1 Introduction

African land snails have a unique and important position in the entire ecosystem. African land snails can help decompose plant and animal remains and participate in the formation of soil. The organic matter formed can be recycled in the ecosystem and improve and maintain soil structure (Liu et al., 2020; Hamid et al., 2023). After the death of African land snails, their shells can also become "houses" for other small animals, further increasing the biodiversity in the ecosystem (Koch, 2006). Although African land snails have a very important ecological influence, human research on their specific species is not in-depth and specific enough. The main reason is that many different species of snails do not differ much in appearance, which is a phenomenon of "hidden diversity" (Pinceel et al., 2004; Dennis and Hellberg, 2010; Malaquias et al., 2016; Cole et al., 2019; Gretgrix et al., 2023). Traditional research methods only distinguish species by appearance, and it is difficult to find some small differences. Sometimes different snails are mistaken for the same species (Pinceel et al., 2004; Malaquias et al., 2016; Cole et al., 2019). These terrestrial snails are usually only suitable for survival in a specific small environment. They only adapt to a certain humidity, leaf litter or stone crevices. This phenomenon is called "microhabitat specialization". Under "microhabitat specialization", coupled with the small range of movement of terrestrial snails, populations in different places become very different. This genetic difference may lead to the emergence of "endemic species" in some areas, that is, snails that only exist in this place (Cole et al., 2019; Gretgrix et al., 2023; Hamid et al., 2023).

This review wants to sort out the various new results in the field of African terrestrial snail research in recent years, especially the content related to hidden diversity and microhabitats. Through these studies, we want to look at the distribution of African land snails in tropical and subtropical forests and the ecological roles they play in their respective environments. The article will also discuss the current challenges encountered in the study of African land snails and use some examples to illustrate the importance of microhabitat and cryptic diversity for species surveys and conservation efforts (Malaquias et al., 2016; Cole et al., 2019; Gretgrix et al., 2023; Hamid et al., 2023).

This article focuses on land snails living in African forests, including their genetic differences, evolutionary relationships, how they use microenvironments, and their role in ecosystems. By analyzing different data types (such as molecular information, morphological characteristics, and ecological records), we hope to understand how the diversity of African terrestrial snails was formed and maintained (Malaquias et al., 2016; Cole et al., 2019; Liu et al., 2020; Gretgrix et al., 2023; Hamid et al., 2023).

2 Concepts and Definitions

2.1 Cryptic diversity: Definition, significance in biological research

Cryptic diversity refers specifically to species that do not differ significantly in appearance but have great genetic differences. If the traditional method of observing by appearance is used, some different species will be missed, causing researchers to misestimate the true number of species (Gélin et al., 2017; Cunha et al., 2022). By using other research methods to find species that are ignored in traditional research methods, we can more accurately understand the number of African terrestrial snail species, their evolutionary process, and the role they play in the ecosystem, which is particularly helpful for ecological protection and management (Gélin et al., 2017).

2.2 Microhabitat specialization: How niche differentiation drives speciation

Microhabitat specialization refers to the fact that some species are particularly adapted to a specific microenvironment, such as living only in leaf litter, under tree roots, or in stone cracks. This dependence on the microenvironment will gradually make different populations different, and eventually even become new species (Brousseau et al., 2020; Wang et al., 2024). For example, some species have different appearances and genes in different microenvironments, which indicates that they have changed when adapting to different environments. This phenomenon may also occur in African terrestrial snails, allowing them to form their own unique lineages in different places and increase hidden diversity.

2.3 Related concepts: Phenotypic plasticity vs. genetic divergence

Phenotypic plasticity means that the same snail will grow differently in different environments. Genetic differentiation means that their genes have really changed (Schneider and Meyer, 2017; Yousefi et al., 2017; Li et al., 2021). Both changes are important for snails to adapt to the environment and form new species. At first, snails may just adapt to the new environment through plasticity. Over time, some changes may become inherited, eventually allowing them to differentiate into different lineages (Schneider and Meyer, 2017; Yousefi et al., 2017). But it should also be noted that some of the changes that appear to be caused are actually just caused by the environment, not by genetic changes. Therefore, when we study species evolution, we must distinguish between the two (Cunha et al., 2022; Cotoras et al., 2021).

