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ABSTRACT We report a complex community of ectosymbionts in the egg mass of the horsehair crab Erimacrus isenbeckii (Brandt, 1848) from Hokkaido, Japan. We examined eight individual crab pleopods containing eggs and 56 pleopods without eggs. The following ectosymbionts were found: 207 turbellarians, 22 copepods, two nematodes, two ciliates, and one nemertean. Our preliminary morphological observation and molecular phylogenetic analysis based on partial sequences of the 28S rDNA suggested that the turbellarians would belong to the genus Ectocotyla Hyman, 1944, which was nested within Monocelidinae in a maximum-likelihood phylogenetic tree along with Minona Marcus, 1946, Monocelis Ehrenberg, 1831, and Pseudomonocelis Meixner, 1943, members of which commonly possess the simplex type of male copulatory bulbs. Further studies on the feeding biology of this monocelidid may lead to its possible impact on the crab egg mass community and/or the host crab’s population dynamics. This study also reveals the crustacean egg mass as a unique habitat for turbellarians.

Life is not a beach for those animals that survive in the rough ecological conditions found in marine sandy beaches – and yet, microscopic animals thrive on them. We explore the drivers for meiofaunal diversity in beaches by analysing taxonomic and functional patterns of 348 flatworm communities across 116 reflective beaches in the western Mediterranean, totalling 152 species (61.2% new to science). First, we confirm that species richness does not differ between beach hydrodynamic levels (swash, shoaling and surf) but rather depends on the characteristics of each beach. Second, we demonstrate that species composition across those levels depends on the species traits, in addition to geographical and abiotic factors. Third, we highlight that the species functional space has a lower richness than expected and a lower redundancy in the wave‐exposed swash level compared to the shoaling and subtidal levels, suggesting a trait‐based ecological filtering. Finally, we show that those differences depend on the higher frequency of hydrodynamics‐related traits in the species of the swash level. Our results suggest that the rough hydrodynamic conditions in the swash level favour a unique combination of species traits, which might be linked to ecological speciation in flatworms but also in other interstitial animals.

Biogeographical and macroecological principles are derived from patterns of distribution in large organisms, whereas microscopic ones have often been considered uninteresting, because of their supposed wide distribution. Here, after reporting the results of an intensive faunistic survey of marine microscopic animals (meiofauna) in Northern Sardinia, we test for the effect of body size, dispersal ability, and habitat features on the patterns of distribution of several groups. As a dataset we use the results of a workshop held at La Maddalena (Sardinia, Italy) in September 2010, aimed at studying selected taxa of soft-bodied meiofauna (Acoela, Annelida, Gastrotricha, Nemertodermatida, Platyhelminthes and Rotifera), in conjunction with data on the same taxa obtained during a previous workshop hosted at Tjärnö (Western Sweden) in September 2007. Using linear mixed effects models and model averaging while accounting for sampling bias and potential pseudoreplication, we found evidence that: (1) meiofaunal groups with more restricted distribution are the ones with low dispersal potential; (2) meiofaunal groups with higher probability of finding new species for science are the ones with low dispersal potential; (3) the proportion of the global species pool of each meiofaunal group present in each area at the regional scale is negatively related to body size, and positively related to their occurrence in the endobenthic habitat. Our macroecological analysis of meiofauna, in the framework of the ubiquity hypothesis for microscopic organisms, indicates that not only body size but mostly dispersal ability and also occurrence in the endobenthic habitat are important correlates of diversity for these understudied animals, with different importance at different spatial scales. Furthermore, since the Western Mediterranean is one of the best-studied areas in the world, the large number of undescribed species (37%) highlights that the census of marine meiofauna is still very far from being complete.

