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Monothalamea
Monothalamea
Monothalamea
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Monothalamea
Image of a deep sea xenophyophore
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Sar
Clade: Rhizaria
Phylum: Retaria
Subphylum: Foraminifera
Class: Monothalamea
Haeckel, 1862,[1] emend. Pawlowski et al., 2013[2]
Orders and subtaxa incertae sedis[3]

Monothalamea is a grouping of foraminiferans, traditionally consisting of all foraminifera with single-chambered tests. Recent work has shown that the grouping is paraphyletic, and as such does not constitute a natural group; nonetheless, the name monothalamea continues to be used by foraminifera workers out of convenience.[4]

Classification

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"Monothalamea" traditionally contains two groups, neither of which is currently considered to be monophyletic:

  • "Allogromiida" traditionally consists of all foraminifera which lack a mineralised test, instead having a test of tectin. Recent work has shown that this grouping is paraphyletic.
  • "Astrorhizida" traditionally consists of all foraminifera with single-chambered, agglutinated tests. Recent work has shown that this grouping is polyphyletic, as agglutinated tests have evolved from proteinaceous tests multiple times throughout foraminiferal evolution.[4]

Recent molecular evidence has revealed that the deep-sea xenophyophores are in fact agglutinated, single-chambered foraminifera.[5] Molecular evidence has also revealed that the freshwater protist Reticulomyxa is in fact a naked, testless foraminifera, and as such it has been included with monothalameans in scientific discussion.[4][6]

A 2013 molecular study using small subunit rDNA concluded that known monothalameans made up at least 22 distinct living clades from marine environments with an additional four clades from freshwater eDNA.[4]

References

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Further reading

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Monothalamea

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Monothalamea is a paraphyletic class of single-chambered (unilocular) foraminifera, comprising testate protists with soft, organic-walled or agglutinated tests, classified within the phylum Foraminifera of the supergroup Rhizaria. These organisms feature a single aperture for pseudopodia extension, granular cytoplasm often containing stercomata (feces aggregates), and in some cases, pseudochambers formed by cytoplasmic folding, distinguishing them from multi-chambered foraminiferal classes like Tubothalamea and Globothalamea. Traditionally divided into orders such as Allogromiida (organic tests) and Astrorhizida (agglutinated tests), Monothalamea encompasses diverse morphologies ranging from small, elongated or spherical forms to giant deep-sea xenophyophores in the superfamily Xenophyophoroidea, which can reach sizes of several centimeters and dominate abyssal megafauna in nutrient-rich sediments. Early molecular phylogenies based on 18S rRNA identified at least 13 clades within Monothalamea (labeled A–M), with subsequent studies revealing higher cryptic diversity (at least 22 clades); multi-gene analyses confirm basal positions for groups like clade C (including xenophyophores), from which multi-chambered forms evolved. Ecologically, Monothalamea are ubiquitous in benthic environments, occurring in marine coastal and deep-sea sediments (where they can constitute up to 50% of operational taxonomic units in high-throughput sequencing surveys), brackish waters, freshwater bodies, and even terrestrial soils, playing key roles in nutrient cycling and as indicators of environmental conditions despite their poor fossilization due to soft s. Their includes multinucleate organization in larger forms, or chitin-based test composition, and a primarily distribution with recent and fossil records spanning from the . Ongoing integrative combining morphology, , and imaging continues to uncover new , underscoring their underestimated global diversity and evolutionary significance.

Overview

Definition and Characteristics

Monothalamea represents a paraphyletic class of foraminiferans within the Foraminifera (, Retaria), defined by their monothalamous tests consisting of a single, unpartitioned chamber. Unlike multichambered foraminiferans such as those in Globothalamea or Tubothalamea, monothalamids lack internal , resulting in a simple, undivided test structure that distinguishes them morphologically from more complex forms. This class encompasses a diverse array of soft-shelled, unilocular that are primarily marine but also occur in brackish and freshwater environments. Key morphological characteristics include tests composed of organic material or finely agglutinated foreign particles, without , which makes them delicate and prone to degradation in sediments. Sizes vary widely, from microscopic individuals measuring 10–100 μm, such as many allogromiids, to giant forms exceeding 20 cm in diameter, exemplified by xenophyophores like Syringammina fragilissima. These organisms are unicellular eukaryotes exhibiting amoeboid locomotion, with their granular, multinucleate extending through apertures or the test wall to form granuloreticulopodia—branching, anastomosing networks of that facilitate movement, prey capture, and uptake. The contains multiple nuclei and typical eukaryotic organelles, including mitochondria with tubular cristae, peroxisomes, and digestive vacuoles, contributing to its granular appearance. Molecular studies have confirmed the paraphyletic nature of Monothalamea, with its lineages branching basally within and including highly derived groups like the abyssal xenophyophores.

