Cave-dwelling gastropods (Mollusca: Gastropoda) of Brazil: state of the art and conservation

. An up-to-date list of exclusively cave-dwelling gastropod species recorded in Brazil is presented including updated taxonomy, detailed geographic information, and illustration of types. The list includes 18 cave-exclusive (troglobitic) gastro-pods encompassing 15 land and three freshwater species, with the status of further species pending additional studies. Their unusual morphology and diversity are discussed, as well as their conservation status and prospects in the current Brazilian environmental and political scenario.


INTRODUCTION
There are circa 700 terrestrial and around 260 freshwater gastropod species reported from Brazil (Simone 2006, Birckolz et al. 2016, Salvador 2019).Most are endemic, naturally, as would be expected from a country with continental proportions and very diverse biomes, but many also inhabit neighboring countries in South America, with a few even extending to Central America (Simone 2006, Birckolz et al. 2016).Even so, knowledge about land and freshwater gastropod species is still incipient in Brazil (Salvador 2019) and that problem is even more pronounced for the cave-dwelling molluscan fauna (Cavallari et al. 2021).
The lists of cave-dwelling (exclusively troglobitic and other categories) invertebrates in Brazil historically had shortcomings regarding gastropods.Most checklists presented identifications reaching family or genus level only, and species-level identifications were in many cases indicated in open nomenclature (cf.) due to uncertainties about those species' identities (Gnaspini and Trajano 1994, Pinto-da-Rocha 1995, Trajano and Bichuette 2009).Although there have been records of more widespread species also inhabiting caves, the first exclusive troglobitic gastropod in Brazil was only described in the 1990s: Potamolithus troglobius Simone & Moracchioli, 1994 -but see the discussion of Zilchogyra paulistana (Hylton Scott, 1973) below.
We hope to start addressing this shortfall in the literature by providing a more complete checklist of exclusive cave-dwelling (troglobites sensu Schiner-Racovitza, 1907) land and freshwater gastropods known from Brazil.We also update the taxonomy of some species, present detailed information on geographic distribution, and illustrate the type specimens.Finally, we propose a discussion about the morphology and diversity of cave-dwelling gastropods in Brazil, the threats they are exposed to, and prospects for conservation and follow-up studies.

MATERIAL AND METHODS
To produce the present list, we conducted a literature survey of the main checklists and catalogues of Brazilian mollusks (Morretes 1949, 1953, Salgado and Coelho 2003, Simone 2006, Birckolz et al. 2016) and troglobitic invertebrates (Gnaspini and Trajano 1994, Pinto-da-Rocha 1995, Trajano and Bichuette 2009, Gallão and Bichuette 2018), as well as more recent species descriptions and regional checklists (see below).We present here an up-to-date inventory of cave-dwelling gastropods in Brazil, excluding troglophilic species (i.e., that also live outside caves).
The list is organized in systematic order, with the classification updated according to Bouchet et al. (2017), Lydeard and Cummings (2019), and more specific revisionary works (Salvador et al. 2020, Simone andSalvador 2021).We also provide information on each species' type locality and distribution, as well as photographs of their type specimens.Additional remarks are provided as needed.
Cave maps were produced using the software QGIS (version 3.6.0;QGIS Development Team 2019), considering the coordinates in the original descriptions and also from MEB expeditions for collections.Besides the occurrences of gastropods, geomorphological information is also detailed in the maps.
Distribution: Known only from the type locality.
Distribution: Known only from the type locality.
Remarks: Given its ample distribution, this species is probably troglophilic and not strictly troglobitic, but specimens have not been recovered on surface environments as of writing.
Distribution: Known only from type locality.Remarks: The genus was recently reassigned to Cochliopi dae (Simone and Salvador 2021).
Distribution: Known only from type locality.
Remarks: Bichuette and Trajano (2018) list 12 morphospecies of Potamolithus in the same overall area in the Upper Ribeira Valley as P. troglobius above and the related troglophilic P. karsticus Simone & Moracchioli, 1994 (known from Calcário Branco cave, including epigean; Fig. 10).Of those 12 morphs, five are deemed to be troglobites, six troglophiles, and one epigean.Potamolithus spp.are restricted to micro-basins and/or caves, showing small areas of distribution and probably a high degree of endemism.Whether they are indeed new species remains to be fully investigated.Type locality: Piauí state; Serra da Capivara, Coronel José Dias municipality, Toca de Cima dos Pilão cave (8°51'47"S, 42°33'27"W).

Eupulmonata
Distribution: Known only from type locality.

