Members of the following marine annelid families are found almost exclusively in the interstitial environment and are highly adapted to move between sand grains, relying mostly on ciliary locomotion: Apharyngtidae n. fam., Dinophilidae, Diurodrilidae, Nerillidae, Lobatocerebridae, Parergodrilidae, Polygordiidae, Protodrilidae, Protodriloididae, Psammodrilidae and Saccocirridae. This article provides a review of the evolution, systematics, and diversity of these families, with the exception of Parergodrilidae, which was detailed in the review of Orbiniida by Meca, Zhadan, and Struck within this Special Issue. While several of the discussed families have previously only been known by a few described species, recent surveys inclusive of molecular approaches have increased the number of species, showing that all of the aforementioned families exhibit a high degree of cryptic diversity shadowed by a limited number of recognizable morphological traits. This is a challenge for studies of the evolution, taxonomy, and diversity of interstitial families as well as for their identification and incorporation into ecological surveys. By compiling a comprehensive and updated review on these interstitial families, we hope to promote new studies on their intriguing evolutionary histories, adapted life forms and high and hidden diversity.

Recent studies show that Gnathifera, comprising Rotifera, Gnathostomulida and Micrognathozoa, constitute the sister group to the remaining Spiralia (containing, e.g. flatworms, segmented worms and molluscs). Therefore, a better understanding of Gnathifera is central for unravelling the evolution of the highly diverse Spiralia. Here, we describe the previously unstudied nervous system and ciliary structures of Micrognathozoa, using immunohistochemistry and confocal laser scanning microscopy. The nervous system is simple with a large brain, paired sub-esophageal ganglia, two trunk commissures, two pairs of ventral longitudinal nerves and peripheral nerves. The paired ventro-lateral nerve cords are confirmed to be a symplesiomorphy of Gnathifera (possibly even Spiralia), whereas the paired ventro-median nerves are not previously reported in Gnathifera. A pharyngeal ganglion is described for Micrognathozoa: a complex structure with two apical tufts of ciliary receptors, now shown to be shared by all Gnathifera. The ventral pattern of external ciliophores is re-described, and protonephridia with multi-ciliated collecting tubules similar to those of Rotifera are confirmed. A range of new details from a simple nervous system and complex set of ciliary structures in a microscopic metazoan are hereby unravelled. The many resemblances with Rotifera corroborate their close relationship, and shed more light on the evolution of Gnathifera.

The Cambrian fossil Amiskwia sagittiformis has puzzled palaeontologists for more than a century, but recent re-investigation of its morphology suggested a close relationship with the bilaterian clade Gnathifera, comprising Rotifera, Gnathostomulida and Micrognathozoa. Since Amiskwia has already been considered closely related to Chaetognatha, this new interpretation of its morphology supports recent molecular and developmental studies finding a close phylogenetic relationship between Chaetognatha and Gnathifera. The recent re-description of the jaw apparatus of Amiskwia with the first phylogenetic analysis by Vinther & Parry ( 2019 Vinther, J. & Parry, L. A. 2019. Bilateral jaw elements in Amiskwia sagittiformis bridge the morphological gap between gnathiferans and chaetognaths. Current Biology, 29(5), 881–888. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar] ) found a rather surprising topology with Amiskwia and Chaetognatha being sister groups and nested inside Gnathifera. Furthermore, a subsequent paper from Caron & Cheung ( 2019 Caron, J.-B. & Cheung, B. 2019. Amiskwia is a large Cambrian gnathiferan with complex gnathostomulid-like jaws. Communications Biology, 2, 164. doi:10.1038/s42003-019-0388-4 [Crossref], [PubMed], [Web of Science ®] , [Google Scholar] ) re-described the jaws of Amiskwia as very similar to the configuration found in Gnathostomulida, but did not include any phylogenetic analysis. Here we test the topology of Vinther & Parry with various parsimony and Bayesian analyses, taking into account the new description of Caron & Cheung with careful re-amendment of the matrix of Vinther & Parry. According to our results and the recent findings on the molecular phylogeny of their extant members, we suggest a new systematization of these taxa. We recovered Amiskwia as a stem-group chaetognath within a clade Cucullophora nov., rejecting the Caron & Cheung hypothesis of its close affinity to Gnathostomulida, and found a more consensual topology with monophyletic Gnathifera sister group to Cucullophora, all together forming the clade Chaetognathifera. Furthermore, Rotifera + Micrognathozoa form a clade named Gynognathifera nov. We discuss the characters supporting each clade and the reasons that account for the topology found by Vinther & Parry. Finally, we hope that this carefully amended matrix focused on Gnathifera, Amiskwia and Chaetognatha, together with a formal classification and robust phylogeny, will be of use for future studies on the palaeontology and morphology of these clades.

We report the results of a faunistic survey focused on freshwater and limno-terrestrial meiofauna to improve biodiversity knowledge in a protected area in the Eastern part of the French Pyrénées: the Massane Forest Reserve (336 Ha). The survey provided 1187 occurrence records from 315 taxa (most resoved at species-level), uploaded as a shared online dataset. The highest number of occurrences and distinguishable morpho-taxon belong to the group Nematoda (775 occurrences, 172 taxa), followed by Rotifera (219 occurrences, 67 taxa), Platyhelminthes (85 occurrences, 32 taxa), Tardigrada (69 occurrences, 25 taxa), and Gastrotricha (39 occurrences, 19 taxa). A diversity of meiofaunal organisms was found, in large numbers, in all the samples screened: from stream biofilms and sediments, to forest floor soils, mosses, and litter, to a broad range of tree-related micro-habitats associated with beech-like epixylic mosses and lichens, tree cavities, woodpecker breeding holes, bark pockets and fruiting bodies of saproxylic fungi. This survey makes the Massane forest one of the few protected areas of the world with a taxa-inclusive meiofauna dataset, which could serve as a standard inventory to further consider micro-invertebrates in forest conservation.

Summary: The causes and consequences of genome reduction in animals are unclear, because our understanding of this process mostly relies on lineages with often exceptionally high rates of evolution. Here, we decode the compact 73.8 Mb genome of Dimorphilus gyrociliatus , a meiobenthic segmented worm. The D. gyrociliatus genome retains traits classically associated with larger and slower-evolving genomes, such as an ordered, intact Hox cluster, a generally conserved developmental toolkit, and traces of ancestral bilaterian linkage. Unlike some other animals with small genomes, the analysis of the D. gyrociliatus epigenome revealed canonical features of genome regulation, excluding the presence of operons and trans -splicing. Instead, the gene dense D. gyrociliatus genome presents a divergent Myc pathway, a key physiological regulator of growth, proliferation, and genome stability in animals. Altogether, our results uncover a conservative route to genome compaction in annelids, reminiscent of that observed in the vertebrate Takifugu rubripes .