Historical Context

The discovery of monothalameans traces back to the , when early microscopists described simple, single-chambered protists with organic or agglutinated tests as primitive forms within the broader group of rhizopods or . These observations laid the groundwork for recognizing their unicellular nature, though initial classifications varied due to limited morphological details. In 1862, proposed the name Monothalamea for single-chambered foraminiferans, distinguishing them from multichambered forms, but the group remained poorly defined amid the era's taxonomic flux. Deep-sea explorations in the late revealed larger specimens, leading to the naming of in 1904 by Franz Eilhard Schulze, who described these giant, agglutinated forms from abyssal environments as a distinct class of rhizopods, separate from typical . Initially, groups like xenophyophores and allogromiids (organic-walled monothalamids) were treated as disparate entities; for instance, early works by Haeckel in 1889 misinterpreted xenophyophores as multicellular sponges or hydroid colonies, while others viewed them as a separate due to their enormous size and complex tests. These misconceptions persisted into the early , with Schulze's classification reinforcing their isolation until morphological and ecological studies in the 1970s, including Ole S. Tendal's influential 1972 monograph, revived interest and tentatively linked them to foraminiferal lineages as giant, sediment-dwelling protists. Key advancements came in the late 20th century, clarifying monothalameans as unicellular foraminiferans rather than multicellular organisms. In 2003, Jan Pawlowski and colleagues analyzed small subunit ribosomal DNA (SSU rDNA) sequences from the xenophyophore Syringammina corbicula, confirming its placement within Foraminifera and resolving long-standing debates about its affinities. This molecular evidence built on 1970s morphological integrations that had begun associating xenophyophores with monothalamid foraminifera in deep-sea surveys. The nomenclature evolved decisively in 2013, when Pawlowski et al. emended Haeckel's Monothalamea to encompass a paraphyletic assemblage uniting xenophyophores, allogromiids, and other single-chambered forms, driven by comprehensive SSU rDNA phylogenies that highlighted their basal position in foraminiferal evolution. Subsequent studies, including phylogenomic analyses in 2022 and new species descriptions in 2025, continue to refine the paraphyletic nature and diversity of Monothalamea.

Taxonomy and Phylogeny

Traditional Classification

In traditional taxonomy, Monothalamea was classified as a subclass within the phylum Foraminifera, encompassing all single-chambered (monothalamous) foraminiferans distinguished by their simple test morphology from multichambered groups. This hierarchy, primarily established through morphological examination, divided Monothalamea into two main orders: Allogromiida and Astrorhizida. The order Allogromiida included species with flexible, organic-walled tests composed of tectin (a proteinaceous material) or entirely testless (athalamid) forms lacking any rigid shell. Representative families within Allogromiida were Allogromiidae, featuring slender, tubular organic tests, and Biomarginidae, characterized by broader, flask-shaped organic walls. A notable example is Reticulomyxa filosa, a freshwater, testless species in the family Reticulomyxidae, known for its extensive, reticulose pseudopodial network that facilitates movement and feeding without a protective test. In contrast, the order Astrorhizida comprised agglutinated forms where the test was constructed from environmental particles, such as sand grains or mineral fragments, bound together by an organic cement. This provided a more robust, granular structure compared to the delicate organic walls of Allogromiida. Key families included Astrorhizidae, with elongated, branching tests resembling rhizopods, and Psammosphaeridae, featuring compact, spherical agglutinated tests. An illustrative taxon is Psammosphaera fusca, a marine with a smooth, spherical test formed by tightly packed grains, often found in deep-sea environments. These morphological criteria—primarily test composition and absence or presence of agglutination—formed the basis for distinguishing the orders, reflecting an evolutionary progression from simple organic or naked forms to more complex agglutinated structures. However, by the early 2000s, studies revealed significant limitations in this system, as overlapping morphological traits and led to artificial groupings. Specifically, both Allogromiida and Astrorhizida were recognized as polyphyletic assemblages, with monothalamous lineages showing greater phylogenetic diversity than morphology alone could resolve. This prompted a shift toward molecular approaches for more accurate classification.