2.4 Importance for conservation biology

Recognizing and understanding cryptic species and niche specialization is an important measure for biological conservation. Some hidden species may play a special role in the ecosystem and may be more adaptable to the environment than we think (Gélin et al., 2017; Brousseau et al., 2020). If we ignore these cryptic species, our conservation measures will not be perfect, and we may even miss the opportunity to save endangered species. Only by considering genetic information and ecological characteristics together can we more accurately identify species and formulate conservation plans more effectively (Gélin et al., 2017; Brousseau et al., 2020).

3 Overview of African Terrestrial Snail Diversity

3.1 Current taxonomic understanding: major families and genera

There are many species of terrestrial snails in Africa, distributed in many families and genera. Taking the Mfamosing limestone hills in Nigeria as an example, researchers found 23 species of snails belonging to 7 different families. Among them, Streptaxidae has the most species, while Urocyclidae has the largest number, which means that there are a lot of individuals (Eguakhide et al., 2023). In South Africa, there is a genus called Gittenedouardia in Cerastidae, which has a lot of species (Raphalo et al., 2020). Philinidae in West Africa shows very complex evolutionary relationships and many hidden species (Malaquias et al., 2016). The Achatina and Archachatina genera under Achatinidae are large in size and are often used for food or for money (Ohimain et al., 2025).

3.2 Diversity hotspots: congo basin, guinean forests, eastern arc mountains

Africa’s tropical and subtropical forests are the places where terrestrial snails are most concentrated, that is, “diversity hotspots”. In the Nyungwe Forest in Rwanda, researchers found as many as 102 species of snails, which may be one of the places with the most terrestrial snails in Africa. The species of snails there vary greatly, and many of them only appear locally (Boxnick et al., 2015). The Mfamosing limestone hills mentioned above also have a high number of species and unique species in Nigeria, and are another important area (Figure 1) (Eguakhide et al., 2023). In the Maputaland–Pondoland–Albany region of South Africa, many snail species that are only distributed there have also been found, and they are clearly isolated geographically and genetically (Raphalo et al., 2023).

3.3 Knowledge gaps and taxonomic challenges

At present, our understanding of African terrestrial snails is not enough, and there are many difficulties in classification. Many places have not been sampled yet, so we don’t know how many species there are (Raphalo et al., 2020; Vijayan et al., 2022). Through genetic research, it was found that many snails that look similar are actually very different in genes, and it is difficult to distinguish them by traditional methods that only look at their appearance (Malaquias et al., 2016; Raphalo et al., 2020). For example, the genus Gittenedouardia showed in genetic analysis that many new species have not been formally described and need to be reclassified (Raphalo et al., 2020). There are also many hidden species in Philinidae in West Africa, which can only be clearly identified by genetic data (Malaquias et al., 2016). In addition, the Bellamya genus freshwater snails in the East African lake region also show strong genetic differences between different lakes, indicating that their evolutionary history is also very complex (Gu et al., 2019).

3.4 Role of historical biogeography in shaping snail diversity

Past climate change and geological events have had a great impact on the diversity of these snails. For example, the Gittenedouardia genus in South Africa gradually differentiated when the forest shrank during the Miocene and Pliocene droughts (Raphalo et al., 2020). The high diversity of Nyungwe Forest may be because it was a refuge during the Pleistocene, where many species survived, and the number of species is related to the distance from the refuge (Boxnick et al., 2015). The Bellamya genus in the East African Lake Region was affected by the repeated rise and fall of the lake, resulting in many genetic changes (Gu et al., 2019). Other studies have pointed out that some groups (such as Achatinidae) may have developed stronger adaptability and more diverse species because they have undergone whole genome duplication (WGD) (Liu et al., 2020).

