INTRODUCTION

Estuaries are characterised by large spatial and temporal variations in the physicochemical environment. The St Lucia estuarine lake exhibits changes in climatic conditions from wet to dry periods, with a duration of four to ten years (Begg 1978). During dry periods, the system is subjected to high evaporation, low rainfall input and low river inflow (Cyrus & Vivier 2006; Pillay & Perissinotto 2008). St Lucia was artificially separated from the Mfolozi River in 1952, which deprived it of its most essential freshwater source during drought periods (Begg 1978; Ngqulana et al. 2010). As a result, during dry periods the northern reaches of the estuarine system become hypersaline (Cyrus & Vivier 2006; Vivier & Cyrus 2009), with salinity levels of >200%o recorded on several occasions. The estuary is also exposed to stochastic disturbances, such as floods (Forbes & Cyrus 1992), which may very rapidly decrease the salinity within the system (Cyrus 1988; Hanekom 1989; Forbes & Cyrus 1993). Forbes and Cyrus (1992) recorded a decrease in salinity from 45 %o to <10%o in approximately two weeks, in large parts of its South Lake during the flood caused by cyclone Domoina in 1984. These changes between hypersaline and low salinity conditions may cause drastic alterations in estuarine structure and function (Cyrus 1988).

The range of conditions that occur between the extremes of floods and severe droughts shapes the estuarine biotic communities (McLusky & Elliott 2004). Specifically, hypersaline conditions and hyposaline events may cause large reductions in species numbers and changes to species composition (Cyrus 1988; Hanekom 1989; Forbes & Cyrus 1992; Pillay & Perissinotto 2008). Flood events can cause mass mortality (Matthews & Fairweather 2004; Nel et al. 2011) and redistribution of benthic bivalve species in estuaries (Forbes & Cyrus 1992). Hill (1981) stated that sessile and slow-moving benthic organisms in St Lucia are particularly susceptible to mass mortality events during periods of escalated salinity levels. This is partially due to their inability to move to areas with lower salinity and a more favourable physico-chemical environment (Hill 1981 ; Ysebaert et al. 2002). During the most extreme of droughts, more than 50% of the St Lucia estuarine lake may dry out and, along with hypersaline conditions, this may drastically alter the distribution of macrofaunal communities (Pillay & Perissinotto 2008). Boltt (1975) suggested that even when a bivalve species is eliminated from a particular area due to unfavourable physico-chemical conditions, it may have the ability to recolonise these areas once favourable conditions return. It is clear that the distribution of benthic macrofauna in an estuary is not static but changes according to the physico-chemical conditions that prevail at any given time.

Benthic macrofauna play an important role in estuarine ecology by acting as intermediate links in food webs and contributing to bioturbation (Hampel et al. 2009; Cyrus et al. 2010; MacKay et al. 2010). Bivalves, in particular, fulfil a vital function in estuaries as they collect and settle a significant amount of sediment while filter-feeding, and may alter phytoplankton biomass and composition in the water-column (Gerritsen et al. 1994; Vaughn & Hakenkamp 2001). The filtering of phytoplankton and suspended solids from the water-column may also affect the water clarity of an estuary (Gerritsen et al. 1994). Bivalve shells provide a settlement medium for algae and other invertebrates, creating a multi-dimensional environment (Vaughn & Hakenkamp 2001). A shift in climatic conditions can alter species numbers and composition, as well as cause the loss of important functional roles played by the bivalves.

Despite the physico-chemical variability observed in the St Lucia estuarine lake, the system has a rich biological diversity (Pillay & Perissinotto 2008; MacKay et al. 2010). It is for this reason that the estuary has attracted a number of investigations concerning its benthic community, under varying physico-chemical conditions (Owen & Forbes 1997). Past work on the benthic community was done during hypersaline (Day et al. 1954; Boltt 1975), marine (Blaber et al. 1983; Weerts 1993), and low salinity conditions (Millard & Broekhuysen 1970; Cyrus 1988; Weerts 1993). Studies on the benthic community found in the Narrows have been fewer, but focused on a longer time frame, from 1983 to 1994 (Owen & Forbes 1997). Although all these surveys mention bivalve taxa, to date no attempt has been made to compile a comprehensive list of the species that occur in the different areas of the St Lucia Estuary and to assess how they respond to the climatic shifts that characterise the system.

