Biodiversity census of Lake St Lucia, iSimangaliso Wetland Park (South Africa): Gastropod molluscs

Abstract The recent dry phase experienced by the St Lucia estuarine system has led to unprecedented desiccation and hypersaline conditions through most of its surface area. This has changed only recently, at the end of 2011, with the onset of a new wet phase that has already caused a major shift to oligo- and mesohaline conditions. The estuary mouth, however, remains closed to the ocean, making the weak connection recently established between the St Lucia and the Mfolozi estuaries the only conveyance for marine recruitment. As a result, only 10 indigenous and two alien aquatic gastropod species are currently found living in the St Lucia estuarine lake. This is out of a total of 37 species recorded within the system since the earliest survey undertaken in 1924, half of which have not been reported in the literature before. The tick shell, Nassarius kraussianus, which was consistently found in large abundance prior to the recent dry phase, appears to have temporarily disappeared from the system, probably as a result of the extinction of Zostera marine grasses inside the lake. Population explosions of the bubble shell Haminoea natalensis, with its distinct egg masses, were recorded seasonally until 2009, but the species has subsequently not been observed again. A molecular DNA analysis of the various populations previously reported as belonging to the same assimineid species, variably referred to as Assiminea capensis, A. ovata, or A. bifasciata, has revealed that the St Lucia assemblage actually comprises two very distinct taxa, A. cf. capensis and a species provisionally referred to here as “A.” aff. capensis or simply Assimineidae sp. In the mangroves, the climbing whelk Cerithidea decollata is still found in numbers, while ellobiids such as Cassidula labrella, Melampus semiaratus and M. parvulus are present in low abundances and all previously recorded littorinids have disappeared. A number of alien freshwater species have colonized areas of the system that have remained under low salinity. These include the invasive thiarid Tarebia granifera, which can be found in concentrations exceeding 5000 ind.m-2, the lymnaeid Pseudosuccinea columella and the physid Aplexa marmorata.