Punctoidea Cystopeltidae
Zilchogyra Weyrauch, 1965 Zilchogyra paulistana (Hylton Scott, 1973) Type locality: São Paulo state.Distribution: The original work of Hylton Scott (1973) did not provide precise locality data for the species and there was scarce additional information accompanying the type specimens (holotype and 2 paratypes MCN 1054; and paratype MACN 27622).Further literature, however, indicates that the species is found in caverns (e.g., Gnaspini and Trajano 1994) in São Paulo state, more specifically in Iporanga municipality, which is part of the Upper Ribeira Valley, famous for its multiple caves (e.g., Salvador et al. 2016).Nevertheless, further specimens have imprecise locality data: Fonseca and Thomé (1993) referred to shells from Cerro Azul municipality in Paraná state (erroneously listed as paratypes by those authors), but no additional information was given.Considering that Cerro Azul is also part of the Upper Ribeira Valley (and just 70 km SW of Iporanga), it is impossible to exclude the possibility that the specimens were collected in one of the many caves in the region.As such, the status of this species as a troglobitic remains uncertain.
Remarks: The placement of this species in Cystopeltidae follows the molecular phylogeny of Salvador et al. (2020), which removed the members of genera Zilchogyra and Lilloiconcha Weyrauch, 1965 from Charopidae and included them in Cystopeltidae.Those authors raised the possibility that a smooth protoconch could be a diagnostic character for South American cystopeltids.Type locality: Piauí state; Serra da Capivara, Coronel José Dias municipality, Toca de Cima dos Pilão cave (8°51'47"S, 42°33'27"W).

Rhytidoidea
Distribution: Known only from type locality.
Distribution: Known only from type locality.

Morphology
There are a variety of reasons that can lead individual animals and populations to inhabit caves, such as exploitation of alternative resources (e.g., food sources, mating sites), usage of caves as temporary or seasonal refugia, and chance (Weigand 2014).Speciation of troglobitic lineages can establish morphological traits that are considered typical of cave environments.
Several of the species listed here show some of those classical troglomorphisms, such as reduction/absence of eyes and lack of body/periostracum pigmentation (i.e., white body and typically translucent shell), well exemplified by H. strangei and S. punctata (Figs 5,7).Such morphological features are common in virtually all terrestrial and freshwater cave-dwelling snail lineages worldwide (e.g., Boeters 1979, Weigand 2013, Delicado 2018), but not mandatory, as not all troglobitic species develop troglomorphisms (Romero 2009, Christiansen 2012, Trontelj et al. 2012).Case in point, many of the troglobitic species listed here (more than half of which are stylommatophorans; Figs 12-22) do not present obvious troglomorphisms; however, we should caution that most are known only from their shells.Besides, physiological and "unconventional" anatomical troglomorphisms (e.g., specializations in sensory organs, elongation of tentacles) have not been investigated yet, so some of these may be identified as new live specimens and additional data become available.
Miniaturization and morphological simplification are also common in troglobitic species (Romero 2009, Christiansen 2012, Gladstone et al. 2021) and can be seen in some Brazilian species in the form of unsculptured and unornamented shells .Potamolithus troglobius (Fig. 11) is an example of miniaturization, where some organs (e.g., intestine, ctenidium) are simplified or absent (Bichuette and Trajano 2018).
However, a few species listed here exhibit "extreme" morphologies when compared to their close relatives, like the twisted body whorl of Clinispira insolita (Fig. 20), the teleoconch sculpture of Spiripockia spp.(Figs 7,8), and the dextral shells of the diplommatinids (unusual in this typically sinistrally-coiled family; Figs 3-6).Similar unusual sculptures and shapes, though uncommon, are also known from troglobitic snails worldwide (e.g., Khalik et al. 2018).Nevertheless, these features may be related to other factors in the snails' life histories that are not exclusive to cave environments.For instance, the shell shape of Clinispira insolita is possibly related to a better attachment to rock surfaces when retracted, as reported from unrelated species with similar morphology (Páll-Gergely and Neubauer 2020).