The purpose of the present paper is to provide a census of the bivalve species that have been recorded from the St Lucia estuarine system. Included is an overview of taxa that have been recorded in the past, and under what conditions, as well as what is currently found or no longer found in the system. Literature and museum collections relating to ecological surveys conducted in St Lucia since the 1920s were reviewed and references to bivalve species recorded. The updated taxonomic status of each species and synonyms are provided and attention is drawn to misidentifications. An annotated and illustrated checklist of all bivalve molluscs conclusively identified during this study is given in the Appendix. The checklist will provide scientists, managers and visitors with means to identify the bivalves of the St Lucia Estuary. This will reduce the risk of erroneous identifications and hopefully stimulate further research interest in these important organisms.

MATERIAL AND METHODS

The 2011 survey

During 2011, bivalves were picked up by hand along the entire accessible shoreline of the estuarine lake. A dedicated survey was carried out from 18–21 March, during which both quantitative and qualitative benthic samples were taken. Quantitative samples were collected using a Zabalocki-type Ekman grab (sampling area 0.0236m²; depth 15 cm). A single sample comprised three grabs and three replicate samples were taken at each site. Water was added to each sample, which was stirred vigorously, thereby suspending the benthic invertebrates. The supernatant was then washed through a 500 µm sieve. This process of adding water, stirring and sieving was repeated five times, and any material retained on the sieve was finally emptied into a plastic jar. This procedure has been shown to extract more than 95 % of the macrofauna in a sample (Cyrus & Martin 1988). The remaining sediment was washed through a 2000 µm sieve in order to recover larger macrofauna such as bivalves, gastropods and crustaceans (Cyrus & Martin 1988). However, the Zabalocki-type Ekman grab may undersample larger bivalves found deeper within the sediment (MacKay et al. 2010). All macrofauna samples were preserved in 10% formaldehyde solution and stained with Phloxin-B. In the laboratory, bivalves were removed and identified to species level using voucher specimens deposited at the NMSA or the identification guidelines provided in Kilburn and Rippey (1982), Willan (1993), Appleton (1996), Branch et al. (2010) and Huber (2010). Qualitative samples were obtained using a stainless-steel D-net (33 cm diameter) with a raking front edge, pushed by hand along 10–20 m transects and penetrating about 5–10 cm into the sediment. Samples collected in this way were preserved and analysed as explained above. Bivalves removed from samples were preserved in 70% ethanol solution. This survey differed from the two previous surveys in that live bivalves were collected. It also took place during a period of constant drought conditions when only those species that were able to withstand the extreme conditions were surviving, while the others had disappeared.

Fig. 1.

Map of the St Lucia Estuary, showing the sampling sites where bivalves were collected from 1925 to 2011; and its geographical position relative to South Africa (adapted from Carrasco et al. 2010).

Some specimens from this and previous serveys are kept at the University of KwaZuluNatal, Durban (UKZD).

RESULTS

A total of 24 bivalve species were recorded in St Lucia between 1925 and 2011. Of these, only 12 have been reported in published literature, viz. Barnea manilensis, Brachidontes virgiliae, Dosinia hepatica, Eumarcia paupercula, Irus irus, Macomopsis moluccensis, Salmacoma litoralis, Siliqua cf. polita, Solen cylindraceus, Soletellina lunulata, Tellina cf. rousi and Theora lata (Table 1). Ten species, identified during the survey of 1982/83 (Table 2), had not appeared in published literature for St Lucia. Only nine of these were considered true inhabitants of the estuary, while Timoclea lavrani was excluded because only a single valve had been found. The clam Meretrix meretrix was first found in St Lucia in 2000 (NMSA) and subsequently recorded in huge numbers during the survey of 2005. This was the only additional bivalve species found in the 2005 survey (Table 3) that had not previously been mentioned for St Lucia. Another two species, not included in the literature, are in the NMSA. These are Mactra cuneata and Martesia striata (Tables 4a, 4b). During the survey of 1982/83,22 bivalve species were found, as empty shells, on the shorelines and islands of the St Lucia estuarine system (Table 2). Only 11 species were recorded during the survey of 2005 (Table 3). A total of 18 and seven species, respectively, from the St Lucia estuarine system are currently in the NMSA and the SAMC (Tables 4a, 4b, 5). As the majority of the bivalves were collected as empty shells, there is some uncertainty regarding the exact time at which the bivalves were alive and growing in the system.