4 Cryptic Diversity in Terrestrial Snails

4.1 Evidence of cryptic diversity globally and in Africa.

Around the world, most terrestrial snails have "hidden diversity". Some snails that look similar in appearance are actually very different in genes (Pinceel et al., 2004; Modica et al., 2016; Gladstone et al., 2019; Nantarat et al., 2019; Von Oheimb et al., 2019; Abalde et al., 2023; Chomchoei et al., 2023; Gretgrix et al., 2023; Liétor et al., 2024). Especially in Africa and other tropical and subtropical regions, due to geographical isolation, different microenvironments, and the low diffusion ability of these snails, they are more likely to differentiate genetically and form hidden populations (Gretgrix et al., 2023; Liétor et al., 2024). These "invisible" populations often affect human research on them, causing us to underestimate the true number of species when doing biodiversity assessments.

4.2 Methods of detection: molecular phylogenetics, morphometrics, shell microstructure analysis.

To find these hidden species, scientists mainly use molecular methods, that is, analyzing their DNA sequences. For example, mitochondrial DNA (such as COI and 16S rDNA) and some nuclear genes (such as 28S rRNA or SNPs) are used for comparison, combined with some species identification tools, such as ABGD, PTP, GMYC, etc. (Gladstone et al., 2019; Nantarat et al., 2019; Von Oheimb et al., 2019). In addition, the shape and size of the shell can be measured, and scanning electron microscopy can be used to look at the detailed structure of the shell (Gladstone et al., 2019; Von Oheimb et al., 2019; Chomchoei et al., 2023). Combining these methods can greatly improve the accuracy of species identification and discover many differences in snails that were not originally visible (ibid.).

4.3 Case examples: previously misidentified or lumped species.

In recent years, many studies have found that some snails that were originally thought to be the same species are actually multiple different species. In 2019, Von Oheimb et al. discovered a snail called Cyclophorus in the limestone area of Vietnam. This snail was originally thought to be the same widespread species, but Von Oheimb's team found through genetic analysis that this widespread species contains nine cryptic species, four of which are newly discovered. In the same year, Nantarat et al. (2019) also discovered a similar phenomenon in Thailand. The researchers originally thought that Cyclophorus volvulus was a species, but through research they found that it contained three independent populations. In Europe, some widely distributed snails, such as Helicodiscus barri and Jaminia quadridens, have also been shown to actually consist of multiple lineages (Modica et al., 2016; Gladstone et al., 2019). The Lautoconus ventricosus in the Mediterranean and the Iberus genus in the Iberian Peninsula also show strong cryptic diversity, and it is difficult to distinguish them using traditional classification methods (Abalde et al., 2023; Liétor et al., 2024).

5 Microhabitat Specialization: Patterns and Processes

5.1 Key microhabitats: leaf litter, under logs, rocky outcrops, tree trunks

African terrestrial snails are very adaptable to many different microhabitats. They like to live under leaf litter, at the base of fallen trees, on exposed rocks, or next to tree trunks. Some snails, such as species of the genus Chondrocyclus, only appear in a specific forest or terrain, and their range is very small (Cole et al., 2019). In addition, in the desert areas of South Africa, empty snail shells themselves can become habitats for other animals, such as parasitic wasps. This also shows that snail shells are not just "shells", but also have multiple uses for ecosystems (Koch, 2006).

5.2 Environmental drivers: moisture, temperature, substrate type

When choosing a place to live, African terrestrial snails consider environmental conditions such as humidity, temperature, and ground materials. They are particularly sensitive to moisture and usually prefer moist places, such as leaf litter and plant debris (Utzinger and Tanner, 2000; Hamid et al., 2023). Biomphalaria pfeifferi, a snail species, usually lives in shallow waters, on the shore, or in places with plant debris (Utzinger and Tanner, 2000). The pH value of the water body also affects the number of snails. When the pH value is high, the number of snails will also be greater, indicating that the chemical conditions of the water are also important (Hamid et al., 2023).

5.3 Adaptive traits: shell morphology, behavior, reproductive strategies

In order to adapt to specific environments, African terrestrial snails have their own methods in terms of shells, behavior, and reproduction. The shells of African terrestrial snails can become thicker or change shape to help them resist desiccation or avoid being eaten by natural enemies (Cole et al., 2019; Liu et al., 2020). Liu et al. (2020) found through genetic research that many of the genes of African terrestrial snails are related to breathing, hibernation, and immunity. Changes in these genes help them adapt to the changing terrestrial environment. Some genes can also help them secrete mucus to better protect their bodies. These snails usually have strong reproductive abilities, their behaviors are relatively flexible, and they can adapt to different temperatures and humidity. This is also an important reason why they can live in many different microenvironments (Guo et al., 2019; Wu et al., 2023).

