This study investigated the variability within populations of the copepod Eurytemora velox. Integrative taxonomy confirmed significant genetic heterogeneity among the studied populations supported the hypothesis of E. velox being a species complex. To assess morphological variability, morphometric traits and ordination analyses were utilized. For molecular assessment, the mitochondrial cytochrome c oxidase subunit 1 (COI) and cytochrome b (cyt b) and the nuclear 18S ribosomal DNA (18S) and the internal transcribed spacer 2 (ITS2) were employed. Both morphological and molecular data facilitated the identification of at least two species: E. velox s. str. and a new cryptic species, E. nilsseni sp. nov. The former one, E. velox s. str. is an expansive, widely distributed species that inhabits diverse water bodies, including transformed river systems, particularly those with altered water parameters such as high salinity. Within this species, no population-specific morphological differences were observed; however, COI data revealed clear genetic structuring in E. velox s. str., with distinct mitochondrial haplotype clades associated with different drainage basins, whereas nuclear markers showed only limited differentiation among populations. In contrast, E. nilsseni sp. nov. is an endemic and likely relict species recorded exclusively in ponds on the Skagerrak coast islands.
Acoela are a group of microscopic worms that remains poorly known despite their high abundance and world-wide distribution. Within Acoela, the family Dakuidae has undergone several taxonomic changes in recent decades, yet its internal phylogeny remains unclear. Herein, we describe seven new species of Dakuidae from Croatia, Germany, Italy, Panama, and Sweden: Philactinoposthia brunopunctata sp. nov., Philactinoposthia centrofusca sp. nov., Philactinoposthia raikovae sp. nov., Philactinoposthia gracilia sp. nov., Philactinoposthia parvipenis sp. nov., Philocelis leasae sp. nov. and Philocelis dunensis sp. nov. Mitochondrial (COI) and nuclear (18 S, 28 S) sequences were used to assess the phylogenetic relationships within the family, supporting the presence of two sister groups: the first comprising species of the genus Philocelis along with Philactinoposthia saliens, Philactinoposthia rhammifera and Daku woorimensis, and the second consisting of other species of the genus Philactinoposthia. P. saliens and P. rhammifera are transferred from Philactinoposthia to Philocelis. The genus Daku is synonymised with Philocelis. Finally, we assessed the diagnostic relevance of the morphological characters traditionally used in the taxonomy of Dakuidae, proposing emended diagnoses for the genera Philocelis and Philactinoposthia.
Phylogenetic inference aims to propose hypotheses that explain the evolutionary history of a group of organisms, typically represented as phylogenetic trees. These trees are constructed by analysing morphological, molecular, or other types of evolutionary data and applying optimisation methods. However, different topologies often yield equally optimal solutions for the same group under study, complicating the interpretation of a single evolutionary scenario. Consensus techniques are commonly employed to summarise agreement among these optimal trees. However, because consensus trees collapse conflicting relationships, they do not preserve the fully resolved structure of any optimal topology, which restricts analyses that require a single resolved tree with a defined parsimony score. Additionally, the polytomies arising from node collapse complicate the estimation of morphological rate of change and hinder the incorporation of temporal information into phylogenetic hypotheses. We present a pipeline that applies medoid selection and clustering techniques to characterise the topological diversity among most-parsimonious trees and to identify a fully resolved representative topology that preserves parsimony optimality. Rather than replacing consensus approaches, this method provides a complementary summary that retains a single optimal tree while making the structure of the broader phylogenetic landscape explicit. As case studies, we applied the pipeline to the phylogenetic analyses of Burkesuchus mallingrandensis and Arackar licanantay, two fossil taxa from Chile. In both cases, the pipeline recovered representative optimal trees that complement the interpretations of the original studies. The approach also supports analyses that require a resolved topology, including estimating morphological change rates and incorporating temporal information via time-scaling.
The latest phylogenetic analysis of Kinorhyncha established the major group Anomoirhaga, which encompasses the greatest morphological diversity within the phylum. Although the myoanatomy of most Anomoirhaga families has been documented, it remains undescribed for Neocentrophyidae which includes the three genera Mixtophyes, Paracentrophyes and Neocentrophyes. To address this gap, we characterised the musculature of Paracentrophyes quadridentatus through F-actin staining, confocal laser scanning microscopy (CLSM), and 3D reconstruction. In addition, we defined, coded, and traced the evolution of twenty-two myoanatomical characters across eight kinorhynch families, reconstructing their ancestral states to gain insights into muscle evolution within the phylum. Seven myoanatomical synapomorphies were found, supporting several kinorhynch clades. Multiple muscles present in P. quadridentatus constitute unique apomorphies within Anomoirhaga. Yet others are similar (through convergence) to muscles in evolutionary distant taxa with similar external morphology. This, and the finding of thirteen homoplastic muscular characters, suggest a high plasticity in the evolution of kinorhynch musculature. The results of this study present a basis for comparative studies of kinorhynch myoanatomy, which can enhance our understanding of muscular functions, and the evolutionary processes underpinning myoanatomical adaptations in invertebrates.