Siliqua cf. polita has only been collected from Picnic Point, Lister's Point and Bird Island in 1982/83 (Table 2), and in False Bay in April 1987 (Table 4b). This appears to be an undescribed species (R.N. Kilburn pers. comm.), which has not been seen again in subsequent surveys. The first record of the mytilid Brachidontes virgiliae at St Lucia dates from July 1948, when it was found at the estuary mouth (Table 5). In December 1962, it was found at Charter's Creek in beds of the submerged macrophyte Zostera capensis (Table 4a). Empty B. virgiliae shells were found as far north as Lister's Point and Bird Island (Table 2) as well as from Charter's Creek to Fanie's Island during 1982/83 and 2005, respectively (Table 3). In 2011, live B. virgiliae were found in numbers at Fanie's Island, Catalina Bay and Charter's Creek. Live B. virgiliae were also present in the Stuckenia pectinata beds along the Narrows. In the quantitative samples, a maximum density of 51 ind.m^-2 was recorded at Fanie's Island in 2011 (Table 6).

Barnea manilensis specimens were first collected at False Bay in July 1948 (Table 5). In the 1982/83 and 2005 surveys, this species was detected throughout the western lake shoreline, at False Bay, and at North and South lakes (Tables 2, 3). Dead B. manilensis were recorded in situ in large numbers (often > 100 m²) in 2011 throughout the western shores, wherever cretaceous sandstone was available for burrowing (Fig. 2).

Soletellina lunulata occurred throughout the St Lucia lake complex at the time of the 1982/83 and 2005 surveys, and is well represented in the NMSA collection (Tables 2, 3, 4a).

The semelid species Theora lata was collected in False Bay, South Lake and the Narrows and deposited at the NMSA in 1981 and 1987 (Table 4b). It was also found in False Bay in 1964 and deposited in the SAMC (Table 5). In 1982/83, it was recorded as being abundant throughout North Lake and False Bay, whereas in 2011, only a few empty shells were encountered, on the western shoreline.

Solen cylindraceus has been reported in a number of studies (Table 1), and has been an abundant feature in all three lakes since 1948 (Tables 2, 3, 4b, 5). It was occasionally present in the Narrows (Table 2). In March 2011, it was found alive at Charter's Creek and Catalina Bay at densities ranging from 14 to 494 ind.m² (Table 6), and was recorded at Fanie's Island and Lister's Point in the qualitative samples.

Fig. 2.

Barnea manilensis, a large concentration of dead shells at False Bay, Lake St Lucia, in April 2011. (Photo: Lynette Perissinotto)

TABLE 1

Bivalve species reported in the literature from the St Lucia Estuary. Species marked with the asterisk have been identified in the estuary during the present study.

TABLE 2

Bivalve species reportedly found dead along the shores of the St Lucia estuary during the Natal Parks Board bivalve survey of 1982/83. Abbreviations: Species: Aa — Anomia achaeus, Ac — Arcuatula capensis, An — Anadara natalensis, Bm — Barnea manilensis, Bv — Brachidontes virgiliae, Cf — Corbicula fluminalis, Cw— Chambardia wahlbergi, Dh — Dosinia hepatica, Ds — Dendostrea sandvichensis, Ep — Eumarcia paupercula, Ff — Fulvia fragilis, Ii — Irus irus, Ml — Salmacoma litoralis, Mr — Macomopsis moluccensis, Sc — Solen cylindraceus, Sf — Saccostrea forskahlii, Sl — Soletellina lunulata, Sp — Siliqua cf. polita, Tb — Tellina s.l. bertini, Ti — Timoclea lavrani, Tl — Theora lata, Tr — Tellina cf. rousi; Abundance: A — abundant, C — common, P — present, X — present but relative abundance unknown. Species underlined represent new records for the St Lucia Estuary.

TABLE 3

Bivalve species recorded as freshly-dead or alive along the shores of the St Lucia Estuary during the Ezemvelo KZN Wildlife survey of 2005. Abbreviations: Aa — Anomia achaeus, Bm — Barnea manilensis, Bv — Brachidontes virgiliae, Dh — Dosinia hepatica, Ep — Eumarcia paupercula, Ff — Fulvia fragilis, Ii — Irus irus, Mm — Meretrix meretrix, Sc — Solen cylindraceus, Sl — Soletellina lunulata, Tr — Tellina cf. rousi. The underlined species represents a new record for the St Lucia estuary.

