The study area includes 15 beaches distributed along the southern part of the Gulf of Mexico and the northern Caribbean Sea (eastern Mexican coast) ( Figure 1 ), spanning low (18° N) and mid-latitude (22°N) and experiencing a diversity of climates conditions along its wide geographic range. In the northern zone (Tamaulipas state), the climate is semi-arid with an average annual rainfall of 800 mm. In the other states, the climate is warm-humid to sub-humid, with the highest levels of precipitation in Veracruz, Tabasco, and Campeche (1,500-3,500 mm) and to a lesser extent in Yucatán and Quintana Roo (600-1,400 mm) (de la Lanza-Espino et al., 2013). The beaches of the states of Tamaulipas, Veracruz and Tabasco are characterized by their terrigenous origin (sediments supplied by rivers), their location in front of a narrow continental shelf and are influenced by the warm current of El Lazo (Silva-Casarín et al., 2011). The sandy coasts that border the Yucatan Peninsula are the product of biogenic genesis, situated on an emergent karst platform composed predominantly of calcium carbonate sediments. Devoid of surface fluvial inputs, these coastal landscapes are intricately governed by the dynamic interaction of the Yucatan and Caribbean currents.

Sampling was conducted within a one-year time frame (March, April, and June 2019, and in June 2020) at 15 localities (beaches) ( Figure 1 ; Table 1 ) representing all states along the eastern Mexican coast except Veracruz, where sampling was precluded by conditions imposed during the SARS CoV-2 pandemic. The sampling design was hierarchical and nested across five spatial scales: regions (kilometers), subregions (kilometers), localities (hundreds of meters), sites (tens of meters), and cores (meters). None of the 15 localities were within 2 km of a river mouth, coastal lagoon, lake, or urban communities. At each locality, three sites separated by 50 m were selected and at each site one of three different sampling designs (A or B) was used ( Table 1 ; Figure 2 ). In design A, at each of the 3 locality sites, two transects were drawn parallel to the beach, the first in the infratidal zone and the second in the intertidal zone. Along each transect, 3 sediment samples separated by 1 m (replicates) were obtained. In design B, the sampling effort was doubled on the replicate scale, i.e., 6 sediment samples per transect (infratidal and intertidal) were collected at each site (1, 2, and 3) for each locality.

The objectives of using three sampling designs were as follows: design A, to increase the number of sampled beaches and enhance geographic representativeness while reducing the time invested in field and laboratory work; design B, to increase the local sampling effort to enable comparisons across different spatial scales, ranging from meters to hundreds of thousands of kilometers, albeit with an increased time requirement for field sampling and laboratory work.

The samples were collected using a PVC sediment corer (diameter 15 cm and length 30 cm; equivalent area 0.0177 m², volume of 0.005 m³). Each sediment core was sieved in the field through a mesh bag (mesh size 500 µm), which retained the macrofauna and sediment particles larger than 500 µm, then sorted organisms were fixed with 4% formaldehyde.

In the laboratory, organisms were separated from the sediment, counted, and identified to the lowest possible taxonomic category using taxonomic keys for each group (Abele and Kim, 1986; LeCroy, 2000, 2002, 2004, 2007, 2011; Hsueh, 2015). The specimens were then preserved in 70% alcohol at the Colección de Crustáceos de Yucatán (YUC-CC) from Unidad Multidisciplinaria de Docencia e Investigación, Sisal, Facultad de Ciencias, UNAM.

Based on the species/taxa identified in the present study, a faunistic list was compiled following the taxonomic classifications proposed by Ahyong et al. (2011) and Lowry and Myers (2017). In addition, the number of collected organisms was counted, and alpha diversity (species richness) per locality was calculated.

The distribution and abundance patterns of species/taxa along the coast were identified with an ordered shade plot. The abundance values were transformed into natural logarithms (plus 1) to downweigh the effect of highly abundant species. The species/taxa were ordered based on their similarity (Bray-Curtis) in their spatial distribution with respect to the locality, using a hierarchical cluster analysis using the PRIMER7 software (Clarke et al., 2014).

To evaluate the two sampling designs (A and B), three extrapolation and rarefaction curves based on sample size, coverage, and completeness were derived using the incidence data of each species per locality, and the incidence data per region (Gulf of Mexico and Caribbean Sea) with the iNEXT package (Chao et al., 2014; Hsieh et al., 2016) in R Studio. These measures allowed us to infer the representativeness of the sampling effort at each locality and each region. The biological data collected were organized into a Darwin Core dataset that is available in the OBIS website (https://doi.org/10.15468/zfgt5w) (Muciño-Reyes et al., 2024).

