Table 3 but which may have been introduced are Melanoides tuberculata (Table 2) and Galba truncatula. Both require molecular analysis to determine whether or not they were introduced. Although M. tuberculata is indigenous to South Africa (Connolly 1939; Brown 1994), the characteristic tuberculate morph illustrated by Van Eeden (1960), Oberholzer and Van Eeden (1967) and Appleton (1996) is rare in the country today. The morph currently common in Durban was illustrated by Pointier et al. (2003) and resembles the Asian genotype/morph shown by Genner et al. (2004) to have invaded Lake Malawi. This is the subject of ongoing investigation.

Van Eeden (1958, 1966a ) proposed that G. truncatula had been introduced to South Africa from Europe though he (Van Eeden el al. 1965) drew attention to Connolly's 1939 records of fossil and sub-fossil shells which suggest that the species previously had a wider distribution across the sub-continent. Recently, however, Correa et al. (2010) presented molecular evidence that G. truncatula originated in North America but has spread widely in recent times. This needs to be confirmed but it is possible that European colonists introduced G. truncatula to South Africa just as they did to the altiplano of Bolivia (Bargues et al. 1997).

Allowing for lag periods, the dates of first collection and regions of origin of the 14 species listed in Table 3 enable the delineation of three phases in introduction: species first collected between the 1940s and mid-1960s, originating mostly in North America, some perhaps coming via Europe; those first collected between the late 1960s and 1980s in South America; and those first collected more recently, between the 1990s and the present, in Asia. The arrival of T. granifera around 1996 appears to have heralded the Asian phase in the introduction of freshwater snails, including R. rubiginosa and G. chinensis, into South Africa.

General trends in South African trade data indicate that imports from Asia have essentially doubled during the period from 1998 (15% of total imports by value) to 2011 (29% of total imports by value), according to South African Revenue Service data on main trade partners for that period. South African records of newly introduced alien invasive gastropods from Asia coincide with this time period. As import frequency increases so does the possibility of alien species introduction. Better control is essential to prevent repeated and further introductions into South Africa (see Robinson 1999).

The spread of freshwater snails has frequently been attributed to the aquarium trade and aquarium plants in particular (Pointier 1999; Strayer 2010). Gyraulus chinensis was probably introduced to South Africa as a passenger on prohibited aquarium plants such as Myriophyllum spicatum L. coming from Malaysia and Indonesia, i.e. following the major South African aquarium plant import trade line. It is difficult to track alien species introductions into South Africa via pet stores, aquarists and the internet-mediated trade; however, Martin and Coetzee (2011) report that a variety of invasive and/or prohibited plants are traded within South Africa and that there is a general lack of knowledge about identification and regulation of alien species. As alien species are continuously moved and traded in the country, recurring local introductions (propagule pressure) will increase and contribute to their establishment and spread. This surely accounts for the appearance of Aplexa marmorata (Guilding, 1828) in the southwestern Cape, more than 1200 km from KwaZulu-Natal and Mpumalanga where it has thus far been confined (Mlambo et aí. 2011). A census of snails in aquarium supply facilities across the country is needed.

The findings of this report draw attention to the ongoing accidental importation of alien aquatic invertebrates into South Africa via the poorly regulated aquarium trade. There is a clear need to improve the regulation of this trade and to educate customs officers and auditors on the ‘best practice’ recommendations for controlling such introductions as outlined by Appleton and Bosman (2000). These include an illustrated database of potentially invasive species and increased public awareness of the harmful effects of invasive organisms in invaded ecosystems (see also Robinson 1999).

The National Lists of Invasive Fresh-water Invertebrates published in the South African Government Gazette of 12 February 2014 are related to the National Environmental Management Biodiversity Act (NEMBA) (Act No. 10 of 2004). List 9 (National List of Invasive Fresh-water Invertebrate Species, p. 45) lists three snail species, Aplexa marmorata, Pseudosuccinea columella and Tarebia granifera, all of which are discussed in the present report. Others listed as invasive in Table 3 should be included. List 10 (prohibited fresh-water invertebrates, p. 72) lists four bivalve species that would undoubtedly present threats should they be introduced. These are Corbicula fluminea (O.F.Müller, 1774), Dreissena polymorpha (Pallas, 1771), Dreissena rostri formis bugensis (Andrusov, 1897) and Potamocorbula amurensis (Schrenck, 1861).