Three tellinid species, Macomopsis moluccensis, Salmacoma litoralis and Tellina cf. rousi, have been reported from St Lucia. Macomopsis moluccensis and S. litoralis were collected at False Bay, North Lake, South Lake and the Narrows, from 1925 to 2005 (Tables 2, 4b). Both were found alive in 2011 at Charter's Creek, while S. litoralis was also found at Makakatana (Table 6). Tellina cf. rousi was present during 1982/83 from the Narrows to False Bay (Table 2), in the South Lake during 2005 (Table 3), and was collected at Shark Basin in 1987 (Table 4b). It also occurred, as shells, at False Bay in 2011. It now appears that it may represent an undescribed species (M. Huber pers. comm.). The highest densities for the three tellinid species M. moluccensis, S. litoralis and T. cf. rousi, were 65, 37 and 80 ind.m² respectively, in 2011 (Table 6).

Three venerid species, Dosinia hepatica, Eumarcia paupercula and Irus irus, also featured prominently in this estuary. Dosinia hepatica was first recorded at Fanie's Island in November 1971 (Table 4a) and was common throughout the surveys done in 1982/83 and 2005 (Tables 2, 3). In 2011, this species was found alive only at Charter's Creek, at a density of 4 ind.m^-2 (Table 6). Specimens of E. paupercula deposited at the NMSA were collected from 1927 to 1981 (Table 4a). During the 1982/83 and 2005 surveys, it was found in abundance throughout the entire estuary, except at False Bay (Tables 2, 3). It was also found in July 1948 and deposited in the SAMC (Table 5). Irus irus was collected from 1949 to 1987 (NMSA and SAMC) and was abundant but dead in situ during all three surveys.

Twelve bivalve species have not been documented previously in any literature on St Lucia. These are Anadara natalensis, Anomia achaeus, Arcuatula capensis, Chambardia wahlbergi, Corbiculafluminalis, Dendostrea sandvichensis, Fulviafragilis, Mactra cuneata, Martesia striata, Meretrix meretrix, Saccostreaforskahlii and Tellina s.l. bertini.

A single shell of A. natalensis was collected at the dredge spoils during 1983, but several dead shells (ca 10) were found at Charter's Creek in April 2011.

The freshwater bivalve C. fluminalis was only found at the Hluhluwe River mouth in 1983 (Table 2). Chambardia wahlbergi, a freshwater mussel, was collected in 1982 at the Hluhluwe River mouth and in the Mkuzi swamps (Table 2).

TABLE 4a

Bivalve species collected from St Lucia and held at the NMSA. Abbreviations: Ac — Arcuatula capensis, Bm — Barnea manilensis, Bv — Brachidontes virgiliae, Dh — Dosinia hepatica, Ep — Eumarcia paupercula, Ff — Fulviafragilis, Ii — Irus irus, Sl — Soletellina lunulata, Tb — Tellina s.l. bertini.

TABLE 4b

Bivalve species collected from St Lucia and held at the NMSA. Abbreviations: Mc — Mactra cuneata, Ml — Salmacoma litoralis, Mm — Meretrix meretrix, Mr — Macomopsis moluccensis, Ms — Martesia striata, Sc — Solen cylindraceus, Sp — Siliqua cf. polita, Tl — Theora lata, Tr — Tellina cf. rousi. Species underlined represent new records for the St Lucia Estuary.

TABLE 5

Bivalve species (in alphabetical order) collected from St Lucia and held at the SAMC. Abbreviations: Bm — Barnea manilensis, Bv — Brachidontes virgiliae, Ep — Eumarcia paupercula, Ii — Irus irus, Sc — Solen cylindraceus, Sf — Saccostrea forskahlii, Tl — Theora lata.

Tellina s.l. bertini was only found in October 1982 at Esengeni (Narrows) (Tables 2, 4a), while A. capensis was collected in numbers in 1981, at Catalina Bay (Table 4a), and was present in both the North Lake and False Bay in 1982/83 (Table 2).

Three oyster species were reported from St Lucia. Anomia achaeus was collected in the North Lake during the Natal Parks Board survey (Table 2) and in the South Lake during the Ezemvelo KZN Wildlife survey (Table 3). Dendostrea sandvichensis occurred throughout the St Lucia system (Table 2). In 2011, it was often seen attached, but only dead, to rocks on the banks of the lake. Saccostrea forskahlii was collected during July 1948 at the estuary mouth. It was also recorded in 1982/83 throughout North Lake (Table 2); and in April 1987, it was collected at False Bay (Table 4b).