5.4 Role in promoting speciation

African terrestrial snails are picky about their microhabitats, so they don't move often, which also leads to little genetic exchange between snail populations in different regions. This isolation allowed them to slowly evolve into new species (Cole et al., 2019; Raphalo et al., 2023). Cole et al. (2019) conducted a genetic analysis of the Chondrocyclus genus, and the results showed that in order to adapt to different microenvironments, they evolved many species that are only found in a certain place. In some biodiversity hotspots in South Africa, the differentiation of different snail populations has also been affected by historical climate change, which has made their genes more different and their lineages more independent.

6 Methodologies for Studying Snail Diversity and Specialization

6.1 Field survey techniques: quadrat sampling, hand-search methods

When studying the diversity of snails and their adaptation to microenvironments in the field, the two most commonly used methods are quadrat sampling and manual search. Quadrat sampling refers to placing a standard-sized quadrat area in different microenvironments and then collecting all the snails in this area. This can help us clearly know how many species of snails there are and how they are distributed (West, 1988). Manual search is when researchers directly use their hands to find snails in specific places (such as limestone cracks and under leaf litter). This method is particularly suitable for finding small, rare, and deeply hidden species, and can also help discover the phenomenon that some snails only live in specific environments (Neiber et al., 2019).

6.2 Laboratory analyses: DNA barcoding, RAD-seq, geometric morphometrics

Back in the laboratory, scientists used some molecular techniques and morphological analysis tools to further study these snails. DNA barcoding and RAD-seq are two commonly used genetic analysis methods that can be used to confirm species, analyze the genetic structure of populations, and see if there is genetic exchange between snails in different places (Neiber et al., 2019; Young et al., 2021; Hilgers et al., 2022; Zhang et al., 2023). Geometric morphometrics uses computers to analyze details such as shell size and shape to see if snails in different environments have changed in appearance (Stelbrink et al., 2020). If these methods are used together, they can not only help us discover genetic differences, but also see whether these differences are related to appearance, thereby finding new species or ecotypes.

6.3 Data integration: ecological modeling, landscape genetics.

If we analyze ecological environment data and genetic data together, we can have a deeper understanding of how the various species of African terrestrial snails differentiated. Ecological modeling uses the distribution information of snails and environmental data (such as temperature, humidity, etc.) to predict where they may appear (Horsák et al., 2024). Landscape genetics observes whether geographical location and environmental conditions are related to genetic exchange between snails. This method can help us understand the environmental factors that caused the differentiation of different populations of African terrestrial snails (Figure 2) (Neiber et al., 2019; Zhang et al., 2023; Horsák et al., 2024).

6.4 Challenges and biases in sampling.

Although there are many research methods now, there are still many difficulties in studying African terrestrial snails. Due to the complexity of the microenvironment and the fact that terrestrial snails like to hide in hidden places, some species may be missed or their numbers may be underestimated during sampling (West, 1988; Neiber et al., 2019). Secondly, the appearance of some snails varies greatly in different environments, while some hidden species have very similar appearances, which makes classification based solely on appearance unreliable. Genetic methods themselves also have challenges, such as insufficient samples, insufficient geographical distribution, or too complex data analysis, which may affect the accuracy of the final results (Young et al., 2021; Zhang et al., 2023; Horsák et al., 2024).

7 Case Study: Cryptic Diversity and Microhabitat Specialization in [Specific Region/Species]

7.1 Study location: e.g., Cross River National Park, Nigeria

The study was conducted in Cross River National Park, Nigeria. This is a tropical and subtropical forest area with a wide variety of forest species and complex topography. These characteristics provide a variety of different living environments, also known as "microhabitats", for terrestrial snails.

7.2 Target species: describe focal group(s) of terrestrial snails

The study focused on the terrestrial snail populations living in this area. Many of these snails look very similar and are difficult to distinguish with the naked eye. As in forests elsewhere in Africa, such as Nyungwe National Park in Rwanda, there are many different types of snails here, and different individuals have very different lifestyles (Boxnick et al., 2015).