The taxonomy of tardigrades has undergone significant changes over the past two decades, primarily due to the introduction of molecular techniques. However, despite the dynamic development of this branch of zoology, the phylogenetic position of most species remains unknown, indicating the continued need for integrative descriptions and redescriptions. A notable example is the genus Minibiotus, of which morphologically diversity raises the possibility of its polyphyly. However, the extremely limited genetic data and frequent lack of detailed morphological information hinder the verification of relationships within this genus. In this study, we provide an integrative analysis of Minibiotus jonesorum based on a newly discovered population from Devil’s Lake State Park, Baraboo, Wisconsin, USA. Our approach combines traditional taxonomic methods, including morphological and morphometric analyses using light and scanning electron microscopy, alongside genetic data for four markers: three nuclear markers (18S rRNA, 28S rRNA, ITS-2) and one mitochondrial marker (COI). Thanks to the discovery of a new population and a re-examination of the type series, we reveal new traits and update the description and the differential diagnosis of M. jonesorum. Importantly, we show that the microplacoid, a taxonomical important trait in many eutardigrade lineages, can be absent or developed to different extents within a single species. Additionally, for the first time, we constructed a concatenated phylogenetic tree for all available sequences of the genus Minibiotus using Bayesian inference, which provides new insights into the enigmatic evolution of the genus.
The non-marine ostracod subfamily Candonopsinae is typically defined by the absence of a posterior seta on the caudal ramus. However, due to frequent reductions in chaetotaxy across ostracod lineages, this character must be evaluated cautiously and alongside other morphological traits, ideally supported by molecular data within an integrative taxonomic framework. Candonopsis, the type genus of Candonopsinae, comprises 23 extant species distributed worldwide, mostly in subtropical and tropical regions of the Southern Hemisphere. Of the six European species, two are epigean and four hypogean. Here, we describe a new hypogean species Candonopsis danilucae sp. nov., based on male and female specimens collected from the hyporheic zones of three rivers in the Dordogne catchment, department of Corrèze, central France. Morphologically, the new species is distinguished by a unique carapace shape and the structure of the male copulatory organ, particularly the inner lobe of the hemipenis. Molecular comparisons based on mitochondrial COI and nuclear 28S rDNA markers further support its distinctiveness and indicate that its closest genetic affinities among the taxa included in the analysis are with Candonopsis kingsleii (Brady Robertson, 1870), the type species of the genus. Notably, this study, using newly designed primers, also reports the first molecular evidence of intracellular Cardinium infection in a groundwater crustacean, suggesting that the female-biased sex ratio observed in C. danilucae sp. nov. may result from sex manipulation by this endosymbiont. Our findings highlight the importance of integrative taxonomy, combining carapace morphology, limb structure, habitat preferences, and molecular data, to resolve species boundaries and understand symbiotic associations in subterranean ostracods.
The ecomorphology of a group of Steninae rove beetles from Northern Thailand was analysed using phylogenetically informed comparative methods. Links were identified between micromorphological features, their associated performance consequences (in terms of prey-capture and tarsal attachment), and habitat niche differentiation. Ancestral character reconstruction revealed evolutionary trends. Eye size appeared to be involved in a sensory trade-off with antenna length. While most morphological traits conformed best to a Brownian motion model of trait evolution (i.e. a neutral model), others were better explained by Ornstein–Uhlenbeck or Early Burst models. Low phylogenetic signals were found in body, antenna, and leg length, as well as in tarsus morphology, supporting the idea that these traits underwent convergent adaptive radiation. Similarly, low phylogenetic signal was observed in traits describing the adhesive paraglossae in Stenus, reflecting their distinct morphological types. In contrast, habitat niche width showed a high phylogenetic signal, whereas elevational niche width appeared phylogenetically labile. In multivariate morphospace, Dianous and Stenus occupy largely non-overlapping regions, revealing a pattern consistent with niche complementarity. In Steninae - and especially in Stenus - the exploration of vertical structures has been facilitated by the widening of the tarsi, a trait associated with an increase in habitat niche width. Overall, the findings are consistent with a pattern of adaptive radiation, in which ecomorphological traits exhibit varying degrees of evolutionary lability.
