Checklist of the fresh and brackish water snails (Mollusca, Gastropoda) of Bénin and adjacent West African ecoregions

Abstract Currently no comprehensive checklist of fresh and brackish water gastropods from Bénin exists, and those for adjacent West African areas are outdated. Yet, such checklists provide essential biodiversity information and a consistent taxonomic and nomenclatural framework for that biodiversity. Here a first checklist of the fresh and brackish water gastropods from Bénin and adjacent West African ecoregions is presented, based on an extensive literature review and field surveys between September 2014 and June 2019 in six major fresh and brackish water ecosystems in Bénin. This inventory includes information on synonymy, species distribution in West Africa, habitats, and conservation status. The fresh and brackish water gastropod fauna includes 60 species, belonging to 28 genera and 16 families. Pachychilidae, Ampullariidae, Neritidae, and Bulinidae were the most diverse families with 9, 8, 7, and 7 species, respectively. However, literature and field data indicated that 23 species observed in West African basins that extend to Bénin do not occur in the territory of Bénin. These species were not detected in our field surveys, most likely because they are rare at collecting sites. Of the 60 species included, five are classified as “Data Deficient”, 43 as “Least Concern”, two as “Nearly Threatened”, one as “Vulnerable”, and six as “Endangered” by the IUCN, whereas the remaining three species were not evaluated. Because the taxonomy of fresh and brackish water gastropods in West Africa is still largely based on morphology, comparative molecular and taxonomic studies may result in substantial revisions of this checklist over the coming years.


Introduction
Mollusca are the second largest animal phylum on Earth, after Arthropoda, and comprise estimated numbers of 50,000-55,000, 25,000-30,000 and 6,000-7,000 of described and valid marine, terrestrial and freshwater species, respectively (Strong et al. 2008; MolluscaBase 2019b). The largest molluscan class, Gastropoda (83% of accepted mollusc species), has repeatedly and successfully colonized continental waters on all continents, except Antarctica (Strong et al. 2008;MolluscaBase 2019b). Despite their economic interest and ecological importance in many aquatic ecosystems (Wanninger and Wollesen 2019), our understanding of their biodiversity is far from complete, especially in developing countries, where expertise, resources and facilities for biodiversity studies are limited (Odountan et al. 2019a). A poor understanding of the biodiversity that underpins ecosystems and their functioning, hampers sustainable management. Indeed, as much legislative work depends on a validated overview of taxonomic biodiversity (Araujo and Jong 2015), biodiversity inventories are essential for the development of monitoring strategies and conservation policies. Moreover, with the growing need to understand natural resources and heritage, biodiversity checklists and databases have become essential tools facilitating communication between taxonomists, naturalist data managers, ecologists, geneticists, museum curators, conservationists, etc. Beyond consolidating taxonomic knowledge, they enable study and management at organismal and ecosystem level, making them essential for national and international conservation (Lydeard et al. 2004;Régnier et al. 2009). As result, there is an increasing demand from policy makers and managers to readily have access to datasets regarding biodiversity (Gofas et al. 2017).