Molecular-Based Classification

The molecular-based classification of Monothalamea relies primarily on analyses of small subunit (SSU rDNA) sequences, supplemented by (eDNA) surveys, which have revealed a complex phylogenetic structure far exceeding traditional morphological groupings. In a seminal 2013 revision by Pawlowski et al., monothalamids—encompassing naked and single-chambered with organic or agglutinated tests—were reclassified as a paraphyletic assemblage comprising 26 distinct clades, with 22 predominantly marine and 4 associated with freshwater or soil habitats. This framework, derived from complete SSU rDNA phylogenies and eDNA metabarcoding, demonstrated that Monothalamea does not form a monophyletic group but instead includes the basal ancestors of all , rendering it paraphyletic relative to multi-chambered lineages. Within the broader phylogeny of , Monothalamea occupies a basal position inside the supergroup , representing an early-diverging assemblage that branched off prior to the radiation of polythalamous groups. Xenophyophores, the giant deep-sea agglutinated foraminifera, are integrated as a derived within this paraphyletic structure, supported by SSU rDNA data linking them to other monothalamids despite their extreme morphological specialization. Key molecular insights highlight the early divergence of monothalamids, with their lineages emerging near the root of the foraminiferal tree, and underscore the of traditional morphological orders; for instance, Allogromiida spans multiple unrelated clades, reflecting in test wall composition rather than shared ancestry. Recent advancements in metabarcoding and reference databases have further refined this , integrating high-throughput sequencing to uncover extensive hidden diversity and cryptic within monothalamid clades. Studies from 2023–2024, such as those employing eDNA surveys of estuarine and deep-sea sediments, have identified numerous novel lineages among soft-walled monothalamids, confirming high levels of cryptic through exceeding 5% in SSU rDNA barcodes while maintaining morphological similarity. The BFR2 ribosomal reference dataset, released in 2024, curates over 250 sequences from monothalamid groups like Clade C and Xenophyophoroidea, enabling better assignment of metabarcodes and revealing that most unidentified foraminiferal operational taxonomic units (OTUs) belong to these basal taxa, thus emphasizing their underestimated evolutionary role. Ongoing research continues to uncover additional clades, potentially exceeding the initial 26 identified in 2013.

Morphology and Anatomy

Test Structure

The test of monothalameans is a single-chambered (monothalamous) structure lacking internal septa or partitions, which serves as a protective envelope for the protoplasm. This simple architecture allows for a unified cytoplasmic body within the test, typically featuring one or more apertures that enable the extension of reticulopodia or filopodia for locomotion, feeding, and interaction with the environment. Monothalamean tests exhibit two primary types based on composition: organic and agglutinated. Organic tests are constructed from tectin, a flexible, proteinaceous material secreted directly by the , providing a soft, translucent wall that is often thin and elastic. These are prevalent in families such as Allogromiidae, where the tectin-based wall lacks rigidity and can appear membranous or finely layered. In contrast, agglutinated tests are formed by assembling exogenous particles, such as grains, , clay, or fragments of other l tests, which are bound together by an organic or, less commonly, a matrix. This type dominates in deep-sea groups like xenophyophores (superfamily Xenophyophoroidea) and many members of Astrorhizida, where the incorporated particles create a robust, opaque, and sometimes coarse-textured wall that mirrors the surrounding composition. Test formation begins with secretion from the cytoplasm, which produces the initial organic lining or template. In organic tests, the is deposited layer by layer to shape the enclosure. For agglutinated varieties, collect and transport suitable particles from the substrate into the cytoplasmic network, where they are coated with an organic envelope before being cemented in place to build the . This selective incorporation ensures structural integrity, with the process often resulting in a multilayered wall that grows incrementally as the expands. The cytoplasm facilitates particle selection and binding, though detailed internal mechanisms are addressed elsewhere. Variations in test morphology are extensive, reflecting adaptations to diverse habitats. Some monothalameans, such as Reticulomyxa filosa, are entirely testless or naked, relying on a delicate cytoplasmic without any hardened covering. Tests, when present, adopt shapes ranging from spherical (e.g., in psammosphaerids) and ovoid to elongate-tubular or highly irregular and branching forms, particularly in xenophyophores that form fan-like or reticulate structures. varies dramatically, from diminutive forms measuring around 20–50 μm in length, typical of soft-walled species, to gigantic xenophyophores exceeding 20 cm in diameter, representing the largest known monothalameans. These extremes highlight the group's morphological plasticity, with larger agglutinated tests often providing stability in soft abyssal sediments.