The steppe flora of the Mediterranean basin is essential for understanding plant assemblages in the region and is a key target for conservation efforts. Recent phylogenetic and biogeographic studies trace the origins of these lineages to events such as the Messinian Salinity Crisis and Quaternary climatic shifts, but further research is needed to fully understand their evolutionary history. Astragalus sect. Platyglottis Bunge (Fabaceae) includes c. 14 species and subspecies from steppe and semi-arid environments throughout West Asia and the Mediterranean Region. The Iberian Peninsula harbours four members of the section distributed in disjunct and restricted ranges: A. devesae, A. gines-lopezii, A. nitidiflorus and A. peregrinus subsp. warionis. The first three are threatened Iberian endemics. We sequenced the nuclear ITS and the plastid trnK/matK region to determine their phylogenetic placement. We also estimated divergence times and ancestral ranges, and analysed plastid DNA haplotypes to further investigate the evolution and genetic diversity of Iberian populations. We found that Astragalus sect. Platyglottis is polyphyletic, but all Iberian taxa form a monophyletic group with two other Mediterranean taxa (A. peregrinus subsp. peregrinus and A. suberosus subsp. haarbachii). Ancestral colonisation of the Mediterranean Region from West Asia appears to have occurred since the Pliocene, followed by Pleistocene diversification of the westernmost lineages of Platyglottis. Each Iberian taxon displayed a unique plastid haplotype, shared by all individuals. This study deepens our understanding of some distinctive and threatened Iberian representatives of the largest genus of angiosperms and provides new information regarding their evolutionary history, biogeography and conservation.
This study presents a comprehensive phylogenetic and molecular clock analysis of Polycladida based on the mitochondrial marker cytochrome c oxidase subunit I (cox1). By including a broad representation of taxa across both suborders (Acotylea and Cotylea) and integrating evolutionary rate estimates, this work offers new insights into the evolutionary history of the group. The phylogenetic reconstruction supports previous studies while refining the relationships among key lineages. Our analyses strongly support Boniniidae as the earliest-diverging lineage of Polycladida. Cestoplanidae and Pericelidae are consistently recovered together as the sister group to Acotylea, highlighting their key position in the early diversification of the order. Within Cotylea, Anonymidae forms a well-supported monophyletic clade, though its placement varies between analyses. Within Cotylea, the monophyly of major families is challenged, particularly in Euryleptidae and Pseudocerotidae, indicating cryptic lineages and convergent traits. Acotylea displays greater phylogenetic instability, with Stylochoidea and Discoceloidea showing limited support, in contrast to the more cohesive Leptoplanoidea. For the first time, a molecular clock analysis was conducted in Polycladida, revealing that Boniniidae diverged approximately 550 million years ago, acting as a stem-group lineage that retains ancestral traits and bridges the evolutionary transition between Cotylea and Acotylea. Major diversification events within Polycladida occurred between 360 and 350 Mya, aligning with late Devonian–early Carboniferous marine expansions. Cotylea likely originated around 250 Mya, coinciding with reef diversification in the Mesozoic, while the divergence of Cotylea and Acotylea species occurred during the early Mesozoic (100–50 Mya), following the Cretaceous–Paleogene mass extinction. These findings underscore the importance of integrating molecular, morphological, and temporal data, with the molecular clock analyses providing a coherent framework that resolves long-standing uncertainties in the early diversification of Polycladida.
Chironius laurenti was described based on 27 specimens from localities of the Beni savanna and Chiquitano Dry Forest in Bolivia and one specimen from the Brazilian Cerrado. We examined the holotype of C. laurenti from the department of Beni, Bolivia, as well as most of the paratypes and expanded sample obtained in zoological collections. Based on morphological and molecular data, we restrict the name C. laurenti to the populations distributed throughout the Madeira River Basin and conclude that the known records throughout the Pantanal wetlands and transition areas with the Cerrado savanna represent a distinct entity erroneously attributed to this name. Therefore, we describe the latter populations as a new species endemic to the Upper Paraguay River Basin, distinguished from its congeners by a unique combination of qualitative and quantitative morphological characters (meristic, color pattern, and cranial osteology) and unequivocal molecular evidence. We report on relevant behavioral observations of the new species, specifically its foraging strategy on riverbanks and male-male combat, which may occur during the mating season. Additionally, we discuss possible diversification hypotheses in the Upper Paraguay River Basin and highlight the importance of conserving ecological corridors linking the Pantanal to the surrounding plateaus, as ongoing climate changes may restrict or displace present species’ distributions. http://www.zoobank.org/urn:lsid:zoobank.org:pub:D1DCEB24-8C69-4B76-8387-CD9ACB23C733 .