Introduction
Lake St Lucia is a large, complex estuarine lake situated on the South African east coast. It has been extensively investigated since the late 1940s, as it is the largest such system in Africa, the oldest protected estuary in the world and a Ramsar Wetland of International Importance since 1986 (Porter 2013). It currently forms a crucial part of the iSimangaliso Wetland Park, which is South Africa's first UNESCO World Heritage Site. During the past century, the system has undergone a number of changes related to anthropogenic activities, which superimposed on an already complex and variable climate have escalated the magnitude of its regular shifts from wet to dry states ). Typically the system experiences sub-decadal alternations of droughts and anomalous wet conditions, at times resulting in severe floods. Recently, the system has undergone a major shift, from a prolonged dry phase during 2002-2011 to flood conditions during 2012-2013, which has resulted in the current predominance of oligohaline conditions through most of its basins (Raw et al. 2013;Peer et al. 2014).
The rich biodiversity of the St Lucia estuarine lake is one of the main drivers of its special conservation status. Species are the building-blocks of any ecosystem, yet in the St Lucia case there are many misidentifications and several groups of invertebrates remain poorly investigated or completely ignored. A few detailed taxonomic studies of selected invertebrate groups have already been undertaken, starting from 2010, using a combination of traditional morphological analyses and molecular DNA barcoding. These have consistently revealed the occurrence of species that were either previously confused with others or completely unknown to science (e.g. Daly et al. 2012;Gómez et al. 2012;Carrasco and Perissinotto 2012;Todaro et al. 2011Todaro et al. , 2013. A systematic approach has been implemented recently, aimed at producing a modern "Biodiversity Census" for the estuarine lake. The main objective of the initial phase of the census is to add accurate checklists of as many aquatic invertebrate groups as possible to those already existing for the vertebrates and the macrophytes. As the diversity census of bivalve molluscs and true crabs have already been completed and published (Nel et al. 2012;Peer et al. 2014), this new contribution focused on the gastropod molluscs is regarded as the third of the series planned within the census.
The class Gastropoda is the most diverse among the molluscs and includes about 55000 extant aquatic species globally (Brusca and Brusca 2003). Gastropods play an integral part in the functioning of aquatic ecosystems (Carlén and Ólafsson 2002). In estuarine ecosystems such as St Lucia, gastropods are important components of food webs and energy pathways. They are mostly detritivores or herbivores, feeding on a large variety of decomposing organic materials or on benthic or epiphytic microalgae, protozoans, bacteria and fungi. There are also a few predatory or scavenger species, especially among the muricids, nassariids and naticids. Gastropods also play a key role in nutrient dynamics in mangrove ecosystems, where they graze on encrusting fauna and microalgae, thus cleaning the pneumatophores (Ghasemi et al. 2011). In turn, both snails and limpets are consumed by a variety of predators, including birds (Whitfield and Cyrus 1978;Hockey et al. 2005;Turpie et al. 2013), fish (Blaber and Blaber 1980;Whitfield 1998;Dyer et al. 2013), crabs (Sousa 1993;Schaefer and Zimmer 2013), leeches, larvae of marsh flies and aquatic beetles (Appleton 1996), as well as anemones (Daly et al. 2012). Some freshwater species living on the fringes of Lake St Lucia, or even entering the estuarine system in times of freshwater dominance, may be of veterinarian/medical importance as they act as vectors of parasites responsible for water-borne diseases, such as bilharzia, fascioliasis and paramphistomiasis (Appleton 1996). In estuaries, the diversity of gastropods is determined to a large extent by changes in physico-chemical conditions and the availability of detritus and microalgae as their primary food sources (Carlén and Ólafsson 2002).
The purpose of this study is thus to provide a comprehensive review of the diversity of gastropod molluscs in the St Lucia estuarine lake. This includes identifying species that are currently present in the system and comparing them with what was collected in past surveys. Changes in diversity over time are related to shifts in environmental and climatic conditions that have occurred during the past century. The compilation of an annotated and illustrated checklist of all gastropod species recorded so far within the system is designed to aid managers, researchers and visitors in the iSimangaliso Wetland Park with the identification of these important molluscs.

Methods
The St Lucia estuarine lake is located on the north coast of KwaZulu-Natal, between 27°52' to 28°24'S and 32°21' to 32°34'E. The system has a surface area of approximately 350 km 2 (Taylor et al. 2006), with a perimeter of approximately 347 km at low water and depth of 0.9 m (Cyrus et al. 2011). It is subdivided into three lake basins, viz. False Bay, North and South lakes, which communicate with the mouth via a narrow channel known as "The Narrows" (Figure 1).
The first gastropod records from the St Lucia system date back to 1924, with specimens reposited in the KwaZulu-Natal Museum (NMSA), in Pietermaritzburg, ever since. Further collections were later undertaken during the two surveys of the University of Cape Town, in 1948Town, in -1949Town, in and 1964Town, in -1965. Specimens collected during these surveys are currently reposited at the Iziko South African Museum (ISAM), in Cape Town. In 1982-1983, a dedicated collecting survey was undertaken throughout the lake system by the provincial conservation authority, the Natal Parks Board (NPB). The same authority, renamed Ezemvelo KwaZulu-Natal Wildlife (EKZNW), completed another similar survey during 2005, at the peak of the most recent drought. A number of publications containing gastropod records have also been published since 1954, mainly by researchers operating at the universities of KwaZulu-Natal, Zululand, Rhodes and Cape Town (e.g. Day et al. 1954;Millard and Broekhuysen 1970;Boltt 1975;Perissinotto 2008, 2013;MacKay et al. 2010). Finally, a dedicated survey during 2012 and 2013 was conducted as part of this study, in order to establish the current status of gastropod diversity within the system.