Cytoplasmic Organization

Monothalamea exhibit a distinctive multinucleate cytoplasmic organization, with individual cells containing hundreds to thousands of nuclei distributed throughout the plasmodium-like body. This condition is particularly pronounced in species such as Reticulomyxa filosa, where nuclei, approximately 5 μm in diameter, undergo synchronous closed without , enabling coordinated cellular function across the expansive . In some cases, pseudochambers are formed by cytoplasmic folding, distinguishing monothalameans from multi-chambered foraminiferal classes. In xenophyophores, a of Monothalamea, the forms a multinucleate enclosed within branching organic tubes known as granellare, facilitating structural support and nutrient distribution. The is characterized by an extensive network of granuloreticulopodia, which are fine, anastomosing that extend from the cell body for locomotion and resource acquisition. These structures feature bidirectional streaming of , transporting vacuoles, products, and other inclusions along the network. In Reticulomyxa filosa, the peripheral granuloreticulopodia are slender and dynamic, supported by an amplified and molecular motors such as myosins, dyneins, and kinesins, which enable efficient cytoplasmic flow. (TEM) reveals a layered cytoplasmic arrangement, with a denser rich in organelles contrasting with the more translucent ectoplasm in pseudopodial extensions. Key organelles in Monothalamea include mitochondria and Golgi apparatus, essential for energy production and secretory processes, respectively. Mitochondria appear oval-shaped (0.5–1 μm) with prominent cristae under TEM, often concentrated near the plasma membrane to support . The Golgi apparatus consists of stacked cisternae producing transport vesicles (70 nm) and larger secretory vesicles (150–200 nm), which contribute to cytoplasmic maintenance. As heterotrophic organisms, Monothalamea lack chloroplasts, relying instead on ingested . In agglutinated forms, biomineralization-related fibrillar vesicles (approximately 500 nm) are abundant, aiding in the of organic matrix for test construction by binding foreign particles. Ultrastructural studies via thin sections highlight vacuolization patterns, particularly in deep-sea xenophyophores, where large vacuoles (10–200 μm) house intracellular barite (barium sulfate) crystals known as granellae. These crystals, unique to xenophyophores, accumulate in the granellare and stercomata (waste aggregates), potentially contributing to cellular ballast or detoxification, though their precise function remains under investigation. In monothalamous species like Ovammina opaca, TEM shows nuclei with distinct lamina and nucleoli embedded in this vacuolated cytoplasm, underscoring the adaptive complexity of Monothalamea cellular architecture.

Biology and Physiology

Reproduction

Monothalamea primarily reproduce asexually through binary or multiple fission, a process facilitated by their often multinucleate . In smaller forms, such as those in the order Allogromiida, multiple fission divides the into numerous daughter cells, each developing its own . Larger, reticulopodial species like Reticulomyxa employ fragmentation, where the extensive cytoplasmic network breaks into 2–3 propagules that grow into new individuals. Sexual reproduction remains poorly documented and appears rare in Monothalamea, with observations limited to a few allogromiid species. In Niveus flexilis, occurs within the gamont, producing numerous small biflagellated gametes that are released through the test into the surrounding for . An alternation of sexual and asexual generations has been hypothesized for some monothalamids, including xenophyophores, but remains unconfirmed due to the lack of complete life cycle observations. Like other , the life cycle of Monothalamea involves an alternation of sexual (gamont) and asexual (agamont) generations, though it is poorly documented in most , featuring direct development from either gametes or asexual propagules without prolonged larval stages. Reproductive processes are modulated by environmental cues, particularly , which influences fission rates; deep-sea monothalamids likely exhibit slow owing to low thermal regimes.