Malacological investigations of fresh and brackish waters are uncommon in West Africa in general and in Bénin in particular. Adanson (1757) and Dautzenberg (1912) investigated the malacological fauna of Senegal and West Africa, respectively, but their works focussed mainly on shells of marine species. The freshwater gastropods of Bénin were studied for the first time by Germain (1917), based on collections by Henry Hubert made around the 1910s. The first identification guide of West African molluscs (from Mauritania to Angola) was published in 1950 (Nicklès 1950), but focused mainly on marine taxa. Towards the end of the 20 th century, several malacological stud-ies have been undertaken on freshwater and brackish taxa in West Africa and in Bénin (Sellin et al. 1980;Danish Bliharziasis Laboratory 1981;Maslin and Bouvet 1986;Le Loeuff 1992, 1993;Brown and Kristensen 1993;Le Loeuff and Zabi 1993). These regions were also covered in the first treatise on African freshwater snails (including some considerations on brackish species) on a continental scale (Brown 1980(Brown , 1994. These taxonomic papers are now becoming outdated, and in several respects inaccurate. Indeed, since the overviews by Brown (1980Brown ( , 1994 much taxonomic and faunistic progress has been made (e.g., Jørgensen et al. 2008;Hayes et al. 2015), but the results of these studies have not yet been compiled in an updated overview. Moreover, most research dealing with mollusc diversity in West Africa, including Bénin, focussed on ecology (e.g., Villanueva 2004;Gnohossou 2006;Adandedjan 2012;Odountan 2017;Zinsou 2017;Koudenoukpo 2018) or the transmission of human diseases (e.g., Ibikounlé et al. 2008Ibikounlé et al. , 2009Ibikounlé et al. , 2013Ibikounlé et al. , 2014aAgboho 2018;Onzo-Aboki et al. 2018). As a result, the taxonomic basis of many of these studies was not up to date. This also applies to the List of non-marine molluscs of Benin in Wikipedia (https:// en.wikipedia.org/wiki/List_of_non-marine_molluscs_of_Benin), which is incomplete, with outdated nomenclature, it does not include synonyms, and does not provide distributional and conservation information. As such, the Wikipedia list does not inspire much confidence (Kittur et al. 2008). Hence, a new solid and comprehensive synthesis is in order. Therefore, we here provide a comprehensive taxonomic overview of the fresh and brackish water gastropods of Bénin and adjacent West African ecoregions and compile an up to date biodiversity checklist for this fauna. This checklist was developed from literature study and verified through field surveys in six major fresh and brackish ecosystems in Bénin. It provides species synonymy, distribution and habitat data for West Africa, and conservation status. As such we hope that it will act as a reference and research tool for future taxonomic and biomonitoring studies.

Study region: Bénin and adjacent ecoregions
Bénin is located in West Africa between 6°15' and 12°25'N latitude and between 0°45' and 4°00'E longitude. Its neighbouring countries are Togo in the west, Burkina Faso in the north west, the Republic of Niger in the north (Niger River), and Nigeria in the east. In the south Bénin has a coastline of ~ 125 km along the Atlantic Ocean. Bénin extends from north to the south for ~ 700 km and its width varies between 125 km (along the coast) and 325 km (at the latitude of Tanguiéta). The country has a surface of 112.622 km² (Adam and Boko 1983) and a fairly large network of more or less permanent rivers and standing aquatic ecosystems. Generally, the rivers (e.g., Oueme River, Mono River) are modest in their flow regime and drain into the southern lentic system (e.g., Lake Nokoue, Lake Aheme). This aquatic network is subdivided into four basins, namely the Niger Basin (shared with Mauritania, Guinea, Algeria, Mali, Ivory Coast, Burkina Faso, Niger, Nigeria, Chad, and Cameroon), the Volta Basin (shared with Mali, Ivory Coast, Burkina Faso, Ghana, and Togo), the Oueme Basin (shared with Togo and Nigeria) and the Mono Basin (shared with Togo). Ecologically these watersheds also contain distinct natural communities, composed of different species with specific ecological dynamics, i.e., they represent distinct freshwater ecoregions (Abell et al. 2008;Graf and Cummings 2011). Sections of the same catchment system are sometimes subdivided into additional ecoregions, and matching freshwater ecoregions that have primarily been established for fish (Abell et al. 2008) with the Transboundary Freshwater dispute Database (https://tfddmgmt.github.io/tfdd/map.html). As such, Bénin and its immediate surroundings are covered by ecoregions 505-508 and 513-519 ( Fig. 1; Table 1), which form the geographical scope of our study.

Literature study
This checklist is based on a careful literature review to construct an up-to-date biodiversity inventory. These literature sources include peer-reviewed articles, books, reports, manuals, dissertations and other grey literature on the gastropods of Bénin, surrounding countries and their shared drainage basins. Indeed, the development of such a corpus of literature requires the collection of heterogeneous, sometimes contradictory, not to say conflictual, taxonomic opinions across a wide variety of publications.