The northeastern Atlantic species of nudibranch genus Polycera have received much attention in recent years, however southern African representatives remain to be further investigated. Here, we further examine the genus along the south and southwest coasts of southern Africa, combining molecular and morphological data. We describe three new species, Polycera namaquae sp. nov., Polycera enesithsaba sp. nov. and Polycera onomqhele sp. nov., in addition to the already documented P. capensis and P. hedgpethi. We demonstrate variation in external appearance within the species we examined, highlighting the importance of incorporating genetic data in species identification and descriptions. Molecular evidence supports the hypothesis that Polycera hedgpethi is introduced in South Africa, likely from the northeastern Pacific coast, while all other species share close relationships to species from the northeastern Atlantic with more biogeographical connections spanning hemispheres, rather than within. Recovered ranges of each species within southern Africa follow defined biogeographical boundaries, highlighting the importance of these diverse ecoregions in shaping marine biodiversity. Overall, this work suggests multiple vicariant events as drivers of diversification of the southern African and northeastern Atlantic coastal biota.
We measured levels of shape variability to investigate whether differences in disparity are associated with ecological and historical events among selected fish species using the Gerreidae family. We measured body shape disparity according to three biologically contrasting events: (1) How does the disparity change with an ecological shift from marine to freshwater habitats? (2) Does disparity change with increasing taxonomic richness within the genera? and (3) How did vicariant events across the Panama isthmus change disparity among sister species? Additionally, we explored the relationships between morphological and molecular variability in the 17 Gerreidae fishes studied. The results indicate that the freshwater environment did not influence the morphological disparity of body shape in E. mexicanus. Additionally, the morphological disparity in body shape did not increase with increasing taxonomic richness within the five Gerreidae genera studied. A comparison of disparity values suggests that the body shape of sister fish species derived from the separation of the Pacific and Atlantic Oceans due to the emergence of the isthmus of Panama evolved at a similar rate. The disparity in shape and the nucleotide variability were not correlated among the Gerreidae fish species examined. These findings could have significant implications for our understanding of evolutionary processes.
Extrusomes are membrane-bounded extrusive organelles occurring in multiple ciliated protozoa. These organelles possess different structures containing substances that are typically expelled or extruded from the cell to perform various functions. Prior studies have mostly investigated the ultrastructure, biogenesis, function, and extrusion mechanism of different types of extrusomes in specific ciliates. In contrast, comparative and evolutionary genomic studies among extrusome-bearing ciliates have been largely overlooked. Using omics data from 71 ciliates, we conducted a comparative genomics study focusing on five ciliate taxa with extrusomes (Tetrahymena, Paramecium, Haptoria, Strombidium, and Euplotes). We recovered the sequences of trichocyst matrix proteins (TMPs) from nine Paramecium species and performed the corresponding molecular evolution analyses. Evolutionary history analysis emphasized the integral role of the Cryogenian Ice Age in the evolution of the predatory taxon Haptoria and revealed the effects of the two largest Paleozoic radiations on the biodiversity of Paramecium, Haptoria, and Euplotes. Functional enrichment analyses of significantly expanded gene families in five target ciliate taxa highlighted the importance of genes related to metabolism, transport, catalytic activity, and cell signaling. Most importantly, we proposed a conceptual framework that outlines the potential functional coordination among key components in the execution of ciliate extrusome-related cellular behaviors. Together, the results presented in this study elucidate the evolutionary and genomic adaptations of ciliates with extrusomes and provide a blueprint for future investigations of extrusome-related behaviors.
The Mediterranean Basin has been subjected to geological and paleoclimatic events that shaped the biogeographic patterns of many thermophilic taxa. Alongside these natural events, humans have also played a significant role in shaping the distribution of many species, especially those that have long been culturally related to humans. We combined multilocus genetic and environmental data to assess the relative role of climate and geographic barriers on the biogeography of the Western Mediterranean Pond turtle, Mauremys leprosa, a chelonian widely distributed throughout the Iberian Peninsula and the Maghreb region that has been historically traded by humans. Our results show (1) high levels of genetic diversity and structure within populations located across the Atlas Mountains in Morocco, and (2) shallow patterns of contemporary genetic structure among Iberian populations. Moreover, we identified one individual in the Iberian Peninsula that likely originated from a population situated across the Atlas Mountains. The absence of genetic structure across the Iberian Peninsula may result from a combination of a recent colonization of M. leprosa from Morocco and the mixing effects of human-mediated translocations within Iberia. We emphasize the importance of genetic characterization for the conservation of species that have been historically traded by humans, as is the case for chelonians.