Feeding and Nutrition

Monothalamea employ as their primary feeding mechanism, using granuloreticulopodia to capture and ingest food particles such as , , and small protists from sediments or the . These reticulose , characterized by their fine, anastomosing threads with bidirectional granular flow, enable efficient particle entrapment and transport to the main cell body. Primarily functioning as deposit feeders, Monothalamea process organic-rich sediments to extract nutrients, with xenophyophores notably ingesting large volumes of detrital particulates evidenced by their production of mineral grain-rich fecal material. Some species display omnivorous tendencies, incorporating microalgae and other small eukaryotes into their diet alongside bacterial prey. Once captured, food particles are enclosed in membrane-bound vacuoles and transported intracellularly via , where digestion occurs through lysosomal enzymes in acidic environments. Indigestible residues are compacted and expelled as stercomata, granular fecal pellets that contribute to reworking. Key adaptations include the high surface area of granuloreticulopodia, which maximizes contact for nutrient uptake beyond mere . In some taxa, such as xenophyophores, bacterial associations on stercomes provide symbiotic benefits by supplementing the detrital diet with labile organic compounds from cultivated microbes. Recent microbiome analyses (as of 2025) reveal associations with electrogenic bacteria in xenophyophores, potentially facilitating and nutrient supplementation.

Ecology and Distribution

Habitats

Monothalamea primarily inhabit marine environments, ranging from intertidal mudflats and coastal zones to abyssal plains at depths exceeding 10,000 meters. In shallow coastal settings, such as fjords, bays, and sublittoral areas, they occupy soft sediments rich in , where species like those in the Allogromiidae thrive in fine-grained, muddy substrates. Deeper populations, including xenophyophores, are found on open ocean floors, often as epifaunal forms adhering to the sediment surface, with records extending to the at approximately 10,600 meters. These deep-sea monothalamids prefer low-energy, stable environments with minimal disturbance, though some tolerate hypoxic conditions near hydrocarbon seeps. Freshwater occurrences of Monothalamea are rare but documented in lakes, ponds, and rivers, where they demonstrate tolerance for low-salinity conditions transitioning from brackish to inland waters. For instance, Reticulomyxa filosa inhabits nutrient-enriched freshwater bodies, such as ponds with decaying vegetation, highlighting their adaptability to non-marine settings despite the group's predominantly affinity. These populations are often overlooked due to low densities and challenging detection in oligotrophic inland systems. Terrestrial occurrences are also rare, primarily in moist environments like damp soils and decomposing matter, with Reticulomyxa filosa reported in such habitats. At the microhabitat scale, Monothalamea frequently burrow into anoxic, sulfidic s, exploiting organic-rich, fine-grained deposits for nutrient availability, as seen in species from the Black Sea's oxic-anoxic interface. In the , xenophyophores like those in the Clarion-Clipperton Zone form surface aggregations on abyssal muds, contributing to localized sediment heterogeneity. Their distribution is cosmopolitan, with elevated diversity in polar regions such as western Svalbard fjords and southwest , as well as equatorial Pacific abyssal plains, reflecting broad environmental tolerance across latitudinal gradients.

Ecological Roles

Monothalamea, particularly the xenophyophore subgroup, play significant roles in deep-sea biogeochemical cycling through sediment bioturbation and nutrient recycling. These organisms enhance the deposition of organic particles onto the seafloor by trapping phytodetritus with their complex test structures, creating localized hotspots of carbon accumulation that stimulate microbial decomposition and mineralization processes. Their tests, often enriched with barite (barium sulfate) crystals, contribute to the fixation of sulfate in sediments, indirectly supporting sulfur dynamics in oxygen-poor environments. By mixing surface sediments during feeding and movement, monothalamids promote the remineralization of organic matter, facilitating nutrient release such as nitrogen and phosphorus back into the water column for primary production. Xenophyophores within Monothalamea provide essential structures in otherwise barren deep-sea sediments, acting as nurseries and refuges for and . Their large, intricate tests offer attachment sites and shelter for juvenile (Liparidae), with observations of up to 20 eggs or embryos per individual, as well as for nematodes, peracarid crustaceans, ophiuroids, and sipunculans. These structures increase habitat heterogeneity, leading to higher local in xenophyophore-dominated areas compared to surrounding sediments, where associated metazoan densities can be elevated due to enhanced food availability from trapped organics. As benthic detritivores, Monothalamea occupy a basal trophic position in deep-sea food webs, consuming organic and to transfer to higher levels. Benthic , including monothalamids, process phytodetritus and serve as primary consumers, linking microbial communities to macrofaunal predators and contributing to overall stability. Their abundance and distribution make them effective indicators of deep-sea , reflecting changes in organic and conditions. Monothalamea face significant threats from deep-sea mining activities, particularly in nodule-rich zones like the Clarion-Clipperton Zone, where sediment plumes and removal can destroy fragile tests and disrupt associated communities. Xenophyophores are designated as indicators of Vulnerable Marine Ecosystems (VMEs) by the due to their slow recovery potential and ecological importance, underscoring the need for conservation measures to protect these sensitive s from anthropogenic disturbance.