Field surveys
We supplemented the literature-based biodiversity inventory with field sampling in Bénin. Sampling was conducted in the Sô River, the Oueme River, Lake Nokoue, the Porto-Novo Lagoon, Lake Aheme and the Coastal Lagoon of Ouidah Grand-Popo. In total, 94 field excursions (24, 22, 12, 12, 12, 12, respectively), each of two days per waterbody, were organized between September 2014 and June 2019. Eight to twelve sampling sites were defined within each waterbody to cover a wide range of subhabitats. These field surveys were performed with an Ekman grab (0.0225 m 2 ) and a long-handled kick net (250 µm mesh). Specifically, we inspected the littoral area, the deeper zones, within/under aquatic macrophytes and other environments suitable for molluscs. Snails were put in formalin in prelabelled plastic containers. These containers were subsequently transported to the laboratory, where the snails were washed, and identified using appropriate identification keys (Nicklès 1950;Durand and Lévêque 1981;Brown and Kristensen 1993;Brown 1994) and compared with reference specimens from Dahomey (former name of Bénin) and Bénin (if available) deposited in the collections of the Royal Belgian Institute of Natural Sciences (RBINS) and the Royal Museum for Central Africa (RMCA).

Data compilation
All taxa included in this study were cross-checked for their original name combination, synonymies, type locality data, habitats and dubious records against MolluscaBase (accessed at http://www.molluscabase.org during October 2019), and the Worldwide mollusc species Data Base (WMSD accessed at http://www.bagniliggia.it/ during October 2019) when MolluscaBase did not provide the required data. In addition, the conservation status of each species was determined from the IUCN red list (accessed at https://www.iucnredlist.org/ during October 2019). The discussion of the occurrence and conservation status of species whose geographical distribution extends beyond the targeted ecoregions, is mainly limited to the ecoregions covered here. We based our systematic arrangement of subclasses and orders on Bouchet et al. (2017), whereas families, genera and species are listed in alphabetical order.

Systematic
Evidence in Bénin. Along the coast of the Atlantic Ocean (Kristensen and Stensgaard 2010e).
IUCN status. Least Concern. https://www.iucnredlist.org/species/40087/10303057 Remarks. The last whorl encloses earlier whorls almost completely and the lip is commonly expanded in two wing-like projections which appear to be most fully developed in freshwater (Pilsbry and Bequaert 1927). The species is widely distributed and observed beyond our region of interest in countries such as Liberia, the DR Congo and Angola (Brown 1994 (Binder 1968;Sankaré and Etien 1991;Brown and Kristensen 1993;Le Loeuff and Zabi 1993;Guiral et al. 1999;Kouadio et al. 2008Kouadio et al. , 2011Olomukoro and Azubuike 2009 Brown 1994;Guiral et al. 1999;Kouadio et al. 2008Kouadio et al. , 2011MolluscaBase 2019d).
Evidence in Bénin. Widespread especially at Cotonou garden ASECNA, Toho-Todougba lake, Sèhouè Hlan lake and Acron (Ibikounlé et al. 2009 Observed in our field data in ecoregion 517. Lanistes varicus is an intermediate host for non-human schistosomes and is often investigated by parasitologists (Ibikounlé et al. 2009). It is an edible species consumed by humans in Bénin (Koudenoukpo 2018). It usually is abundant in natural permanent water bodies. Lanistes guinaicus mutation depressa Germain, 1917 (513-514) from Région des Tchis, cercle de Mono and Tchaourou (misspelled as Ichaourou)] is an unavailable name because of its infrasubspecific nature (Code, Art. 45.6, Glossary) (Cowie 2015). This taxon seems to be referable to L. varicus, but specimens of L. varicus from the localities mentioned by Germain (1917) should be further studied to elucidate the status of L. guinaicus mutation depressa. L. varicus as in Adandedjan (2012)  Remarks. This species is common in the Niger River from Mali to Nigeria and therefore could be present in Niger tributaries in Bénin, namely the Mékrou, Alibori and Sota. It seems that reports from coastal environments in West Africa are misidentifications. Köhler and Glaubrecht (2006) designated a lectotype (Museum für Naturkunde, Berlin: ZMB 1335), a paralectotype exists apparently at the Museo Nacional de Historia Natural, Santiago, Chile (MNHNCL). This species has, like P. ovata, a wide geographical distribution, which, however, needs to be examined with molecular data.

Type locality. Alexandria, Egypt.