Museum and literature data
Specimens and data of gastropods collected at St Lucia in past surveys were obtained from the KwaZulu-Natal Museum in Pietermaritzburg and the Iziko South African Museum in Cape Town. Particularly rich collections from St Lucia were undertaken by the NMSA in 1987. Reference to specimens from either museum are here complemented with their accession numbers. Literature involving past macrobenthic surveys undertaken in the St Lucia Estuary (e.g. Day et al. 1954;Millard and Broekhuysen 1970;Boltt 1975;Weerts 1993;Pillay and Perissinotto 2008;MacKay et al. 2010) were reviewed to obtain information about gastropod diversity and the environmental conditions in which species were collected. Information such as synonyms, common name, size distribution and other records about the various species were used to annotate the illustrated checklist.

Historical surveys
Both NPB 1982-1983and EKZNW 2005 collecting surveys were conducted at the onset of extreme drought conditions, when organisms within the estuarine lake were experiencing mass kills in response to hypersaline conditions and lake fragmentation/ desiccation. Dead gastropods were mainly found washed up along the shorelines of St Lucia. The Natal Parks Board surveyed the banks of the whole lake from December 1982 to April 1983, collecting freshly dead specimens. These were later identified to species level by the late R.N. Kilburn. Ezemvelo KZN Wildlife surveyed the St Lucia banks in 2005, taking samples at fixed points along a number of transects in both South and North lakes ( Figure 1). On this occasion, gastropods were identified by D. Herbert and R.N. Kilburn. In all cases, no specialized equipment was used and specimens were collected by hand at the surface or within the sediment by using spades and/ or mechanical grabs.

2012-2013 survey
Two surveys were conducted in March and July 2012. The survey in March was conducted at Fani's Island, St Lucia Mouth, Hell's Gate, Makakatana, Lister's Point, the Bridge over the Narrows and along a transect from Catalina Bay to Charter's Creek. The Back Channel, Shark Basin and Mpate Mouth were also visited ( Figure 1, Table 1). Macrobenthic samples were taken using a Zabalocki-type Eckman grab with a sampling area of 0.0236 m 2 and depth of 15 cm. Each sample was made up of three grabs, with three replicate samples taken at each site. Replicate samples were emptied into 20 L buckets and water was added in each sample. Each sample was stirred vigorously to suspend benthic organisms and the supernatant was passed through a 500 µm sieve. This procedure was repeated five times for each replicate sample. Material that was retained on the sieve was emptied into plastic jars. Sediments that were left in the bucket were washed through a 2 mm sieve. Samples were preserved in 4% phloxin-stained formalin. Qualitative samples were also collected with a D-net. At each site, the net was pushed over the sediment surface for a distance of approximately 5 m. At least one D-net sample was collected from each site. Macrofauna retained on the D-net were emptied into plastic jars and 4% phloxin-stained formalin was added. Both sampling methods (grab and D-net) were used at Fani's Island, St Lucia Mouth, Hell's Gate, Makakatana and along a transect from Catalina Bay to Charter's Creek, while only D-net samples were collected from the Bridge and the Back Channel ( Figure 1, Table 1).
The survey also included the manual collection of dead gastropod shells along the shoreline of the lake, within close proximity of the sampling stations. Further collections on an opportunistic basis were undertaken throughout 2013. In the laboratory, each sample was emptied into a sorting tray and gastropods were separated and identified with the aid of specialized literature and, where necessary, external taxonomy experts. Suitable specimens and shells were photographed in a standardized way, so as to show morphological characteristics that aid in their identification in an illustrated checklist.