We present here a first comprehensive database on the diversity of proseriate flatworms (Platyhelminthes: Rhabditophora: Proseriata) on Western Mediterranean microtidal, wave dominated beaches. We sampled 116 stations in two years, through Spain (22 beaches, including Balearic Islands), France (25 beaches, including Corsica), Italy (63 beaches, including Sardinia, Sicily, and Lampedusa), and Tunisia (6 beaches). In each beach, we sampled at three depths, corresponding to the swash, shoal, and subtidal zones. For each sample, we obtained environmental data. The research yielded a total of 152 species, of which 93 were new to science. For each of the species found, we coded and described 16 functional traits. We discuss the functional meaning of the selected traits, as well as on diversity patterns and emerging biogeographic signals across the investigated regions. We particularly focused on the most widespread and dominant species in our dataset, concentrating on their putative adaptations to high energy environments; as well as the high number (58) of the species only found once. Finally, we discussed the coverage of our sampling by estimating the diversity at each investigated region and comparing it to the actual diversity. All information provided is available through the Global Biodiversity Information Facility (GBIF) and the Open Science Framework (OSF) following the Darwin Core Standard.

Oceanic islands, characterized by high levels of endemism and distinct faunas when compared to neighbouring continents, represent natural evolutionary laboratories for biologists to understand ecological and evolutionary processes. However, most studies on oceanic islands have focused on terrestrial and marine macrofaunal organisms, and ignored microscopic animals. We present here an inventory of all soft-bodied meiofaunal organisms collected during a 2-week workshop on the oceanic island of Lanzarote, Canary Islands. Our checklist included 239 species, with 88 of them endemic to the archipelago. The number of endemic species was lower in groups with a higher proportion of parthenogenetic species, while it was not significantly affected by body size and percentage of species with dispersal stages. A higher percentage of endemic species was found in isolated habitats and environments, with only annelids showing significantly higher number of endemic species in anchialine caves. Our results might be biased by the high number of indeterminate species found in our samples and the lack of knowledge of the meiofauna of the African coast. Our findings, however, provide the first insight of patterns of diversity of soft-bodied meiofauna in Atlantic oceanic islands, suggesting that island endemic species might also exist amongst microscopic animals.

Free-living flatworms, also known as turbellarians, are abundant and key components of aquatic habitats. They play vital roles in food webs and contribute to ecosystem health. However, our understanding of their diversity remains limited, even from comparably well-studied regions of Europe. In this work, we summarise the available records on aquatic turbellarians from Hamburg, Germany, provide new records based on material obtained during exploratory collecting, and place selected species in a molecular phylogenetic context. We sampled four localities in Hamburg, two urban and two suburban, and three other localities in Germany and Switzerland, collecting submerged vegetation, litter, and mosses. Prior to our investigation, Hamburg had documented 26 species of aquatic turbellarians. Our collections have led to the first recorded instances of Stenostomum gotlandense and Castrella alba in Germany, along with four new species for Hamburg: Macrostomum rostratum , Microdalyellia schmidtii , Krumbachia hiemalis , and Polycelis tenuis . Additionally, we provide updated information for five species previously recorded from Hamburg: S. leucops , Prorhynchus stagnalis , Microdalyellia armigera , Gyratrix hermaphroditus , and Planaria torva . The phylogenetic analysis revealed cryptic diversity within S. grande and S. leucops , with S. grande comprising two distinct clades (Brazil and Japan + Germany), and S. leucops consisting of four clades (Sweden + Germany, two from Finland, and Brazil). One of the Finnish clades, S. leucops aquariorum , is both morphologically and molecularly distinct, and we recognise it as a valid species, now S. aquariorum . Future research is needed to further clarify the relationships of the remaining clades of these two species, particularly with material from their type localities in the United States. Among rhabdocoel microturbellarians, we identified cryptic diversity within Castrella truncata (with European and North American lineages) and Microdalyellia armigera (encompassing Finnish and Spanish lineages). We also provided the first molecular evidence supporting the monophyly of Krumbachia following the sequencing of K. hiemalis . Conversely, species of Bothromesostoma were found to be nested within a clade containing species of Mesostoma , leading us to propose synonymising these genera. Overall, our study underscores the rich biodiversity potential of urban and suburban ecosystems for detecting freshwater turbellarian species, while highlighting the need for further research in Hamburg and across Germany to clarify species distributions. Moreover, molecular phylogenetic analyses have uncovered cryptic diversity in several species of catenulids and rhabdocoels, emphasizing the importance of future efforts to describe potential new species.

