Background: Sequencing and annotating genomes of non-model organisms helps to understand genome architecture, the genetic processes underlying species traits, and how these genes have evolved in closely-related taxa, among many other biological processes. However, many metazoan groups, such as the extremely diverse molluscs, are still underrepresented in the number of sequenced and annotated genomes. Although sequencing techniques have recently improved in quality and quantity, molluscs are still neglected due to difficulties in applying standardized protocols for obtaining genomic data. Results: In this study, we present the chromosome-level genome assembly and annotation of the sacoglossan sea slug species Elysia timida, known for its ability to store the chloroplasts of its food algae. In particular, by optimizing the long-read and chromosome conformation capture library preparations, the genome assembly was performed using PacBio HiFi and Arima HiC data. The scaffold and contig N50s, at 41.8 Mb and 1.92 Mb, respectively, are approximately 30-fold and fourfold higher compared to other published sacoglossan genome assemblies. Structural annotation resulted in 19,904 protein-coding genes, which are more contiguous and complete compared to publicly available annotations of Sacoglossa with respect to metazoan BUSCOs. We found no evidence for horizontal gene transfer (HGT), i.e. no photosynthetic genes encoded in the sacoglossan nucleus genome. However, we detected genes encoding polyketide synthases in E. timida, indicating that polypropionates are produced. HPLC–MS/MS analysis confirmed the presence of a large number of polypropionates, including known and yet uncharacterised compounds. Conclusions: We can show that our methodological approach helps to obtain a high-quality genome assembly even for a "difficult-to-sequence" organism, which may facilitate genome sequencing in molluscs. This will enable a better understanding of complex biological processes in molluscs, such as functional kleptoplasty in Sacoglossa, by significantly improving the quality of genome assemblies and annotations.

The genus Dendrodoris is ascribed to the family Dendrodorididae, whose members are characterized lacking spicules in the dorsum as well as a radula, one of the most important taxonomic traits to distinguish and define nudibranch species. The absence of this taxonomic character makes difficult to delineate species based on morphologic traits, and consequently, several cryptic species may remain unreported resulting in a potential underestimation of the real diversity of the genus. Species delimitation analyses based on molecular data constitute an important tool to unveil cryptic species, especially when morphology may be misleading. However, for the genus Dendrodoris, only two molecular studies have been performed to date, both based on Indo-pacific species and the molecular marker COI. In the present study, we performed a multilocus approach using the molecular markers COI, 16S and H3 based on 47 Dendrodoris specimens from the North Atlantic Ocean and Mediterranean Sea. As a result, we detected monophyly for five of the previously accepted species plus the presence of an unidentified one. Using morphological and anatomical data from 14 additional individuals and a review of the literature, we verified this unidentified species is the previously synonymized species Dendrodoris temarana Pruvot-Fol, 1953, and we redescribe it morphologically (chromatic pattern, gills, rhinophores and anal papilla), anatomically (neural, circulatory, digestive and reproductive systems), and biologically herein. D. temarana is differentiated by its chromatic pattern, previously thought to be intraspecific variation of D. grandiflora (Rapp, 1827), D. limbata (Cuvier, 1804) and D. herytra Valdés & Ortea in Valdés, Ortea, Avila & Ballesteros, 1996, and therefore it is considered a pseudocryptic species. This study increases the number of valid species in the North Atlantic and Mediterranean to nine and confirms the importance of integrative taxonomic approaches to resolve the identification of cryptic species in complex groups.

Geodia hentscheli, a species forming sponge grounds in the North Atlantic and Arctic Oceans, is a common deep-sea organism, that plays a fundamental role in forming biogenic habitats. However, there is little information about gene flow and adaptation patterns of this species, which is crucial to develop effective management/conservation plans under current global change scenarios. Here, we generated ddRADseq data from 110 specimens of G. hentscheli, together with microbial profiling, transcriptomics, and metatranscriptomics for a selection of specimens to investigate their genetic diversity, molecular connectivity, and local adaptations. Sampling covered the species' entire distribution within a wide bathymetric range. We obtained 1,115 neutral SNPs and identified long-distance genetic connectivity among regions separated 1,000s of km, but strong genetic structure segregating populations by depth at ca. 1,300 m, in line with our microbial analyses. Coalescent analyses inferred the split of these depth-related genetic entities ∼10 KYA, coincident with the last postglacial maximum. Analyses of SNPs under selection, combined with transcriptomic and metatranscriptomic data highlight the presence of several sponge genes and microbial metabolic pathways involved in adaptation to depth, including heat shock proteins and fatty acids, among others. The physiological plasticity of the sponge and its microbiome as a function of depth suggest the existence of a host-microbiome metabolic compensation for G. hentscheli. This study provides a multiscale paradigmatic example of the depth-differentiation hypothesis, a phenomenon mainly caused by changes in environmental conditions at different depths, mainly related to the presence of water masses with different characteristics that drive local adaptations.