Results
A total of 20 families and 37 species of gastropods have been found in the St Lucia estuarine system since 1924, with half of the species not previously recorded in the literature. The earliest records of gastropod specimens collected in the St Lucia system are from the KwaZulu-Natal Museum (NMSA) and date back as far as 1924. Seventeen species originating from St Lucia are currently reposited in its collections, mostly obtained during dedicated surveys conducted in 1987 (Table 2). Of these, 11 are among the new records reported here, as they were never included in previous reports or publications on Lake St Lucia. Another two of the new records were collected during surveys undertaken by the provincial conservation authority, EKZNW, while the other six were only revealed during the latest survey of 2012-2013. Although three among the latter were only recorded as dead shells (i.e. Cerithium dialeucum, Murex brevispinus, Purpura bufo), they were found in sufficient number and deep enough in the upper reaches of the estuarine lake to suggest with reasonable confidence they were once established within the system, rather than accidentally introduced there.
Only 12 species were found alive within the system during the recent 2012-2013 survey. These include Aplexa marmorata, Assiminea cf. capensis, Assimineidae sp., Bulinus natalensis, Cassidula labrella, Cerithidea decollata, Lymnaea natalensis, Melampus parvulus, M. semiaratus, Melanoides tuberculata, Pterotrachea cf. hippocampus and Tarebia granifera ( Figure 2). Among them, five are freshwater dwellers (A. marmorata, B. natalensis, L. natalensis, M. tuberculata and T. granifera) that have entered the system only recently, in response to the establishment of a new wet phase after the prolonged drought of 2002-2011. Melanoides tuberculata was found in high abundance at Shark Basin, at shallow depths in permanently submerged channels, as well as in a tributary stream (Mpophomeni) at False Bay (Tables 1-2, Appendix). Two of these freshwater species, i.e. A. marmorata and T. granifera, are actually alien invasives that have spread from colonies initially restricted to the seepage points around Catalina Bay (Appendix). T. granifera was recorded in high abundance at Makakatana in March 2012, spreading subsequently throughout the Narrows to the south and at least as far as Charter's Creek to the north-west ( Figure 1, Table 1). Table 2. Gastropod species originally recorded from the St Lucia estuarine lake. Reference codes: B: Boltt (1975); BKJC: Blaber, Kure, Jackson and Cyrus (1983); DMB: Day, Millard and Broekhuysen (1954) Weerts (1993). The classification scheme follows Bouchet and Rocroi (2005). (* = New record for Lake St Lucia). Cassidula labrella, Cerithidea decollata, Melampus parvulus and M. semiaratus are the only mangrove species that have been able to survive within the system, despite the closed mouth conditions that have prevailed since 2002. Cerithidea decollata was the only one among them to be found in abundance at all mangrove sites, including the St Lucia Bridge, Back Channel, Honeymoon Bend (Narrows) and Shark Basin near the St Lucia Mouth (Figure 1, Table 1). On the other hand, C. labrella, M. parvulus and M. semiaratus were only found in the mangroves at Shark Basin and in very low numbers, on shaded mud surfaces and under fallen wood.

Species (original record)
Of the typical estuarine species recorded in all surveys undertaken in the past in Lake St Lucia, only Assiminea cf. capensis and Assimineidae sp. persisted through the latest survey . Large aggregations of A. cf. capensis and Assimineidae sp. were found in various areas of the estuarine system, with the former generally preferring salinities below 30 and the latter dominating under hypersaline conditions, mainly at Lister's Point (False Bay). In July 2012, accumulations of hundreds of thousands of live individuals, mainly belonging to Assimineidae sp., were observed along the shoreline of Lister's Point, apparently washed up by wind-driven wave motion (Figure 3). Similar aggregations, but this time dominated by A. cf. capensis, were observed the following year, in May 2013, in the bay just south of Lister's Point, fol- Among the mangrove dwellers that were present in the past but have recently disappeared entirely from the system are all the Littoraria species, i.e. L. coccinea glabrata, L. intermedia, L. pallescens, L. subvittata and L. scabra (Table 2, Figure 2). Although the estuarine lake has experienced a large freshwater inflow since late 2011, several freshwater species that were previously found within the system were not recorded alive in 2012-2013. These include both Neritina species, i.e. N. gagates and N. natalensis, as well as Bulinus tropicus, B. forskalii, Pseudosuccinea columella and Oxyloma patentissima (Figure 2, Appendix).
Typical estuarine and/or coastal marine species that are also among the new records may have entered the system only on sporadic occasions and/or for short periods of time under open mouth conditions. They include Afrolittorina africana, Alaba pinnae, Ergalatax heptagonalis and Phalium areola. All of them are represented in past collections from the KwaZulu-Natal Museum ( Table 2). The common coffee-bean snail Melampus lividus, the estuarine limpet Siphonaria oculus and the opisthobranch Bursatella leachii were already reported in the earliest surveys of the University of Cape Town (Day et al. 1954;Millard and Broekhuysen 1970) (Table 2, Figure 2, Appendix). A second opisthobranch species, Stylocheilus striatus, was only recorded at Catalina Bay for a few months, immediately after the seaward mouth breach of March 2007. Finally, coastal marine species that were clearly once established in the northern lakes and have only been recorded in the latest survey from dead shells include whelks such as Murex brevispinus and Purpura bufo, and the cerithiid Cerithium dialeucum (Table 2, Appendix).