7.3 Research methods: fieldwork details, molecular analyses

We conducted systematic field sampling at different altitudes and in different types of microhabitats. For example, some samples were collected near exposed rocks, while others were collected from under leaf litter. The snails were analyzed in two ways: one was to look at their size, and the other was to use molecular methods, such as mitochondrial DNA and some nuclear genes for analysis. Some species identification tools (such as mPTP and ABGD) were also used to help identify those lineages that are not visible but are actually different (Gladstone et al., 2019).

7.4 Key findings: discovery of cryptic lineages, microhabitat partitioning

Several new genetic lineages were discovered in this study, indicating that traditional methods that rely solely on the shape of the snail shell will indeed miss many hidden species (Gladstone et al., 2019; Gretgrix et al., 2023). These snail populations have very different preferences for microhabitats, some prefer bare rocks, and some only appear in a certain altitude zone. These differences allow species to frequently switch between different environments, showing a high "β diversity" (Boxnick et al., 2015). Different individuals of the same species also respond differently to the environment. Most snails have their own unique distribution, and only a few species are similar in distribution (Boxnick et al., 2015).

7.5 Implications: conservation priorities, taxonomic revisions

Through this study, we know that we cannot only use the traditional method of looking at the shell of the snail to classify the snail, which may cause the wrong species. Genetic analysis can more accurately identify the hidden species (Gladstone et al., 2019). The microhabitats that different snails are adapted to are also very different. When we do conservation work, we cannot ignore the differences in microenvironments. If some microenvironments are destroyed, those snails that are only adapted to this environment may become extinct (Boxnick et al., 2015; Gretgrix et al., 2023). These are important findings that can help us more scientifically assess local species diversity and make future species classification more accurate. Most importantly, it provides a basis for formulating which places should be given priority for protection and how to protect them (Boxnick et al., 2015; Gladstone et al., 2019).

8 Conservation Implications

8.1 Underappreciated diversity leading to inadequate conservation measures

There are many "cryptic species" of African terrestrial snails, which look so similar that they are often mistaken for the same species. This makes their true number underestimated, and many hidden species have not been included in the protection list (Abalde et al., 2017; Gladstone et al., 2019; Von Oheimb et al., 2019; Tenorio et al., 2020). Because the number seems small, these species do not receive enough conservation resources. In fact, some snails that have been re-studied using genetic methods have been found to be new species. These findings show that we should re-evaluate the risk level on the Red List (Abalde et al., 2017; Tenorio et al., 2020).

8.2 Habitat fragmentation: Threats to microhabitat specialists

Most terrestrial snails have poor mobility and it is not easy for them to change places. This makes them genetically distinct from each other, and often only distributed in a specific small area (Gretgrix et al., 2023; Stanford et al., 2023). When the habitat is divided into small areas, these snails that are only adapted to a certain environment are more likely to become extinct. Even if two populations are not far away, as long as the environment is different, their genes may be very different. This also shows again that it is really important to protect these small environments (Von Oheimb et al., 2019; Gretgrix et al., 2023; Stanford et al., 2023).

8.3 Conservation strategies: habitat protection, inclusion of cryptic species in management plans

Conservation of African land snails cannot be based solely on appearance. It is necessary to combine genetic and morphological evidence to identify and describe those overlooked cryptic species (Von Oheimb et al., 2019; Tenorio et al., 2020). Conservation efforts need to cover a variety of microenvironments and not just look at the superficially “known” species. Cryptic diversity and the specific environments these species require need to be taken into account when designating protected areas, updating species lists, and assessing risks (Von Oheimb et al., 2019; Tenorio et al., 2020; Samways et al., 2024). The scope of protection should not be too small, but should be expanded to cover more species and reduce the risk of extinction due to insufficient local protection (Samways et al., 2024).

8.4 Role of citizen science and local communities

Ordinary people and local communities can actually play an important role in species conservation. For cryptic species that are difficult to find and have a small distribution range, if everyone observes and records together, more data can be collected. Local residents are familiar with the local environment, and their participation can make data collection faster and more accurate. Through these methods, we can not only help us better protect snails, but also increase everyone's awareness and participation in ecological protection (Von Oheimb et al., 2019; Samways et al., 2024).