Microturbellarian flatworms comprise a diverse assemblage amongst meiofauna. These animals primarily exhibit carnivorous feeding habits, preying on various organisms, such as crustaceans, annelids and even other microturbellarians. However, details of their diet are poorly known. This study represents the first documentation of a proseriate preying upon a rhabdocoel. The proseriate was extracted from the sediment and studied alive. Within its digestive tract, structures of the reproductive systems of its prey were observed and identified as belonging to Phonorhynchopsis haegheni , a predatory turbellarian as well. No remains of any other organisms were detected. This finding underscores the relevance of Proseriata as top-level predators within the meiofaunal trophic web, a role that warrants further consideration beyond what has been previously acknowledged.

Five new species of Monocelididae (Platyhelminthes, Proseriata) are described from Eastern Atlantic. Three new Archilina species came from the Canary Islands. Two of them have the cirrus provided with a stylet, cylindrical in Archilina regina sp. nov. and spike-like in Archilina imperatrix sp. nov. Both species lack a vaginal pore: ventrally to the bursa, an area of vacuolar parenchyma is present. Archilina coronata sp. nov. lacks a stylet and has a vaginal pore. The three species are sister taxa to Duploperaclistus circocirrus and Duplominona miranda sp. nov., from Brittany (France), characterized by an extremely long, tubular stylet, about 150 μm in length. A further Archilina is described from Cabo Verde Islands, Archilina regisfilia sp. nov., with a funnel-shaped cirrus and without a vaginal pore. The finding in the Canary Islands of closely related species, occurring abundantly in the same habitats and differing for presence/absence of stylet and vaginal pore, suggests a role of the stylet in allowing internal fertilization in species without external vagina. The phylogeny presented, based on rDNA 18S and 28S genes, does not reflect present systematics of the Monocelididae, and the monophyly of most genera is not supported.

Two new species of Otoplana (Proseriata: Otoplanidae) from the Canary Islands are here described: Otoplana norenburgi sp. nov. and Otoplana didomenicoi sp. nov. These new species are distinguished from their congeners by unique features of the sclerotized structures of the copulatory organ, in particular of the aculeus, swollen proximally in O. norenburgi sp. nov. and lanceolate in shape in O. didomenicoi sp. nov. Specimens of the latter species were also found in South Portugal. Canarian and Portuguese specimens show poor genetic distinction based on the markers used, hinting to a remarkable dispersal power for a mesopsammic organism. New information is given on the “accessory male canal”, a putative autapomorphy for the genus Otoplana . Its functions and connections with genital systems are discussed, in the light of new data on the molecular phylogeny of the family Otoplanidae presented. The Mediterranean specimens of Otoplana sequenced, morphologically attributed to O. truncaspina , O. bosporana , O. falcataspina , and O. labronica , did not show genetic distinction, urging for a reconsideration of the status of the Mediterranean taxa.

Given the pending biodiversity crisis, species delimitation is a critically important task in conservation biology, but its efficacy based on single lines of evidence has been questioned as it may not accurately reflect species limits and relationships. Hence, the use of multiple lines of evidence has been portrayed as a means to overcome identification issues arising from gene/species tree discordance, morphological convergence or recent adaptive radiations. Here, the integrative taxonomic approach has been used to address the study of the Monocelis lineataspecies complex. The taxonomic resolution of the complex is challenging, as the species lacks sclerotised copulatory structures, which as a rule of thumb aid identification in Proseriata. Eighteen populations, which encompass most of the geographic range of the complex, were studied using morphology, karyology, crossbreeding experiments and molecular analysis. These different markers provided evidence of four (karyology) to eight (morphology) discrete entities, whereas crossings showed various degrees of intersterility among the tested populations. Molecular species delimitation revealed a different number of candidate species, spanning from five (ABGD and K/θ) to 11 (GMYC). Such incongruences reflect the multifaceted evolutionary history of M. lineata s.l.and hamper the full taxonomic resolution of the complex. However, two candidate species were consistently validated by all of the markers and are described as new species: Monocelis algicolanov. sp. and M. exquisitanov. sp. The latter species appear to have a restricted distribution, and the possibility that meiofaunal taxa may be of conservation concern is discussed.