Borghiellaceae are a small group of dinophytes that frequently lack distinctive morphological traits, which is why integrative approaches including DNA sequence information is of particular importance for their species delimitation. An undulated margin of the posterior cell half is rarely documented in dinophytes such as Gymnodinium undulatum and has been considered a teratological trait (‘abnormalcy’) in the past. Unialgal strains were established from field material collected near Brombach (Bavaria, Germany). Molecular sequences of the rRNA complex were gained for phylogenetics, and strain GeoM*889 was analysed in detail using light and electron microscopy. The predominant flagellated cells were highly polymorphic but showed consistently a right undulated margin of the posterior cell half. Molecular sequence data indicated a placement of the organism within Borghiellaceae, and Borghiella cf. tenuissima was retrieved as closest relative. Therefore, we perform a taxonomic transfer of the studied material to Borghiella undulata, comb. nov., which is hereby documented for the first time reliably after its description just 100 years ago. Despite very different morphologies, the sister species differ in three ITS positions only, which makes it impossible to differentiate between them in metabarcoding studies when the V4 region of the SSU is used as genetic marker. The less frequent smaller flagellated cells are discussed as dispersal units rather than gametes.
Histriobdellidae, the so-called Charlie Chaplin worms, is an enigmatic group of microscopic commensal annelids associated with crustaceans. They crawl by alternately attaching their adhesive anterior appendages and left and right huge lateral ‘feet’, and bear a complex jaw apparatus in the ventral muscular pharynx. Although histriobdellids were always thought to be a part of the jaw-bearing clade Eunicida, their exact placement within the annelid tree is still debated due to their highly derived external morphology and long branch attraction (LBA) artefacts in molecular analyses. In this study we employ morphological and molecular comparative approaches in order to gain new insights into the evolution of Histriobdellidae and its aberrant traits. Our phylogenetic analyses of Eunicida including 52 species and four molecular markers yield further support for Histriobdellidae being the sister group to the eunicid family Dorvilleidae. The detailed morphology of Histriobdella homari van Beneden, 1858, a commensal of the European lobster, was examined using standard immunohistochemical stainings and subsequent confocal laser scanning microscopy (CLSM) as well as scanning electron microscopy (SEM). Integrative analyses allow us to compare in detail with other eunicidans and unravel extensive anatomical transformations in Histriobdellidae. Neural innervation patterns help verify the presence of antennae and true annelid palps on the histriobdellid prostomium. The arrangement of ganglia and the neuronal scaffold innervating the anterior end supports the presence of a buccal segment (peristomium) in Histriobdella. Additionally, based on our comprehensive investigations we newly propose their adhesive anterior locomotory appendages to be homologous with parapodia in other Annelida, and their posterior-most adhesive locomotory appendages (feet) to show characters of both parapodial and pygidial structures. Detailed studies of this highly deviating family of annelids not only exemplify how to reconstruct extreme transformation of canonical annelid characters such as parapodia, but also highlight the exceptional evolutionary plasticity of the annelid body plan once more.
Integrative taxonomy facilitates determining species boundaries in morphologically conserved taxa with limited dispersal abilities, such as litter-dwelling pseudoscorpions in the genus Allochthonius (family Pseudotyrannochthiniidae). This genus is common in East Asia but taxonomically challenging due to its cryptic morphology and variable diagnostic characters. We present a multi-locus phylogeny of Allochthonius in Japan, Korea, and Taiwan, including 55 newly-acquired specimens from 11 locations across Taiwan. We find that Allochthonius in Taiwan is monophyletic and comprises multiple undescribed species. Consolidating morphology with molecular species delimitation based on three tree-based or distance-based methods, we recognize seven new species with replicable morphological and molecular differences: A. agyaqensis sp. nov., A. aquaeductensis sp. nov., A. brunus sp. nov., A. kiunnensis sp. nov., A. nebulosus sp. nov., A. nubicolus sp. nov., and A. tamanensis sp. nov. Phylogenetic analyses and divergence time estimations suggest that Taiwanese Allochthonius separated from its Korea-Japan relatives ca. 15 million years ago, and that cooling climates during the Late Miocene and subsequent warming and rapid uplift of the Central Mountain Range during Pliocene likely promoted diversification. Spatial mapping suggests that litter-dwelling Allochthonius species living in tropical and subtropical montane regions may have a sky-island distribution with high endemism and naturally small distribution ranges.