Gastropod diversity and hydrological phases
Major climatic events and hydrodynamic processes control the gastropod species richness and abundance in the St Lucia estuarine lake (Figure 2). The highest diversity reported so far coincides with the period Jul 1964 -Jan 1965, when the second survey by the University of Cape Town (UCT) was conducted on the system. On that occasion, 12 gastropod species were recorded at a time when the estuarine lake was under tidal influence, with a normal salinity gradient decreasing from the estuary basin to the northern lakes (Millard and Broekhuysen 1970). Other major surveys undertaken by UCT in 1948-1951and by NMSA in 1987 in total records of 7 and 8 species, respectively (Day et al. 1954; Figure 2). These were periods characterized respectively by the first hypersaline event recorded in the system and a flood peak flow occurrence (Stretch et al. 2013).
During the last decade, St Lucia has undergone some of the most dramatic shifts ever recorded in the region. These have caused an unprecedented crisis and triggered a burst of fresh research activity on the system. It is thus not surprising that of the total 37 species of gastropod recorded within the estuarine lake, 19 are new records arising from the recent escalation in analyses and collecting efforts. During the latest dedicated gastropod survey, undertaken between Jan 2012 and Nov 2013, a total of 15 species were recorded, with only 12 found still alive and four in reasonable abundance, even if intermittently. Among the latter group, two are actually alien invasive species, i.e. Tarebia granifera and Aplexa marmorata (Figure 2).

Response to recent dry and wet phases
In 2002, a sand berm closed off the St Lucia Estuary from the ocean, leading to a prolonged period of mouth closure, which still persists currently. The mouth was breached from the seaward side for a brief period of six months, between March and August 2007, by a combination of extreme events linked to Cyclone Gamede (Whitfield and Taylor 2009). Between 2002 and 2011, prolonged closure and low rainfall resulted in frequent periods of hypersaline conditions in the northern lakes, with complete desiccation of some areas at times ). These events led to the virtual disappearance of the entire gastropod community, with the exception of few mangrove dwellers and the most tolerant estuarine species, namely Assiminea cf. capensis, Assimineidae sp. and Haminoea natalensis. These were actually able to thrive, as little competition for resources remained at the onset of the harsh conditions (Figures 2-4). Alien invasive species, such as Tarebia granifera and Aplexa marmorata, were also able to take advantage of this situation and occupy vacant habitats and under-utilized resources in freshwater seepage areas.
Since the end of 2011, the system has entered a wet phase, with above average rainfall leading to occasional flooding and the prevalence of oligo-to polyhaline conditions throughout the extent of the system. This was compounded by the excavation of a beach spillway in July 2012, which has since contributed substantial freshwater inflow from the Mfolozi River into the St Lucia system and also partial exchange of water with the open ocean (Nel 2014, van Elden et al. in press). These changes have led to the appearance of a number of new gastropod species of brackish to freshwater origin, including Bulinus natalensis, Cassidula labrella, Melampus parvulus, M. semiaratus and Melanoides tuberculata (Figure 2). While the presence of the heteropod Pterotrachea cf. hippocampus in recent plankton collections clearly indicates marine penetration into the St Lucia, although the beach spillway connection to the Mfolozi mouth has failed so far to result in significant recruitment of typical coastal gastropod species from the ocean.

