Marine biodiversity plays a vital role in maintaining the health and resilience of ecosystems, particularly in tropical regions where coral reefs, lagoons, and other complex habitats serve as hotspots for various marine species. Gastropods, as one of the most diverse and ecologically significant groups of marine organisms, contribute significantly to the structure and function of these ecosystems [1] . Despite their importance, the marine gastropod fauna of many remote and isolated islands, including Agalega, remains largely undocumented.

The Agalega Islands are an integral part of the Republic of Mauritius, situated at coordinates 10˚25'S, 56˚38'E. Agalega comprises two islands, namely North Island and South Island. The dimensions of North Island are 12.5 km by 1.5 km, while South Island measures 7 km by 4.5 km [2] [3] . The islands have a total land area of 2600 hectares [4] and are surrounded by a 500 m wide lagoon [3] . The lagoon is very shallow, with a maximum depth of up to 2 meters. Agalega is inhabited by a small community with no significant human disturbance to the marine environment [2] [3] . A fringing reef encircles the islands, with the exception of the port landing area, where the reef lies in close proximity to the shore [2] . The Agalega Islands are characterized by unique ecological features, including enclosed lagoons, sandy shores, and coral reef systems [3] . These habitats offer a variety of niches that can support a rich diversity of marine gastropods. Given the isolated nature and unique environmental conditions of the Agalega Islands, understanding the diversity of gastropods is crucial for both conservation and management efforts. This study aimed to establish the first inventory of marine gastropods inhabiting the Agalega Islands.

Gastropod specimens were collected from five stations located in the coastal and lagoon areas around Agalega between December 2017 and March 2019. A total of 50 days of fieldwork was conducted in Agalega, with each station investigated over a 10-day period. The surveys were designed to cover a wide range of marine habitats, such as coral reefs and seagrass beds. These ecosystems support diverse gastropod species from various ecological zones. The shoreline at each site was explored for dead shells and hermit crabs.

The coastal areas at each station were visually surveyed by walking for one hour along the beach zone. At each site, lagoon surveys were conducted using a systematic grid-based sampling approach [5] , with a 10-man-day effort allocated to each site. The subtidal survey involved one hour of skin diving, during which gastropod specimens were hand-picked.

The gastropod shells were washed, cleaned, and air-dried for several days. The dried samples were then stored in zip-lock bags. Shells measuring ≥ 10 mm in size were retained for this study and classified into Operational Taxonomic Units (OTUs). Photographs of each species were taken, along with their identification codes, for further analysis.

The marine gastropod species were identified to the lowest possible taxonomic level using a combination of scientific literature, field reports, and reference materials, including Marine Molluscs of Mauritius [6] and Compendium of Seashells [7] . Taxonomic nomenclature was standardized in accordance with the principles outlined in the World Register of Marine Species [8] and adhered to International Commission on Zoological Nomenclature guidelines for nomenclatural consistency. Data analysis was conducted using Microsoft Excel and PRIMER 7.0 software [9] [10] .

The gastropod fauna of Agalega had not been thoroughly documented, and a comprehensive list of gastropods for this region had yet to be published. The only existing literature is attributed to Poisson [13] , who conducted a study in Agalega focusing on a broader spectrum of molluscs, including bivalves, cephalopods, and gastropods. Poisson’s [13] study reported the presence of approximately 24 species of gastropods. Several key gastropod families, including Cypraeidae and Conidae, were recorded in Poisson’s [13] work. These gastropod species are typical inhabitants of coral reef environments. However, due to taxonomic uncertainties, these records were excluded from our comparative analysis.

This study marked a pioneering effort to document the molluscan fauna across selected sites within the Agalega Islands, having uncovered a total of 119 marine gastropod species. The gastropod community was dominated by three primary families: Cowries (Cypraeidae), Cone Snails (Conus), and Muricidae, which together accounted for 46.2% of the observed diversity. This pattern aligned with general trends reported in tropical marine environments [14] [15] , where these families thrived due to their broad ecological adaptability and diverse life history strategies. The dominance of Cypraeidae could be attributed to their widespread distribution in tropical and subtropical regions, as well as their ability to inhabit temperate waters. Their reproductive strategy, involving planktonic veliger larvae with extended developmental periods, enhances their dispersal potential across vast oceanic distances, enabling the colonization of varied habitats, particularly coral reefs in the Indo-Pacific [16] . Ecologically, cowries play a critical role as herbivores, grazing on algae to regulate algal growth and maintain the structural integrity of coral reef ecosystems [17] . This balance is vital for supporting other reef-associated organisms, further emphasizing their ecological importance. Conus species, another key component of Agalega’s gastropod community, are globally diverse, with over 700 species occupying approximately 25% of the world’s marine environments [18] . The Indo-West Pacific region serves as a biodiversity hotspot for Conus, hosting 60% of the global species [19] [20] . Their predatory nature and adaptive diversification across various marine ecosystems contribute to their ecological significance. As predators, Conus species feed on fish, polychaete worms, and molluscs, regulating prey populations and maintaining the stability of marine food webs. This regulation underscores their role in sustaining ecosystem health and highlights their importance in conservation planning [21] . Similarly, the Muricidae family demonstrates significant adaptability, with species distributed from intertidal zones to deeper marine environments. The family includes genera such as Coralliophila, which act as parasites on anthozoan cnidarians [22] , showcasing a specialised ecological niche. The carnivorous feeding habits of muricids and their influence on prey populations underline their role as key predators, driving benthic community dynamics through top-down control. Other gastropod families, such as Strombidae, Terebridae, and Naticidae, exhibited moderate representation at the surveyed sites, with each family comprising 5 - 7 species. These families displayed diverse feeding strategies and habitat preferences, reflecting the varied ecological niches within the Agalega marine ecosystem. In contrast, families like Ellobiidae, Modulidae, and Hipponicidae, each represented by one or two species, highlighted the ecological specialisation and rarity of certain taxa in this region. The observed species composition was consistent with records from other parts of the Indo-Pacific, including Mauritius and Réunion, with no endemic species identified during the study. The limited number of survey sites and the brief duration of the study restricted the ability to conduct a comprehensive comparison of gastropod diversity with other tropical islands in the Western Indian Ocean.