In general, Excirolana braziliensis exhibited the widest distribution across beaches from Tabasco to Quintana Roo. However, the restricted distribution of other taxa, with 19 out of 24 species recorded at a single locality, underscores the influence of local environmental conditions and habitat specificity. For example, the amphipod Mexorchestia carpenteri raduloviciae was the most abundant species, accounting for 66.41% of individuals and primarly associated with Sargassum deposits on Quintana Roo’s beaches ( Figure 6 ). The association of this talitrid amphipod with macroalgal wrack underscores the critical role of stranded algae in providing food and refuge, enhancing habitat complexity, and supporting high macrofaunal abundance (Parker et al., 2001; Tanaka and Leite, 2003; Ali et al., 2018).

From a regional perspective, the environmental transition between regions such as Campeche and Yucatán, characterized by a mix of calcareous sediments and diverse water masses, emerged as a macrocrustacean biodiversity hotspot (Escobar-Briones and Jiménez-Guadarrama, 2010). This finding aligns with biogeographical theories that highlight the importance of transition zones in fostering species richness by combining elements of neighboring ecoregions and hosting endemic species (Smith et al., 2001). However, besides the potential effect of this transitional region, the division of sampling into beach zones (supratidal, intertidal, infratidal) further expanded the ecological distribution records of species such as Melita planaterga Kunkel, 1910 and the order Cumacea, illustrating the relevance of different spatial scales in determining the distribution of species. Particularly, Melita planaterga and other Cumacea species, previously known for inhabiting intertidal zones (Paz-Ríos et al., 2013a; Hidalgo, 2017), were now observed in infratidal areas.

In addition, this study revealed potential cryptic diversity within some taxa. Specifically, morphological differences in the isopods Exosphaeroma sp., Excorallana sp., Exosphaeroma cf. productatelson, and Ancinus sp. compared to known species, suggest the need for molecular analyses to confirm their taxonomic status.

First, the isopod Exosphaeroma sp. found in this work in P. Morelos is very similar to E. antillensis. Both species have two broadly subconical protrusions on the pleotelson. However, Exosphaeroma sp. has a pleotelson with an entire posterior margin while E. antillensis has a pleotelson with an emarginate posterior margin. Furthermore, in the former the margins of the uropodal exopods and endopods are smooth, whereas in the latter these margins are crenulated (Kensley and Schotte, 1987). Second, the isopod belonging to the genus Excorallana sp. collected in Sisal is very similar to E. subtilis, previously documented in Florida (Boyko et al., 2019). However, Excorallana sp. has two subconical protuberances on the pleotelson that E. subtilis does not have. Third, Exosphaeroma cf. productatelson Menzies and Glynn, 1968 is distinguished by a fully crenulated outer margin of the uropodal exopod, and a frontal lamina that is equally long and wide. Fourth, two specimens (females) of the genus Ancinus found on the beach of Boca Paila (Caribbean Sea) were very similar to A. depressus (Say, 1818), a species reported from the northern Gulf of Mexico (Schotte et al., 2009). However, in Ancinus sp. the dorsal integument granules are much smaller and the two pairs of antennae are also smaller than in A. depressus: antenna 1 does not extend beyond the fourth thoracic segment and antenna 2 does not extend beyond the third thoracic segment.

Such taxonomic identification efforts of potential new species from sandy beaches would enhance our understanding of species richness and evolutionary processes in these underexplored ecosystems (Kensley and Schotte, 1987; Boyko et al., 2019).

The state-level disparities in faunal knowledge highlight the importance of localized conservation efforts informed by robust baseline data ( Figure 8A ). For instance, the increased species richness documented in Campeche and Quintana Roo emphasizes the need for targeted protection of these biodiversity hotspots. Conversely, the limited data available for Tabasco and Tamaulipas call for prioritizing these regions in future research and conservation planning. Much of the knowledge about macrocrustaceans on Mexican beaches along the eastern coast has been generated in recent years, and the present study represents a significant contribution ( Figure 8B ). However, there is still a considerable information gap for many Mexican beaches. This is also the case for beaches on the western Mexican coast (Pacific) where only three studies have documented macrocrustaceans of sandy beaches (Dexter, 1976; Torres and Lowry, 2011; Torres-Alfaro et al., 2012). Given the regional ecological and economic importance of sandy beaches, integrating this biodiversity data into state and national coastal management strategies is essential. The deposition of specimens in public collections and the open availability of data through the Ocean Biodiversity Information System (OBIS) ensure that this study’s findings can support evidence-based decision-making and foster collaboration among stakeholders.