Historical trends
Historical collections and surveys have, however, recorded numerous species of euryhaline marine and estuarine species, even in the uppermost reaches of the estuarine lake. For instance, the tick shell Nassarius kraussianus is present in all museum collections from St Lucia and has been recorded as abundant in all previous studies in the area (Table 2, Figure 2). Despite the numerous dead shells retrieved during the past few years throughout the system, it has not been found alive since 2006(MacKay et al. 2010. Nassarius kraussianus is known as being generally associated with Zostera beds (Marais and Seccombe 2010) and its recent disappearance from the system has coincided with the extinction of the Zostera beds inside the lake basins after 2005, following the closure of the mouth in 2002 (Adams et al. 2013). Similarly dependent on marine grasses are typical estuarine species, such as Alaba pinnae, which is generally attached to Zostera blades (Kilburn and Rippey 1982), and the top-shell Jujubinus suarezensis and the whelk Murex brevispinus, both tidal mudflat dwellers living among submerged seagrasses (e.g. Thalassodendron in Mozambique, Kilburn and Rippey 1982). Only old specimens of these species were found reposited in the KwaZulu-Natal Museum (Table 2) and only dead shells were retrieved in the latest survey. Therefore, it seems most likely that these too may have died out shortly after the closure of the St Lucia mouth. Other mouth closure episodes have been recorded in the past, e.g. 1959-1961 and 1992-1993 (Figure 2), however they never persisted uninterruptedly for such a long period of time and never created conditions severe enough to cause the complete extinction of the Zostera beds from the system (Millard and Broekhuysen 1970;Adams et al. 2013).
Apart from causing the disappearance of marine grasses, prolonged mouth closure would also lead to the eventual death of barnacle and oyster beds (Nel et al. 2012), on which several species of gastropods depend for their food. For instance, the two whelks retrieved at False Bay and Charter's Creek (South Lake) as dead shells in recent collections, Ergalatax heptagonalis and Purpura bufo, are known to be associated with barnacles and most probably depended on the dense barnacle beds that proliferated on the Cretaceous rock platforms prior to mouth closure (Kilburn and Rippey 1982;Marais and Seccombe 2010).

Mangrove communities
Among the 20 species not previously recorded from the St Lucia estuarine lake are typical mangrove dwellers, such as Littoraria intermedia, L. pallescens, L. subvittata, L. scabra and Melampus parvulus. St Lucia mangroves have undergone significant deterioration since the mouth closed in 2002, as persistent low salinity in the Narrows and near the mouth has favoured the development of reeds at the expense of mangrove vegetation (Adams et al. 2013). This has inevitably impacted on the once rich mangrovedependent gastropod community (Day et al. 1954;Millard and Broekhyusen 1970). Indeed, of all the typical mangrove species reported here, only the resilient Cerithidea decollata was found in reasonable numbers during the latest survey. The other three species still present, M. semiaratus, M. parvulus and Cassidula labrella, occurred sporadically in very low numbers and only near the St Lucia Mouth. Surprisingly, the giant mangrove whelk, Terebralia palustris was never recorded at St Lucia, despite having been reported as well-established and common both to the south (e.g. Richards Bay, Durban Bay) and to the north (Mgobezeleni, Kosi Bay) of this system, at least in the past (Macnae 1963;Berjak et al. 2011).