Invasive species are one of the main threats to global biodiversity and, within marine ecosystems, tunicates feature some prominent examples. Styela plicata is an ascidian species inhabiting harbours in all temperate oceans and seas, thus being considered a thriving invasive species. However, this species' adaptive mechanisms, introduction history, and population structure have never been completely elucidated. Here, by genotyping 87 S. plicata individuals from 18 localities worldwide with 2b‐RADseq, we confirm the global presence of four chromosome inversions, demonstrate population structuring on this species, detect local adaptation signals, and infer historical demographic events. We show that North Carolina individuals constitute an unrelated population, Atlanto‐Mediterranean and Pacific localities form their own genetic clusters with substructuring, being the most evident the split between northern and southern Atlantic localities. The locality of South Carolina presents an intermediate genetic position between North Carolina and the other two groups pointing to a hybrid origin with recurrent gene flow. We generate and test demographic models, providing evidence of two independent introduction events to the Atlantic and Pacific, and an admixture that originated the population of South Carolina. Finally, we identify candidate loci for adaptation, with functions involved with cell processes, metabolism, development, and ion transport, among others. Overall, this study highlights the complex historical processes of S. plicata, which have led this species to its current distribution, population structure, and local adaptation footprint in oceans worldwide.

Animals, including invasive species, are complex entities consisting of a host and its associated symbionts (holobiont). The interaction between the holobiont components is crucial for the host’s survival. However, our understanding of how microbiomes of invasive species change across different tissues, localities, and ontogenetic stages, is limited. In the introduced ascidian Styela plicata, we found that its microbiome is highly distinct and specialized among compartments (tunic, gill, and gut). Smaller but significant differences were also found across harbors, suggesting local adaptation, and between juveniles and adults. Furthermore, we found a correlation between the microbiome and environmental trace element concentrations, especially in adults. Functional analyses showed that adult microbiomes possess specific metabolic pathways that may enhance fitness during the introduction process. These findings highlight the importance of integrated approaches in studying the interplay between animals and microbiomes, as a first step toward understanding how it can affect the species' invasive success.

Population genomic studies have increased in the last decade, showing great potential to understand the evolutionary patterns in a great variety of organisms, mostly relying on RAD sequencing techniques to obtain reduced representations of the genomes. Among them, 2b-RAD can provide further secondary reduction to adjust experimental costs by using base-selective adaptors, although its impact on genotyping is unknown. Here we provide empirical comparisons on genotyping and genetic differentiation when using fully degenerate or base-selective adaptors and assess the impact of missing data. We built libraries with the two types of adaptors for the same individuals and generated independent and combined datasets with different missingness filters (presence in 100%, 75% and 50% of the libraries). Our approach, by exploring locus-by-locus, found 92% of identical genotypes between the two libraries of the same individual when using loci shared among 100% of the libraries, which decreased to 35% when working with loci present in at least 50% of them. We show that missing data is a major source of individual genetic differentiation. The loci discordant by genotyping were in low frequency (7.67%) in all filtered files. Only 0.96% of them were directly attributable to base-selective adaptors reducing heterozygosity. An underestimation of heterozygosity in 6.44% of genotypes was found in libraries generated with fully degenerated adaptors, of which ca. 70% had <10 reads per locus indicating that sufficient read depth should be ensured for a correct genotyping. Base-selective adaptors reduce the number of loci, increase their coverage, and provide a reliable genotyping with no heterozygosity underestimation at a lower sequencing cost. We demonstrate that missing data is a major concern for a correct identification of genetic differentiation, and should not be neglected in genomic studies. Thus, we conclude that 2b-RAD libraries using base-selective adaptors are a robust tool to use in population genomics regardless the target species genome size, promoting their use in future studies.