Fulvia fragilis was reported as common during 1983/83 and 2005 in the North Lake and False Bay (Tables 2, 3, 4a). It was also present in the South Lake (Table 2), but in 2011, only a few empty shells were found at Charter's Creek. Mactra cuneata was only collected at Makakatana, in July 1987 (Table 4b), while M. striata was sampled in numbers from decaying wood at Makakatana in April 1987 (Table 4b). Meretrix meretrix was recorded for the first time in July 2000, at Charter's Creek (Table 4b). It was also found in abundance in 2005 and 2011, but only dead (Table 3).

For two bivalve species, there are only single records. These are Anodontia edentula and Timoclea lavrani. The latter was recorded as a single valve at the Link Canal in 1982/83 (Table 2), while A. edentula was found as an empty shell at Charter's Creek in April 2011.

DISCUSSION

Twenty-four bivalve species were identified with high confidence by comparing them with museum specimens and in consultation with leading world experts. Identification was based on specimens obtained in the St Lucia estuarine system during surveys undertaken from December 1982 to April 2011, and on specimens deposited at the NMSA and SAMC. This is fewer than the number of 26 reported for St Lucia previously (Forbes & Cyrus 1993), but equal to it if the two species for which only a single valve was retrieved are included. The bivalve taxa Tivela natalensis, Pitar abbreviatus and a third species similar to Tivela compressa were reported by MacKay et al. (2010) from a survey of St Lucia that spanned 2004 to 2008. The three species were not recorded in any of the major surveys done by the Natal Parks Board and Ezemvelo KZN Wildlife (Tables 2, 3). There is also no St Lucia-associated specimen of any of them in the museum collections (Tables 4a, 4b, 5). The records may be fortuitous or represent erroneous identifications of Meretrix meretrix, which is extremely variable in colouration and has more than ten synonyms in the literature (Boshoff 1965; Branch et al. 2010). Alternatively, these three taxa might have been situated near the mouth, when it was open in March-August 2007, and could have been carried into the estuary by inflowing water, because all three species are typically found in the surf zone of sandy beaches (Kilburn & Rippey 1982). Hiatella arctica was recorded at Fanie's Island and in the North Lake during the first benthic survey of St Lucia, between 1948 and 1951 (Day et al. 1954; as Saxicava sp.). This species is not found in estuaries and it has a very similar appearance to Irus irus, which is common in St Lucia, often being present in great abundance (Kilburn & Rippey 1982). It is, therefore, almost certainly a misidentification of I, irus (Tables 2, 3). Large numbers of empty I. irus shells, embedded in cretaceous sandstone, were also encountered during 2011 from Charter's Creek to Lister's Point. Live individuals were found in False Bay between July 1964 and January 1965 (Millard & Broekhuysen 1970) and the species is considered to be uncommon in South Africa, except at St Lucia (Kilburn & Rippey 1982). Day et al. (1954) and Pillay and Perissinotto (2008) reported Tellina trilatera (= triangularis) as present in areas of the estuarine lake. However, this species is distributed from the Orange River to the former western Transkei (Kilburn & Rippey 1982) and considering that St Lucia does not fall within its distribution range, this suggests that T. trilatera might have been confused with Tellina cf. rousi (confirmed for the specimens of Pillay & Perissinotto 2008), which is distributed from Mozambique to Stillbaai (Kilburn & Rippey 1982). Indeed, T. cf. rousi was recorded in abundance during the surveys done in 1982/83 (Table 2) and 2005 (Table 3). The species was also collected in 1987 at Shark Basin (Table 4b) and many empty shells were observed in 2011, from Charter's Creek to False Bay. In March 2011, T. cf. rousi was found alive in numbers at Charter's Creek and Fanie's Island (Table 6) and is regarded as an undescribed species (M. Huber pers. comm.).

Day et al. (1954) stated that Brachidontes semistriatus was considered locally common around the “Point” area. Millard and Broekhuysen (1970) also wrote that B. semistriatus was found in St Lucia, on damp stones along the shore of the estuarine basin. Davies (1980) showed that there have been many misidentifications between the three mytilid species, B. virgiliae, B. semistriatus and Arcuatula capensis. He suggested that the reported B. semistriatus from St Lucia may have been B. virgiliae. Brachidontes semistriatus is commonly found on exposed marine rocky shores, while B. virgiliae is a true estuarine species (Davies 1980). Both Millard and Broekhuysen (1970) and Day et al. (1954) mentioned Saccostrea cucullata as being the main oyster species recorded from St Lucia. However, this species is normally replaced by S. forskahlii in estuaries (R.N. Kilburn pers. comm.) and in view of the fact that no specimens of S. cucullata were obtained in the surveys described in this study, it seems likely that these old identifications were erroneous.