Key and indicator species
Population explosions of the bubble shell Haminoea natalensis with its distinct egg masses were recorded seasonally until 2009 (Figures 2 and 4). The observed trend of population explosions followed by high mortality and dwindling numbers is typical for opisthobranchs. Environmental conditions during the different seasons as well as the recruitment of H. natalensis, are the most important factors driving its population biology (Malaquias and Sprung 2005;Miranda et al. 2011). Haminoea natalensis has not been observed in St Lucia again after 2009. However, when favorable higher salinity conditions return, the species will probably again be found along the shores in shallow water, feeding on microphytobenthic mats (Miranda and Perissinotto 2012).
The taxonomy of assimineids, or sentinel snails, is poorly understood and currently under revision in South Africa. In the St Lucia Estuary, there are inconsistencies in the literature in terms of what species of Assiminea occur in the system. This is not surprising given the morphological and ecological similarities as well as spatial overlap between different assimineids. Earlier literature refers to A. bifasciata as the only species present in the system (Day et al. 1954;Millard and Broekhuysen 1970;Boltt 1975;Whitfield and Blaber 1978). Recent literature reports A. ovata (Miranda et al. 2011, Daly et al. 2012) and A. globifera (Taylor et al. 2006) and A. durbanensis (Weerts 1993) have also been reported from St Lucia. Three species of assimineids are listed by MacKay et al. (2010) in St Lucia, while Pillay and Perissinotto (2008) and Owen et al. (2010) make reference to Assiminea sp.. Millard and Broekhuysen (1970) reported the occurrence of A. bifasciata as well as Syncera sp. Syncera Gray, 1821 is nomen nudum and a synonym of Assiminea (Fukuda and Ponder 2003). The most recent genetic and morphological study conducted in the St Lucia Estuary has confirmed the existence of two species: A. cf. capensis and Assimineidae sp. ("A." aff. capensis in Miranda et al. 2014). These two species exhibit patterns of spatial overlap that appear to vary depending on environmental parameters, particularly salinity. Assimineidae sp. is an assimineid in the broader sense, but belongs in an as yet unnamed genus and subfamily (Winston Ponder pers. comm.). Perhaps what Millard and Broekhuysen (1970) reported as Syncera sp. was in fact Assimineidae sp.. This false sentinel snail seems to prefer the more saline conditions in the northern parts of St Lucia and has been previously referred to as Coriandria durbanensis or A. durbanensis (Raw et al. 2013;Weerts 1993). It is clear that A. cf. capensis and Assimineidae sp. have been misidentified and confused in the past because of poor taxonomic knowledge and their morphological variability and similarities.

Alien invasive species
Three of the five predominant alien invasive freshwater gastropods in South Africa have been recently recorded from St Lucia: Aplexa marmorata, Pseudosuccinea columella and Tarebia granifera. Previously, under hypersaline conditions these species were restricted to freshwater seepage areas on the Eastern Shores of the South Lake and along the Narrows (Miranda et al. 2010). However, the oligohaline conditions which currently persist following an increased volume of freshwater entering the system potentially favour the expansion of these species to new areas. This has already been observed with T. granifera, which due to its unexpected high salinity tolerance (Miranda et al. 2010) has recently been recorded, albeit at low densities, from Charter's Creek and Makakatana on the Western Shores of South Lake.
As the freshwater-dominated phase of St Lucia continues, the potential for alien invasive gastropods to enter and spread within the system increases. The consequences of these expansions vary depending on the species. Pseudosuccinea columella in South Africa is susceptible to the liver flukes (Fasciola spp.) which infect livestock, although it has not been confirmed as an intermediate host (Appleton 2003). Aplexa marmorata is widespread in KwaZulu-Natal, however its potential impacts are largely unknown. Similarly, although not reported from St Lucia, is Physa acuta (de Kock and Wolmarans 2007) and Helisoma duryi (Appleton 2003). The latter has however been recorded from artificial environments in the region. The greatest potential impact is that of Tarebia granifera, which displaces native gastropods and attains very large densities (Miranda et al. 2011;Raw et al. 2013). As T. granifera is so successful, this species poses a threat to native malacofauna, such as the Bulinus species group, which would be expected to expand their range during a freshwaterdominated phase.
In addition to the threats from freshwater alien invasive species, estuaries are also threatened by the invasion of marine species from coastal sources (Nehring 2006). The majority of marine alien invasive species are introduced through ship fouling or ballast water (Picker and Griffiths 2011) and the proximity of an estuary to intensive international shipping increases its risk to introduced species (Nehring 2006). For St Lucia, the largest threat comes from the shipping activities at the industrial port of Richards Bay. Presently, there are no records of marine alien species in St Lucia due to the prolonged closure of the mouth to the Indian Ocean. The recent re-establishment of a connection through the beach spillway with the Mfolozi River may allow previously reported native marine species to re-enter the system, but also increase the risk of introduction of alien species. The continuous monitoring and assessment of potential impacts that these species may have on the ecosystem at large is necessary, given the significance of St Lucia as the largest estuarine lake in Africa and South Africa's first World Heritage Site.