Diversity and Evolution

Major Groups and Clades

Monothalamea represents a paraphyletic assemblage of single-chambered characterized by organic or agglutinated tests, encompassing traditional groups such as Allogromiida and Astrorhizida, as well as the deep-sea Xenophyophoroidea. A molecular phylogenetic analysis based on small subunit sequences identified 26 distinct s within Monothalamea, including 22 marine clades and four freshwater clades designated A through D. These marine clades encompass diverse environmental lineages, such as the xenophyophore (clade 18), which includes large, agglutinated deep-sea forms, and the psammosphaerid , featuring spherical, sand-grain tests. Major groups within Monothalamea align with both morphological traits and molecular data, though the assemblage's complicates traditional . Xenophyophoroidea form a prominent subgroup of giant, agglutinated species adapted to deep-sea environments, often exceeding several centimeters in size and constructing elaborate tests from particles. Remnants of Allogromiida include soft-walled, organic-tested forms typically found in shallow-water marine and freshwater settings, with simple, uninucleate or multinucleate cytoplasmic organization. Derivatives of Astrorhizida comprise tubular or branching agglutinated structures, bridging monothalamean simplicity with more complex multichambered forms in related lineages. Morphological features correlate with specific clades, highlighting the integration of traditional morphology and . For instance, 7 contains Reticulomyxa-like naked or thinly walled forms with reticulopodial networks, resembling freshwater reticulose species but occurring in marine contexts. 18 distinctly groups xenophyophores, characterized by their massive, multinucleate tests and sediment-agglutinating stercomata. These correlations underscore the basal position of Monothalamea in foraminiferal evolution. The basal diversification of Monothalamea traces to the , approximately 600 million years ago, marking an early radiation of single-chambered rhizarians before the emergence of multichambered tests in the . (eDNA) surveys via high-throughput sequencing have revealed substantial undescribed diversity, with Monothalamea comprising up to 50% of foraminiferal operational taxonomic units in deep-sea sediments, far exceeding morphologically known .

Notable Species and Recent Discoveries

Among the notable species exemplifying Monothalamea diversity is Rhizammina algaeformis, a large abyssal foraminifer related to xenophyophores that can attain sizes up to 20 cm and constructs its agglutinated test from sediment particles, highlighting adaptations to deep-sea environments. Another significant example is Vellaria solenta, a soft-walled monothalamean described in 2021 from hypersaline sediments in Sivash Bay of the (connected to the basin), characterized by filling the entire test and a simple . In Antarctic waters, species of the genus Psammophaga, such as P. magnetica, represent agglutinated monothalamids that incorporate grains into their tests, aiding in magnetic orientation and found in coastal shelf habitats of . The freshwater species Reticulomyxa filosa serves as a key for studying foraminiferal biology, with its sequenced to reveal insights into and life-cycle complexity in non-marine settings. Recent discoveries underscore the ongoing exploration of Monothalamea. In 2024, Gooday et al. described several new xenophyophore species and morphotypes from the eastern Clarion-Clipperton Zone in the equatorial Pacific, expanding knowledge of abyssal in nodule-rich areas. From 2022 surveys in fjords, Gooday et al. identified two new monothalamean species and a new genus within distinct clades, based on samples from the fjord system. Metabarcoding of coastal sediments in 2022 revealed high monothalamean diversity, with numerous operational taxonomic units indicating previously unrecognized lineages linked to water mass circulation. In 2025, Holzmann et al. described the new genus Flaviatella, a monothalamous foraminifer with a wide geographical distribution. Additionally, two new freshwater monothalamean species were described from habitats. Current research trends emphasize integrative taxonomy, combining morphological observations with to resolve cryptic diversity; for instance, surveys often show approximately 50% of monothalamean morphospecies remaining undescribed, particularly in deep-sea and polar environments.

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