In addition to the field work carried out in this study, this work is also the first effort to compile faunal data on macrocrustaceans in the sandy beaches of the Mexican coast of the Gulf of Mexico and the Caribbean Sea and their distribution in three zones of the beach (infratidal, intertidal and supratidal) ( Table 3 ). Of 14 publications traced on this topic, nine are scientific articles, four are undergraduate theses and one is a doctoral thesis. The first published record on the macrofauna of sandy beaches along these coasts (Dexter, 1976) reported eight benthic macrocrustaceans from Tamaulipas, Veracruz and Campeche. A further 18 species were added (Arriaga-Becerra, 1985; Méndez-Ubach et al., 1985) followed by hiatus until information regarding macrocrustaceans was contributed by studies in Veracruz (Pacheco-Ríos, 2010; Rocha-Ramírez et al., 2010, 2016; Miranda, 2012; Martínez, 2013; Hidalgo, 2017), Yucatán (Paz-Ríos et al., 2013a; Guerra-Castro et al., 2020) and Quintana Roo (Wildish and Lecroy, 2014; González et al., 2017): this gradually increased the species/taxa total to 59, to which the present findings have added a further 19 species/taxa ( Table 3 ; Figure 8 ).

From a methodological perspective, using a core sampling method and comparing two designs at the local scale provided valuable insights into sampling efficiency ( Figure 7 ). Design B, involving 36 cores per locality, achieved a balance between observed and extrapolated richness, making it the preferred approach for regional-scale studies. However, including additional techniques, such as baited traps and manual collections, would likely enhance the detection of elusive or cryptic species. For example, Exosphaeroma sp., collected in P. Morelos, and Excorallana sp., found in Sisal, presented morphological traits distinct from those in existing taxonomic keys, underscoring the potential for discovering cryptic or undescribed species (Kensley and Schotte, 1987; Munilla et al., 1998; Beyst et al., 2001; Boyko et al., 2019).

Temporal variability remains an essential but unexplored aspect of this study. Seasonal changes in beach morphology and associated fauna have significantly influenced diversity and abundance patterns (McLachlan and Defeo, 2018). Addressing this limitation would require long-term monitoring, which, while resource-intensive, is essential for understanding the temporal dynamics of sandy beach ecosystems. Future studies could adopt a hierarchical approach, balancing spatial and temporal scales to identify the key processes shaping biodiversity patterns. The geographic extension of this study allowed for the identification of significant spatial variations in species richness and composition. This spatial pattern highlights the need for future studies to harmonize sampling efforts across all regions of the eastern Mexican coast.

While this study provides a foundational understanding of macrobenthic crustacean diversity, several avenues for future research emerge. Expanding the geographic scope to include historically well-sampled regions like Veracruz and extending temporal coverage to capture seasonal dynamics are critical next steps. Incorporating environmental variables, such as sediment grain size, organic content, and hydrodynamic conditions, would enhance the explanatory power of biodiversity patterns observed in this study.

Additionally, advancing molecular techniques could resolve taxonomic ambiguities and uncover hidden diversity within cryptic taxa. Such approaches would complement traditional morphology-based identifications and provide a more comprehensive picture of biodiversity. Collaborative efforts across disciplines, integrating ecological, genetic, and oceanographic data, hold promise for addressing the complex challenges facing sandy beach ecosystems.

This study marks a significant milestone in understanding the diversity of macrobenthic crustaceans along the Mexican Atlantic coast. Integrating historical and novel data establishes a comprehensive baseline for future research and conservation. The insights gained from this sampling effort provide a roadmap for optimizing biodiversity assessments in sandy beach ecosystems in Mexico. As coastal environments face increasing human activities and climate change pressures, this work underscores the urgency of protecting these vulnerable yet invaluable habitats. We take a crucial step toward sustainable management and preservation by fostering a deeper appreciation for their biodiversity and ecological roles.