Diversity
While there are many gastropod species known to inhabit caves in Brazil, most are not restricted to cave environments.Typically, those species are widespread in surface environments, but are also able to inhabit caves -some even down to the aphotic zone (e.g., Salvador and Simone 2016, 2021, Salvador et al. 2016, 2017, 2021a, Bichuette and Trajano 2018).There are relatively few (around 1.5%) exclusively subterranean species known from Brazil, as our results show.
Despite that reduced number, there are five exclusively subterranean genera, distributed among five families.Those genera are: Clinispira, Habeas, Habeastrum, Lavajatus, and Spiripockia.Only the latter inhabits freshwater habitats; all other genera are terrestrial.This does not reflect a high diversity, however, as these genera (except for Habeas) are either monotypic or include very few species.Effectively, this apparent high number of troglobitic genera simply indicates that the differences in morphology and anatomy found in those animals have been deemed large enough to justify the creation of new genera (Simone 2012, 2013, 2018, 2019, Simone and Casati 2013).
In total, there are 15 land and three freshwater exclusively troglobitic snail species known from Brazil; no slug or semi-slug has ever been reported as troglobitic in the country.These numbers are potentially larger, pending the assessment of Zilchogyra paulistana and the undescribed morphs Potamolithus spp., excluded from the total above.Nevertheless, this is a small number in comparison to circa 950 known species of continental gastropods in Brazil (Simone 2006, Salvador 2019).Moreover, given the extension of the Brazilian territory and a large number of caves (circa 20,000, over half of which are calcareous; CECAV 2020; Figs 23-28), this number is certainly a gross underestimation.As alluded to by Salvador (2019), caves are among the least explored habitats in malacological studies in Brazil, a bias that is also present in other countries (e.g., Gladstone et al. 2018, Czaja et al. 2020).In Brazil, our knowledge is biased toward a relatively low number of caves in the eastern portion of the country (44 caves, Fig. 29).When that number is considered taking the total number of caves and the sheer territoral extension of the country into account (Fig. 29), the lack of malacological works focused on cave environments in Brazil becomes immediately evident.As such, it can be surmised that many species remain undiscovered (e.g., Bichuette and Trajano 2018, Salvador 2019).Still, there is a vast quantity of material (both recently-collected or already in museum collections and not fully processed yet) to be studied in more detail.
Caves are potential hotspots for molluscan diversity, especially of freshwater snails (Culver and Sket 2000, Sket 2006, Grego et al. 2000, Gladstone et al. 2021), and there have been studies pointing toward cryptic species in cave systems in Brazil.For instance, in caves in the Upper Ribeira Valley in southeastern Brazil (Figs 23,25,28), Bichuette andTrajano (2003, 2018) have identified several morphotypes of Potamolithus sp., though species limits need to be investigated further to assess their validity, preferably using molecular data.As cave environments are being better explored, the rate of description of troglobitic species is also picking up, as seen by the description dates of most species listed here (largely, from the past decade).Hopefully, this trend will continue to improve.
An interesting feature of cave molluscan fauna in Brazil is that the most diverse and abundant taxa are Subulininae and Scolodontidae (Salvador et al. 2016(Salvador et al. , 2021a)), contrary to the "regular" fauna of the country, which is dominated by Orthalicoidea (Salvador 2019).Nevertheless, all the subulinine and scolodontid species reported so far from caves are widespread species; not a single one is exclusively cavernicolous.Meanwhile, the present list counts six orthalicoid species that so far are known only from caves.

Threats and conservation prospects
As alluded to above, there likely are many new troglobitic gastropod species awaiting discovery and formal description.A similar scenario has been proven true for other troglobitic taxa such as isopods, spiders, and fishes (Gallão and Bichuette 2018).Cave species typically have a narrow geographic distribution, with many being endemic to one or very few caves or aquifers, which makes them immediately prone to extinction.Given that cave environments can be fragile ecosystems and that they are being progressively explored in Brazil (Gallão andBichuette 2018, Cavallari et al. 2021), troglobitic species throughout the country might be facing several threats and require protection.Some impacts are widespread in Brazil (see Gallão and Bichuette 2018), such as pollution of waters due to agriculture and mining activities, poorly controlled tourism, deforestation, changes in local hydrology, etc.Another important but often overlooked threat to the subterranean fauna is over-collection (Bichuette and Trajano 2010), which must be avoided given that population sizes of these species are typically small.Presently, there are only three cave-dwelling gastropod species listed on the Brazilian Red List (Gallão andBichuette 2018, ICMBio 2018), categorized (according to the guidelines of IUCN 2012) as follows: Spiripockia punctata (a troglobitic species), endangered (EN); and Potamolithus karsticus (troglophilic) and P. troglobius (troglobitic), critically endangered (CR).Even though other troglobitic gastropod species (and other invertebrates) are not on the list of threatened fauna, they certainly face several threats (see Gallão and Bichuette 2018 for details).For example, the region of Serra do Ramalho in Bahia state (Fig. 24) has threats mainly related to agricultural expansion and mining; the genus Spiripockia is endemic to caves of that region, as well as Kora nigra and some species of Habeas.Another region with a high diversity of gastropods is Presidente Olegário in Minas Gerais state (Figs 26, 27;Salvador et al. 2021a), for which several impacts have been observed, such as pollution by pesticides and large-scale deforestation for plantations (e.g., Eucalyptus, sugar cane; M.E.Bichuette, pers.obs.).The Upper Ribeira Valley, home of Gonyostomus elinae and Bahiensis ribeirensis, is threatened by poor use of natural resources and uncontrolled ecotourism (Salvador et al. 2016).Very recently, a new government decree allows the destruction of any cave in Brazil for "public utility enterprises" (e.g., mining, energy, transport), which is in line with the destructive policies of the current far-right Brazilian administration (Luiz 2022).
To enforce the protection of this particular and fragile cave-dwelling fauna, systematic collection, taxonomic works, and long-term monitoring projects are fundamental (Cavallari et al. 2021), such as has been done for P. troglobius (Bichuette and Trajano 2003).Those projects also must be allied to outreach and educational initiatives aiming to develop public awareness, since science communication about invertebrates can play a critical role in their conservation (Salvador et al. 2021b).