Habitat. Freshwater. Distribution. The species is widely distributed in the northern hemisphere part of sub-Saharan Africa, and along the Nile (Brown 1994). In West Africa it occurs in Burkina Faso and Nigeria (Gadzama 2012;Gadzama et al. 2015;Ouedraogo et al. 2015Ouedraogo et al. , 2018 Observed in our field data in ecoregion 517. Previous records of this species were all in freshwater habitats, whereas the specimens reported in Bénin occurred in brackish water. Brown (1994) suggested that Bithynia tournieri Binder, 1955 may be conspecific, which is followed here, but in the absence of molecular studies the systematics of Gabbiella are very poorly known. The contrast in habitat between previously recorded specimens and those from Bénin could be indicative for specific differences, but until compelling evidence indicates otherwise, we consider the Bénin specimens conspecific.
Synonyms. -. Type locality. South East shore at Bol in Lake Chad, Chad.
Habitat. Freshwater. Distribution. Tchad and Nigeria (Brown 1994 Remarks. This species occurs in the catchment of Lake Chad, including the Komadugu Yobe River. This catchment falls beyond the ecoregions under study here, but the taxon is considered to have had a more extensive Late Pleistocene-Holocene distribution in the Chad Basin (Van Damme 1984). Additionally, recent specimens have been reported also from Lake Léré on the border between Cameroon and Chad (Brown 1994), which is part of the Niger Basin and the reason for inclusion here. As for Bithyniidae, the lack of knowledge on the anatomy of hydrobiid species combined with a lack of molecular studies currently hampers confident systematic placement of African Hydrobiidae (see e.g., Seddon et al. 2011). Binder, 1955 Original combination. Hydrobia guyenoti Binder, 1955. Synonyms. -. Type locality. Toupah Bay in Lagune Ebrié, Ivory Coast.

Hydrobia guyenoti
Habitat. Fresh and brackish water. Distribution. Ivory Coast in Lagune Ebrié (Brown 1994 Remarks. This species endemic to West Africa is not mentioned by MolluscaBase but is included in WMSDB and regional reports (Smith et al. 2009;Adandedjan 2012). It may be more widespread than previously assumed. It is one the smallest species of the genus: 2.7×1.8 mm. The whorls are strongly convex with a deep suture. The central radular tooth has a single basal denticle on each side and long lateral lobes (Brown 1994 Observed in our field data in ecoregion 517. Only one specimen was observed and that was empty shells, not a living specimen. The species is known from fossils only according to MolluscaBase (2019c), but some authors reported extant specimens (Smith et al. 2009;Badahoui et al. 2010). The species requires taxonomical study (Seddon et al. 2011 (Rosewater 1981). Evidence in Bénin. Cotonou (Rosewater 1981). IUCN status. Not Evaluated.
Remarks. Observed in our field data in ecoregion 517. Specimens from mangroves of the Coastal lagoon of Ouidah, Grand-Popo that have been assigned to Littoraria scabra (Linnaeus, 1758) by Adandedjan et al. (2012) seem to be referable to Littoraria angulifera (Lamarck, 1822). Littoraria scabra is very polymorphic, but endemic to the Indo-West Pacific region (Reid et al. 2010).
Subcohort Cerithiimorpha Golikov & Starobogatov, 1975 Remarks. A temporary order named Caenogastropoda has been established (Mollus-caBase 2019a) to group a number of superfamilies that belong to the Subclass Caenogastropoda but not to the Order Littorinimorpha. This group has previously been referred to as the subcohort Cerithiimorpha (Bouchet et al. 2017 (Brown 1994). The phylogenetic affinities of taxa from both regions are currently unknown. Especially the West African taxa, several of which occur in the ecoregions under study here, display high morphological disparity. Many of the endemic Potadoma species from Cameroon fall just beyond the boundaries of the ecoregions considered here, such as P. zenkeri (von Martens, 1901 (Brown 1994;Owojori et al. 2006;Kouadio et al. 2008).