Conclusions
Throughout its history, the St Lucia estuarine lake has experienced drastic shifts in hydrological states, from extreme dry conditions accompanied by hypersalinity and desiccation, to floods followed by freshwater dominance. The state of the mouth has also varied from an extended open bay joined to the Mfolozi River to extreme constriction and prolonged closure. The latest period of closure has been unprecedented and virtually uninterrupted since 2002. Although the monitoring of gastropod diversity within the system has been erratic until recently, there are clear indications that higher diversity has coincided with periods when the estuarine lake was under tidal influence, with a normal salinity gradient decreasing from the estuary basin to the northern lakes (e.g. 1964-1965). Drastic declines were observed when the system experienced hypersaline (e.g. 1948-1951) or flood conditions (e.g. 1987), with a closed mouth state compounding the problem by preventing any recruitment from the ocean. During the last decade, St Lucia has undergone some of the most dramatic shifts ever recorded in the region. Despite the intense, dedicated gastropod surveys undertaken in 2012-2013, only 12 species were found still alive, with four in reasonable abundance. Among these, unfortunately two are actually alien invasive species, i.e. Tarebia granifera and Aplexa marmorata, with the first spreading at alarming rates as low salinity conditions now prevail throughout the system.

Neritina natalensis Reeve, 1855
Common name. Spotted nerite. Size. Maximum shell height 20 mm (2) . Remarks. Reportedly occurs within mangroves (2) and on Phragmites stems in estuaries. Distribution. East African coast from Somalia extending south to the Umzimkulu River on the KwaZulu-Natal coast of South Africa (2) . Also reported from the Mtamvuna River (3) . St Lucia records. Not previously reported from St Lucia; collected in 1924 and 1987 (NMSA) from False Bay. Not recorded in the recent survey.

Melanoides tuberculata (Müller, 1774)
Common name. Red-rimmed melania. Size. Maximum shell height 45 mm (3) . Remarks. Predominantly parthenogenetic species (3) which inhabits fresh and brackish water (2) . Distribution. Natural Indo-Pacific distribution includes much of Africa with a southern limit at Port Elizabeth (2) and extending east to southern Asia and northern Australia (3) . Introduced to regions of North America (4) and New Zealand (5
Remarks. Typical high intertidal species which occurs in large numbers usually on exposed rocks (1 Remarks. Characteristic mangrove and estuarine species found above high water (1) . Usually found on trunks, roots (23) and branches of mangrove trees (1) . Distribution. Tropical and sub-tropical Indo-Pacific distribution extending from the eastern African coast across to Samoa and Hawaii (23) . The southern limit of the species is Algoa Bay at Port Elizabeth in South Africa (1,19) . Lucia in 1948Lucia in -51 (20) , 1964Lucia in -55 (21) and 1972 however the species has not been recorded recently.

St Lucia records. Reported from St
Littoraria subvittata (Reid, 1986) 18 mm, Lake St Lucia Burnup Collection. . The specimen collected was probably exposed after excavation of the Link Canal as it is a Pleistocene subfossil. Not found during the recent survey, however the species may re-colonize St Lucia if the marine connection is maintained.