The World Register of Marine Species (WoRMS) [8] database, as of 2024, did not contain specific records of gastropod species for the Agalega Islands. However, 247 gastropod species had been documented within the broader Mauritian Exclusive Economic Zone (EEZ). These findings suggested that Agalega might harbour a substantial diversity of marine gastropod species, highlighting the need for further research to better understand its unique marine biodiversity.

The lagoon and reef ecosystems of the Agalega Islands function as ecological microcosms, characterized by a rich diversity of habitats. These systems exhibit a complex mosaic of microenvironments, distinguished by variations in current patterns and substrate types, which are particularly conducive to supporting diverse gastropod assemblages. These gastropods have evolved to exploit the varied ecological niches available within these environments. This study investigated gastropod diversity across five survey stations, revealing that these differences were closely linked to habitat complexity and environmental conditions. La Fourche station demonstrated the highest biodiversity, contributing 29% of the total species and 38% of the specimens recorded. The high diversity at this station could be attributed to a mosaic of habitats, including coral reefs and sandy substrates, which provided ecological niches supporting a broad array of gastropod families, such as Conidae and Cypraeidae. In comparison, the Bato Cassee station, characterized by its predominantly sandy bottom, exhibited the lowest diversity, with only 6% of species and 4% of specimens. This site was primarily dominated by burrowing taxa like Naticidae and Terebridae. Similarly, the Simin Henri and La Pointe Sud stations, characterized by seagrass meadows and soft sediment substrates, supported gastropod communities such as Naticidae and Cerithiidae. These stations accounted for 24% and 23% of the total specimens recorded, respectively. In contrast, the Vingt Cinq station exhibited a distinct ecological profile shaped by tidal fluctuations, with water depths ranging from near exposure at low tide to levels up to 1.5 meters at high tide. This dynamic interplay of tidal activity, bedrock substrates, and strong water currents created a challenging yet diverse environment that supported a unique community composition. Furthermore, the intricate structures of coral colonies and seaweed beds at this station provided a diverse array of microhabitats and refuges for gastropods, significantly shaping the observed gastropod assemblages.

Both living specimens and dead shells were included in the inventory of marine gastropods for the Agalega Islands. This approach was essential due to the specialized lifestyles and habitats of marine gastropods. Many species inhabit environments that are difficult to access, such as surf reef areas or small, hidden crevices. Previous studies had highlighted the challenges associated with collecting species from these unique habitats due to their inaccessibility [23] [24] . Incorporating dead shells in the inventory provided a more comprehensive understanding of the gastropod diversity of the islands [1] . For example, an inventory of the molluscan community in Kuwait conducted by Glayzer et al. [25] revealed that 55% of the community was represented by dead shells. Additionally, gastropod species are often more accessible during specific periods of the year or at nighttime, further complicating sampling and studies conducted within the limited duration of surveys.

Coral reef ecosystems are known to harbour highly diverse molluscan assemblages, including many undescribed species [26] . However, these vital ecosystems are increasingly threatened by global warming. Although no specific information is available on the impact of bleaching events on the Agalega Islands, studies by Ateweberhan and McClanahan [27] , Gudka et al. [28] , McClanahan et al. [29] , Obura [30] , and Sheppard et al. [31] , and have shown that countries in the Western Indian Ocean (WIO) region frequently experience bleaching events of varying severity. Numerous studies have demonstrated that such events could significantly impact marine diversity [32] - [37] . A major bleaching event in 2016 resulted in a 20% decrease in coral cover across six WIO countries [28] . This event significantly altered habitat structures and reduced food availability, potentially affecting the composition and abundance of gastropod species. Since this study was conducted post-bleaching, these factors might have influenced the observed gastropod communities.

Several additional factors likely influenced the species richness observed in this study. The spatial coverage of lagoon areas might have been insufficient to capture the full heterogeneity of gastropod fauna across Agalega. Offshore waters and off-reef areas were also excluded from the study. Furthermore, the experience level of collectors might have limited species detection [38] - [40] .

This study represented a critical step in enhancing our understanding of gastropod diversity within the Agalega Islands, providing foundational baseline data essential for informed conservation strategies. The relatively low levels of human impact on Agalega’s marine ecosystems present a unique opportunity to protect and preserve its biodiversity. A key finding of this study was the identification of a diverse assemblage of Conus species. The ecological significance of these species, coupled with conservation concerns for several members of the genus, highlighted the necessity of integrating them into biodiversity management plans. Future research should build upon these findings by investigating unexplored habitats, such as deeper reef slopes. Additionally, prioritising the study of micro-molluscs would be vital to achieving a more comprehensive understanding of the region’s biodiversity. Such efforts would support the development of targeted conservation initiatives, ensuring the long-term sustainability of Agalega’s unique marine ecosystems.

The authors express their sincere gratitude to the University of Mauritius and the esteemed staff of the Faculty of Agriculture for their continuous support throughout the study. Special thanks are extended to the late Mr. Jayan Sadasing, Mrs. Indrani Sadasing, and Mrs. Simla Sadasing for their assistance in sorting the samples.