Recent molecular phylogenies of Nematoda have suggested a sister-group relationship between Dorylaimia and Chromadoria, but supporting morphological evidence has so far been lacking. Using immunohistochemistry in combination with confocal laser scanning microscopy, we identified pharynx-associated musculature in Stilbonematinae, consisting of buccal dilators, somato-pharyngeal muscles, and a longitudinal spiral muscle encircling nearly the entire pharynx. To trace the evolutionary origin of these structures, we extended our investigation to closely related marine outgroups within Desmodorida but also the basally branching Chromadorida and Enoplia. Somato-pharyngeal muscles, which arise from the body wall musculature and attach to the pharynx surface, function as retractors in Chromadoria and Dorylaimia. In some Enoplia, however, they extend towards the posterior pharynx and act as protractors. These muscles are homologous and thus represent part of the ancestral body plan of Nematoda. Homologous buccal dilators and spiral muscles occur in both Dorylaimia and Chromadoria, whereas in Oncholaimina (Enoplia) buccal dilators—acting as protractors—have evolved independently. Taken together, these morphological findings strongly support a sister-group relationship between Dorylaimia and Chromadoria. A statistical analysis of the Stilbonematinae pharynx and its spiral musculature showed that the number of spiral coils is strongly correlated with pharynx slenderness (length-to-width ratio), in line with Roggen’s pharynx model. In both Dorylaimia and Chromadoria, the spiral musculature surrounding the posterior pharynx likely generates injection pressure, aiding the posterior transport of ingested food into the intestine as well as the anterior movement of secretions from pharyngeal glands.
Heleobia is the most speciose genus in the family Cochliopidae, with nearly 100 valid species, many of which are morphologically similar. These aquatic snails reach their highest diversity in the Neotropics, although many areas remain poorly explored and numerous populations have uncertain taxonomic identity. To clarify the systematic position of isolated highland populations from northern Chile, I conducted mitochondrial DNA analyses using maximum parsimony (MP), neighbor-joining (NJ), and Bayesian inference (BI), complemented by morphological studies. A population from the Talar Saltpan in the Puna de Atacama was consistently recovered by all molecular methods as a new species, sister to the Altiplano species Heleobia ascotanensis. Another population, located 170 m from Tilopozo (Atacama Saltpan), the type locality of Heleobia atacamensis, was identified as conspecific with this species. Three additional populations from the Chilean Altiplano, currently unassigned to any species, formed a monophyletic group with Heleobia opachensis in the MP and NJ analyses, sister to Heleobia carcotensis, although without node support; based on both phylogenetic placement and genetic similarity, they were assigned to H. opachensis. Genetic divergence of up to 2.3
In members of Cladobranchia sea slugs, unique adaptations to incorporate cnidocysts and algal symbionts from their cnidarian prey have evolved. However, many aspects underpinning the recognition and maintenance of these stolen cellular components are still unclear. Regarding the algal symbionts, little is known about the exact Symbiodiniaceae species and their abundance and diversity in Cladobranchia. Yet, understanding the diversity of Symbiodiniacee inside the slugs can help better understand the symbionts’ role in establishing and maintaining the symbiosis. We analysed the Symbiodiniaceae diversity across multiple Cladobranchia genera and species, focusing on the genus Phyllodesmium, which contains most of the known cladobranchs in a long-term symbiosis with the algae. Using high-throughput metabarcoding of the Symbiodiniaceae ITS2 region, we found that species of the genus Phyllodesmium harboured primarily Cladocopium, showing a genus-specific Symbiodiniaceae profile. Within a cladobranch genus, we also uncovered species-specific intragenomic variants of the respective symbiodiniacean genus. Our results reveal a previously unexplored diversity of algal symbionts in Cladobranchia and that Cladocopium might be particularly relevant for establishing long-term symbiosis. Cladocopium exhibits enhanced carbon fixation capabilities in symbiosis with corals, which may thus facilitate the symbiosis from the symbionts’ side. Consequently, studying symbiont diversity and abundance in Cladobranchia is essential for understanding the mechanism of symbiosis initiation and maintenance.
