Mauricio Bonilla-Flores

Ontogenetic information remains limited for many cypridoid ostracods despite its importance for reliable taxonomy, species identification, and the interpretation of ecological patterns based on size and morphology. This is particularly relevant in paleoecological studies, where, usually, only valves are preserved. We investigated post-embryonic development in three asexual species of Heterocypris inhabiting ephemeral aquatic environments: Heterocypris exodonta from the Tibetan Plateau, Heterocypris incongruens from Mexico, and Heterocypris salina from Germany. Based on laboratory cultures maintained under controlled conditions, egg morphology, juvenile valve development, and ontogenetic growth were quantified using measurements of valve length and height. All three species exhibited nine developmental stages, comprising eight juvenile instars and one adult stage. Eggs possess a rough external surface and a distinct internal eggshell structure. Early juvenile stages display a marked polygonal reticulation pattern on their valves that progressively weakens and disappears in adults. Growth ratios, defined as the proportional increase in valve size between successive developmental stages, averaged values close to those expected under geometric growth. However, considerable variation is reported among species and developmental stages. The results provide reference data on ontogenetic development and diagnostic morphological characters, improving the distinction between juvenile stages and both intraspecific and interspecific variation and supporting the interpretation of modern and fossil assemblages.

A new ostracod species, Heterocypris exodonta Bonilla-Flores & Karanovic, sp. nov. is described from a high-altitude temporary pond near Nam Co, Southern Tibetan Plateau. Detailed morphological analyses of valves, soft body, and the partial mitochondrial COI (cytochrome c oxidase subunit I) sequence distinguish this species from its close congeners, Heterocypris incongruens and Heterocypris salina . Key taxonomic features include the morphology of the female genital lobe and the structure of the internal opening of normal pores, particularly variations in the female genital aperture at the intersection of the seminal duct and the oviduct. Pore openings in H. exodonta Bonilla-Flores & Karanovic, sp. nov. and H. incongruens exhibit a turbine shape, while in H. salina they display a simple aperture. Phylogenetic analysis based on the COI dataset supports the new species description and suggests potential synonymy and cryptic species within the H. salina and H. incongruens complexes. Ecologically, these species mainly inhabit temporary water bodies with high oxygen levels and likely follow an r-type ecological strategy, being opportunistic in colonizing new habitats, reproducing via parthenogenesis, and undergoing rapid population growth. This study enhances the understanding of Southern Tibetan Plateau ostracod biodiversity, providing crucial morphological and molecular data for species identification, particularly for parthenogenetic species lacking standard male morphological characters. Additionally, this research offers methods to prevent misidentifications and misinterpretations in (paleo-)ecological studies, contributing significantly to the broader knowledge of taxonomy and ecology of ostracods from the Southern Tibetan Plateau. Due to the high morphological plasticity of Heterocypris incongruens , our findings highlight the need for caution when identifying similar species, as external resemblance may conceal genetic divergence.

Taxonomic clarification of Tonnacypris stewarti comb. nov. (= Herpetocypris stewarti), a non-marine ostracod first described by Daday in 1908, was required due to the existence of various synonyms without detailed morphological descriptions. Our study examined specimens from Mang-tsa, Nam Co (Tibetan Plateau) and Lake Band-e Amir (Afghanistan). We identified seven definitive synonyms (Herpetocypris stewarti Daday 1908, Ilyodromus estonicus Järvekülg 1960, Eucypris afghanistanensis Hartmann 1964, E. gyirongensis Yang 1982, E. rischtanica Schneider 1963 in Mandelstam & Schneider 1963, Tonnacypris estonica (Järvekülg 1960), and T. gyirongensis (Guo et al. 2016). After a detailed study of the morphology of valves and soft parts supplemented by a literature review, we propose combining the genus Tonnacypris with the specific epithet stewarti, resulting in the new name Tonnacypris stewarti. We documented the ontogenetic series of T. stewarti in surface sediment samples from Nam Co. Only female specimens were found, consistent with observations in Mongolia, where they were considered asexual populations. Sexual populations were reported from Afghanistan and the southern Tibetan Plateau, suggesting geographic parthenogenesis in the Palearctic region. Because of widely missing males, we explored the use of female genital lobes as a distinguishing feature for taxonomy. Considering current ecological knowledge, T. stewarti could serve as a (paleo)—bioindicator of cold, oligotrophic, and fresh to slightly brackish habitats, including rivers, lagoons, and lakes. This study emphasizes the importance of resolving taxonomic issues for future paleolimnological research. Moreover, our investigation highlights the potential use of sclerotized internal female genital lobes as distinctive characteristics for species differentiation, especially in asexual populations.

Lake Tzibaná is one of the largest (1.27 km²) and deepest (Z_max = 52 m) karstic lakes in the UNESCO’s Biosphere Reserve “Nahá-Metzabok” and in the Lacandon Forest, southeastern Mexico. It archives sediments from multiple sources and the inflowing Nahá River forms deltaic deposits. In 2019, the water level in Lake Tzibaná declined by ~ 15 m, persisting for 4 months and exposing the Nahá River Delta. A geophysical profile on the exposed delta revealed an accumulation of ~ 20 m of such deposits. Three sediment outcrops from an inactive channel in the Nahá River Delta, which ranged in height from 0.6 to 1.43 m, were sampled and a multi-proxy analysis of biological remains and geochemical variables was conducted. Four facies were observed: (1) massive-coarse sand, (2) fine sand, (3) dark leaf litter and (4) massive silty clay, each characterized by specific microcrustacean, testate amoebae and diatom taxa. Six dark leaf litter horizons were radiocarbon dated and revealed a complex depositional history including inverted ages making the establishment of an age model difficult. Nevertheless, past lake-level changes and the formation of the four facies match three characteristic water-level stages, which can also be observed on recent satellite images: (1) Massive-coarse sand deposits, with compositional and sedimentological characteristics of a shoreline environment and fluvial lateral banks, were formed during large-magnitude reductions in the lake level, similar to the one in 2019, (2) Interbedded layers of fine sand and dark leaf litter, currently found in low-energy fluvial environments, were formed during shorter and less pronounced decrease intervals, and (3) Massive silty clay, with distinctive microorganisms from low-energy lacustrine environments, is deposited during high water-level stages, when the delta is covered by water. Our findings illustrate how hydrological changes alter sedimentary dynamics in deltaic areas of lakes. Despite the complexity of their depositional processes, deltaic records can serve as a complementary source of paleolimnological information to records from distal zones due to their sensitivity to variations in water level, especially during extreme and prolonged desiccation events. Future research should attempt to combine evidences from deltaic and sediment sequences from deeper zones of Lake Tzibaná to reconstruct water-level variations during the entire Holocene. Understanding past lake-level reductions is not only relevant for the local indigenous communities but also crucial for the conservation of this ecosystem of international importance.