Evidence in Bénin. Reported by Kristensen and Stensgaard (2010e). IUCN status. Least Concern. https://www.iucnredlist.org/species/175120/7099504 Remarks. P. freethi is the type species of the genus, by original designation (Gray 1847). Observed in our field data in ecoregion 517. Several subspecies, such as P. f. dykei Spence, 1925 and P. f. guineensis Reeve, 1860 have been described and these are included here. Two other subspecies have been described from Central Africa (DR Congo), i.e., P. f. tigrina Connolly, 1938 and P. f. graptoconus Pilsbry & Bequaert, 1927, but it seems doubtful these would belong to P. freethi given the disjunct distribution of the genus Potadoma. Melania conulus Lea & Lea, 1851, is another species described from Fernando Po of which the original description is similar to P. freethi, but more research is required before we can confirm it to be a synonym. Therefore, M. conulus is considered to be a "taxon inquirendum" (MolluscaBase 2019a: taxon 1115355).
Evidence in Bénin. Observed during our field data in ecoregion 517. IUCN status. Least Concern. https://www.iucnredlist.org/species/175131/7106773 Remarks. Observed in our field data in ecoregion 517. Cleopatra bulimoides is conchologically a highly polytypic species, with many nominal species in possible synonymy, such as C. cyclostomoides (Küster, 1852) and C. congener Preston, 1913. The species boundaries of C. bulimoides need to be explored by molecular methods.  (Brown 1994).
IUCN status. Least Concern. https://www.iucnredlist.org/species/165594/17211568 Remarks. Observed in our field data in ecoregion 517. Some authors have erroneously used "1834" as the year of publication. The species is native to Asia, and has been introduced into West Africa by man (Kristensen and Ogunnowof 1987). Originally, these introductions were to Ivory Coast and Nigeria, but our data suggest that the taxon is spreading in West Africa. Although Indoplanorbis hosts Schistosoma species that parasitise domestic livestock in Asia, no evidence exists to our knowledge that it transmits schistosomes in Africa.  (Gadzama 2012;Koné et al. 2013;Salawu and Odaibo 2014;Diakité et al. 2017).
Evidence in Bénin. Djèffa and Ganhatin (Assogba and Youssao 2002), Acron, Baaka, Cotonou ASECNA garden, Cotonou beach temporary ponds, Lake Nokoue, Lake Toho-Todougba, and Sèhouè bridge (Ibikounlé et al. 2009;Agboho 2018), Nikki, Pehunco and Pèrèrè towns (Ibikounlé et al. 2014a, b) IUCN status. Least Concern. http://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T165761A120112796.en. Remarks. Observed in our field data in ecoregion 517. Some authors have erroneously used "1948" (Agboho 2018) as year of the publication of the name. The species occurs throughout most of Africa, including Madagascar, several islands in the Indian Ocean and Arabia. It usually lives in permanent waters and it is rare in seasonal habitats, unless they are directly connected to permanent waters. Molecular work is required to examine whether R. natalensis indeed has a very wide geographical distribution or whether it consists of several cryptic species that have been lumped together.  (Okafor and Ngang 2008;Salawu and Odaibo 2014;Diakité et al. 2017;Abe et al. 2018;Ouedraogo et al. 2018).
Evidence in Bénin. Widespread especially at Djèffa and Ganhatin (Assogba and Youssao 2002), Toho Todougba Lake, Kpinnou Lake, Sonon, Nikki, Péhunco and Pèrèrè towns, Sô Ava (Ibikounlé et al. 2009(Ibikounlé et al. , 2013(Ibikounlé et al. , 2014aAgboho 2018  , and it was never found in the same site as B. pfeifferi (Brown, 1994 Remarks. Observed in our field data in ecoregion 517. The taxonomy of African Gyraulus is poorly known, but Meier-Brook (1983) found the African species G. costulatus and G. connollyi to have distinct anatomical characteristics that warranted him to place them in the subgenus Caillaudia Bourguignat, 1883. So far, this alternate representation has not been formally accepted.