Only a single record for each of the two bivalves, Anodontia edentula and Timoclea lavrani was obtained at St Lucia. Anodontia edentula is distributed from the tropical Indo-Pacific to as far south as Knysna (Kilburn & Rippey 1982; Branch et al. 2010). Kilburn and Rippey (1982) stated that this species was on a trajectory to extinction in southern Africa. A single valve of A. edentula was found at Charter's Creek in April 2011, suggesting that the species may have lived inside St Lucia in the past. However, since it was not present in any of the historical or current bivalve collections from this estuary (Tables 2, 3, 4a, 4b), it is possible that the valve may have been accidentally introduced into the system. Similarly, T. lavrani was recovered as a single valve in the Link Canal during the 1982/83 survey. The taxon is considered an offshore species with a distribution from northern Mozambique to the South Coast of KwaZulu-Natal (Kilburn & Rippey 1982). Given the small size of the valve concerned, it is considered likely that this was advected into the estuary from the ocean by tidal inflow.

TABLE 6

Abundance (ind.m^-2) of bivalve species reportedly found alive at the St Lucia Estuary during the survey of March 2011. Abbreviations: Bv — Brachidontes virgiliae, Dh — Dosinia hepatica, Ml — Salmacoma litoralis, Mr — Macomopsis moluccensis, Sc — Solen cylindraceus, Tr — Tellina cf. rousi.

All surveys undertaken by specialists in past and recent years have failed to produce evidence of the existence in the estuary of small species such as nuculids, nuculanids, philobryids, micro-tellinids and micro-venerids. Furthermore, no previous literature on St Lucia has mentioned the families Cardiidae, Carditidae, Chamidae, Corbulidae, Donacidae, Glycymerididae, Limidae, Mesodesmatidae, Myidae, Pectinidae, Pinnidae, Pteriidae and Spondylidae, which usually dominate shallow marine ecosystems elsewhere.

One of the twelve bivalve species not previously reported in the literature for St Lucia is Meretrix meretrix. It has a general Indo-Pacific distribution, reaching as far south as Maputo Bay along the East African coast (Scarlet 2005; Branch et al. 2010). The Maputo Bay population is thought to have been introduced for food from further north in East Africa (Scarlet 2005; R.N. Kilburn pers. comm.). Boshoff (1965) commented that M. meretrix was common in Inhaca Island, Mozambique. Kilburn and Rippey (1982) stated that this species does not live south of Mozambique. Steyn and Lussi (1998) report a South African distribution range for this species from Kosi Bay to around Port Alfred. However, as their atlas is based on the occurrence of “beached specimens”, it is likely that no live individuals/populations were involved in this work. The first record of M. meretrix in St Lucia is from July 2000 at Charter's Creek (Table 4b) (details of the specimen are at the NMSA). In 2005 and 2011, freshly dead M. meretrix were found in great abundance in both the South and North lakes (Table 3). This is a highly visible species because of its thick and robust shell. Specimens ranged in size from 0.5 to 7 cm, suggesting that a viable population was at some stage thriving within the estuary. Notwithstanding the equivocal report of Steyn and Lussi (1998), this represents the first confirmed record for this species in South African waters. However, the timing and mode of its introduction into the St Lucia Estuary remain unresolved. Two possible scenarios are that: (1) it might have been introduced as veligers, transported from Maputo Bay via the Agulhas Current; or (2) it was artificially introduced into St Lucia (R.N. Kilburn pers. comm.). Additionally, the ability to live further south may be due to warming of the oceans globally (R. Willan pers. comm.). The absence of live M. meretrix in the bivalve survey of 2011 and the huge numbers of dead M. meretrix observed then, most of them still with the periostracum intact, suggest that the species may have died out in the system only recently.