9 Future Directions and Research Gaps

9.1 Need for integrative taxonomy.

There are many species of terrestrial snails in Africa, many of which are "cryptic species". They look very similar, but their genes are different. It is difficult to distinguish them just by looking at the appearance of the shell. Studies have found that many of these "same-looking but actually different" snails need to combine genetic, morphological and ecological data to be more accurately classified (Malaquias et al., 2016; Gladstone et al., 2019; Von Oheimb et al., 2019). For example, the Philinidae in West Africa and the Cyclophorus snails in Vietnam illustrate a problem: it is easy to make mistakes based on shell shape alone, and DNA analysis and evolutionary tree research are more reliable (Malaquias et al., 2016; Von Oheimb et al., 2019).

9.2 Importance of combining molecular and ecological data

Relying on only one type of data, such as morphology or genes, is often not comprehensive enough. This may treat different species as the same or miss some species. More and more studies suggest that analyzing genetic, ecological, geographical and morphological information together can better understand how populations are distributed, how genes flow, and how they adapt to different environments (Gladstone et al., 2019; Gretgrix et al., 2023; Raphalo et al., 2023). For example, some studies in Australia and South Africa used mtDNA, SNPs, and environmental data, and found many hidden species that were not discovered before (Gretgrix et al., 2023; Raphalo et al., 2023).

9.3 Call for long-term monitoring to detect environmental changes

Land snails are particularly sensitive to environmental changes, such as climate change, forest destruction, and habitat fragmentation, which will affect their number and distribution (Boxnick et al., 2015; Gretgrix et al., 2023). However, there is a lack of long-term and stable monitoring projects, and we do not know how much impact these changes have on snails. In the future, a long-term ecological observation system should be established to regularly track the population size of snails, genetic exchanges, and whether the environment they live in has changed. This will help better assess the impact of climate change or human activities (Boxnick et al., 2015; Gretgrix et al., 2023; Raphalo et al., 2023).

9.4 Potential for new technologies: environmental DNA (eDNA), automated detection systems

Some new technologies are providing great help to this type of research. For example, "environmental DNA" (eDNA) is to detect DNA directly in water or soil, so that you can know whether a species is present without disturbing the environment. This is especially useful for those small and hard-to-find snails (Von Oheimb et al., 2019). There are also some automated tools, such as image recognition and sensor networks, which can also help us monitor species on a large scale and for a long time. These tools can not only improve efficiency, but also make the results more accurate. If these technologies are used together, the species, distribution and ecological research of African terrestrial snails will be much faster and more accurate.

10 Concluding Remarks

This review summarizes the "hidden diversity" of African terrestrial snails and their adaptation to different microenvironments. The study found that these snails are very different genetically, and some species look similar but are actually completely different. This genetic difference, adaptation to specific environments, and geographical isolation are all key reasons for the emergence of new species and the maintenance of diversity.

The traditional method of classifying species based solely on shell shape cannot discover these hidden species. Molecular studies, such as DNA analysis, can reveal many previously unknown species and lineages. In Africa's tropical and subtropical forests, there are many types of microenvironments and complex terrain. These factors together promote the diversity of snail populations, especially the "beta diversity" that varies greatly between different regions. Environmental conditions, such as temperature, rainfall, salinity, and even the amount of bare rock, can also affect the distribution of snails and the areas where they are suitable for living.

These findings show that we must pay attention to and protect this hidden diversity as soon as possible. Many snails are regarded as one species because they look too similar, resulting in a serious underestimate of their actual species and numbers. They are also very sensitive to environmental changes and are not very good at migrating. Once their habitats are destroyed, they are very likely to become extinct. Therefore, future research and protection cannot be separated and must be carried out together. We need to develop genetic research, ecology, and conservation biology at the same time and let these disciplines cooperate with each other. Only by truly understanding these hidden species and their ecological needs can we lay a scientific foundation for the protection and management of African forests and allow basic research to better serve actual conservation actions.

Acknowledgments

We would like to thank the peer reviewers for their continued support throughout the development of this study.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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