Genus Hovorbis Brown & Mandahl-Barth, 1973 The genus was formerly known as Afrogyrus Brown and Mandahl-Barth, 1973, which however is an invalid junior homonym of the coleopteran genus Afrogyrus Brinck, 1955. Özdikmen andDarilmaz (2007) (1757). Several potential synonyms are mentioned in Brown (1994), but more study of these taxa is required to verify their status. Remarks. This species can be readily distinguished from S. angustus by its depressed shell with strongly carinated periphery. It is sometimes found together with S. angustus, but also occurs in temporary waters (Brown 1994

Discussion
This study provides the first checklist of fresh and brackish water gastropods in Bénin and adjacent ecoregions, i.e., ecoregions 505-508 and 513-519 of Abell et al. (2008). It comprises a total of 60 species, classified in 28 genera. More specifically, Architaenioglossa, Cerithiimorpha, Cycloneritida, Ellobiida, Hygrophila, and Littorinimorpha comprise 9, 19, 7, 1, 17, and 7 species, respectively. From the 16 families listed, Pachychilidae, Ampullariidae, Neritidae, Bulinidae, and Thiaridae were the most diverse with 9, 8, 7, 7, and 6 species, respectively. Of the 60 species listed, 37 are recorded (sometimes uncertain) in Bénin (~ 62 %), indicating a considerable species richness. The high richness in Pachychilidae relates to the diversity within the genus Potadoma, whereas the high richness in Ampullariidae relates to the diversity within the genera Pila and Lanistes throughout (sub-)tropical Africa (Cowie 2015). However, almost half of the Pachychilidae and one third of the ampullariids that are recorded in this study have not been recorded directly from Bénin. The fact that only a small part of Bénin's aquatic environments, especially around the Niger River, the largest river in West Africa, have been sampled might explain why some species that are broadly distributed in West Africa such as Pila wernei (endemic to the Niger River basin) and Bulinus senegalensis O. F. Müller, 1781 have not been detected in our sampling. Our findings with literature-based data also provoked some taxonomic concerns, because several papers on the fresh and brackish water malacofauna of Bénin or West-Africa, contained several (nomenclatural) errors. A case in point is the erroneous listing of Codakia orbicularis (Linnaeus, 1758), Cardita calyculata (Linnaeus, 1758), Thais coronata califera (Lamarck, 1822), Thais nodosa (Linnaeus, 1758), Turritella Lamarck, 1799, Polinices Montfort, 1810, Patella Linnaeus, 1758 as non-marine gastropod species in Bénin (e.g., Adandedjan 2012). These taxa were excluded in this study. In addition, we were unable to find information on Melanoides anomala (Smith, 1877) reported from Bénin , and this taxon was consequently omitted. This identification seems to refer to Melanoides anomala (Dautzenberg & Germain, 1914), which has its type locality in the DR Congo and is endemic to the Congo Basin (Brown 1994). The difference in authorship and the report of the species outside its known region is suspect and calls for verification. Similarly, records that cannot be checked, e.g., Lanistes ovum (Agblonon Houelome et al. 2017) because specimens have not been illustrated and nor deposited in publicly accessible institutions, should be treated with caution. Hence, until compelling evidence indicates otherwise, we regard such doubtful species records in the literature as misidentifications.
Although only four species are threatened (Endangered/Vulnerable), a significant number of species has been assessed as Data Deficient, Not assessed or Not applicable. One of the main reasons for Data Deficiency in molluscs is taxonomic uncertainty and poor geographic knowledge (Seddon et al. 2011). Moreover, in West Africa, there are only few, reliable, recent survey data available, so that more species were marked as Data Deficient (Seddon et al. 2011). Therefore, a large field inventory is required that should focus on diverse habitats of fresh and brackish water from North to South with the possibility of molecular analyses. Moreover, species such as Afrophysa brasiliensis (Küster, 1844), Lanistes guinaicus mutation depressa Germain, 1917, Lanistes chaperi (Kobelt, 1912, Lanistes ovum Troschel, 1845, Physa (Aplecta) waterloti Germain, 1911, Pila ovata (Olivier, 1804, and Radix natalensis (Krauss, 1848) need further taxonomic study.
Bénin and its transboundary basins present a diversified fresh and brackish water gastropod fauna. The current checklist contains information on 60 species. However, many of these species require more detailed taxonomic and phylogenetic scrutiny, our current knowledge remains in its infancy. This checklist is hence an updated baseline for further taxonomic and ecological studies of the fresh and brackish water gastropods of Bénin and adjacent West African ecoregions.