Millard and Broekhuysen (1970) reported the occurrence of dead Barnea manilensis shells in St Lucia. This species was also recorded at St Lucia by Barnard (1964). It was commonly found from South Lake to False Bay (Tables 2, 3, 4a), but only dead in 2011, embedded in the cretaceous sandstone exposed by the drought along the western banks of the estuarine lake. This is in agreement with Kilburn and Rippey (1982), who described the bivalve as boring into hard clay and soft sandstone. The extremely large numbers (>100 ind.m²) of dead specimens observed lately along the entire western shoreline of the estuarine lake indicate that until recently, the species was a major filter-feeder and thus a role-player in the settling of suspended silt within the system. Although Irus irus is found in the same localities as B. manilensis, it nestles in pre-existing crevices or boreholes already dug by other bivalves and isopods (R. Willan pers. comm.).

Currently, most of the filtering action within the lake appears to be fulfilled only by Solen cylindraceus, which despite the adverse conditions prevailing at present, can still be found in concentrations exceeding 200 ind.m². It is likely that this light-shelled species is able to disperse easily and grow rapidly. Moreover, it has a fairly wide salinity tolerance range of 15 to 65 %o (Nel et al. 2011). Day et al. (1954) mentioned that S. capensis was found in the Narrows, Fanie's Island and North Lake between 1948 and 1951. However, as the distribution range of S. capensis is from the Olifants River to the Eastern Cape (Kilburn & Rippey 1982; Branch et al. 2010), a misidentification of S. cylindraceus was almost certainly involved. During the 1964/65 assessment of the entire St Lucia estuarine system (Millard & Broekhuysen 1970), S. cylindraceus was found alive and in abundance at Hell's Gate, Makakatana Bay and Brodie's Crossing (Millard & Broekhuysen 1970). In January 1972, S. cylindraceus was only found in the South Lake, while the system was experiencing hypersaline conditions (Boltt 1970). In July 1972, as salinity dropped below 35 %o, S. cylindraceus penetrated North Lake and in January 1973 it moved even further up into the North Lake (Boltt 1970). From 1981 to 1982, the South Lake exhibited stable salinities of 35 %o and S. cylindraceus was found alive at all sampling sites (Blaber et al. 1983). It was rare in the Narrows in 1983; thereafter, it became common as a result of a flood flushing juveniles from the South Lake into the channel, yet this population did not persist (Owen & Forbes 1997). The mouth of the estuary closed in 1992, resulting in hypersaline conditions and the reappearance of S. cylindraceus in the Narrows (Owen & Forbes 1997). Pillay and Perissinotto (2008) investigated the benthic community of St Lucia during the 2005 droughtyear. A total of 14 sites were sampled from the Mouth to False Bay and salinities ranged from 0-125.6 %o within the system (Pillay & Perissinotto 2008). On that occasion, live S. cylindraceus occurred at six of the fourteen sites, i.e. Fanie's Island, Charter's Creek, Makakatana, Catalina Bay, Bridge and Mouth (Pillay & Perissinotto 2008). Solen cylindraceus was also found alive in St Lucia from 2004 to 2008 (MacKay et al. 2010). In 2010, St Lucia exhibited hypersaline conditions, with living S. cylindraceus found only at Catalina Bay and Charter's Creek (Nel et al. 2011). In January 2011, salinities had reduced dramatically due to heavy rainfall, which resulted in a new cohort of S. cylindraceus (size range: 0.5–1.5 cm) being found alive and in large numbers at Charter's Creek, Catalina Bay, Fanie's Island and even reaching Lister's Point (Table 6). Apart from this species, Brachidontes virgiliae, Dosinia hepatica, Macomopsis moluccensis, Salmacoma litoralis and Tellina cf. rousi were the only bivalves found alive in the estuarine lake during the latest survey, in March 2011.

In conclusion, 12 species from St Lucia have been conclusively reported in this study for the first time. The species Brachidontes semistriatus, Saccostrea cucullata, Pitar abbreviatus, Hiatella arctica, Solen capensis, Tellina triliatera, Tivela compressa and T. natalensis, despite having been mentioned previously in the literature for St Lucia, are not considered to be part of the bivalve fauna of the estuarine lake. Meretrix meretrix is a new record for South Africa, but the mode of its introduction into St Lucia is still unresolved. The large numbers of empty Barnea manilensis shells observed along the entire western shoreline suggest that this species has played a major role in settling suspended silt within the system. This role is now fulfilled only by Solen cylindraceus. Apart from this species, B. virgiliae, D. hepatica, M. moluccensis, S. litoralis and T. cf. rousi are the only bivalves that were found alive in the estuarine lake in 2011.