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Chromosome-level genome assembly of the Vermilion Snapper (Rhomboplites aurorubens).

Vermilion Snapper (Rhomboplites aurorubens, Lutjanidae) inhabits deep waters (20-300 m) from North America to Brazil and supports significant commercial and recreational fisheries. Despite its economic importance, the understanding of its basic biology remains limited. Classified as Vulnerable on the Red List due to overfishing, populations have declined by over 30% in recent generations. We assembled and annotated the first chromosome-scale genome of this species by combining PacBio long reads, Illumina short reads, and Hi-C data. The resulting assembly is 987.5 Mbp, with a scaffold N50 size of 41.3 Mbp, and includes 135 contigs clustered and ordered onto 24 chromosomes with 34,496 predicted genes. The high-quality assembly and annotation contained about 98% complete and single-copy BUSCO genes. It is the most complete, chromosome-level genome assembly of an Atlantic snapper to date. The genome assembly and supporting data are valuable tools for ecological and comparative genomics studies of snappers and other valuable commercial species within the family.

Chromosomes

Chromosomal-level genome assembly of Trypanosoma carassii, the etiologic agent of a recent outbreak of trypanosomiasis in cage-cultured large yellow croaker (Larimichthys crocea) in China.

Trypanosoma carassii, a typical freshwater fish trypanosome, has recently been identified as the etiological agent of a trypanosomiasis outbreak in cage-cultured large yellow croaker (Larimichthys crocea) in China and has been designated as T. c. larimichthys. To date, publicly available genomic data for trypanosomes have been limited to terrestrial species, particularly those of medical importance. Here, we present a chromosome-level genome assembly of T. carassii, the first genome of an aquatic trypanosome, generated using PacBio HiFi long-read sequencing and Hi-C scaffolding technologies. A preliminary genome survey based on Illumina sequencing data estimated the genome size at 56.38 Mb with a heterozygosity of 1.17%. The final assembled genome spans 48.55 Mb, with contig N50 and scaffold N50 values of 139.15 Kb, and achieves 100.00% BUSCO completeness. Hi-C data resolved the assembly into 34 chromosomes and 9 unanchored scaffolds. Repetitive elements account for 53.29% of the genome (approximately 25.87 Mb). A total of 11,584 protein-coding genes were predicted, 95.36% of which were functionally annotated. Synonymous substitution rates analysis of paralogous genes indicates a recent burst of gene duplication, which likely corresponds to a whole-genome duplications. This high-quality genome assembly provides invaluable resources for understanding the evolution and host adaptation of aquatic trypanosomes.

Animals

A gap-free, telomere-to-telomere chromosome-scale genome assembly of the mangrove red snapper, Lutjanus argentimaculatus.

The mangrove red snapper (Lutjanus argentimaculatus) is a commercially important marine fish species in the Indo-Pacific region. Despite its significant economic value for aquaculture, existing genomic resources remain fragmented, limiting the advancement of molecular breeding and functional genomic studies. Here, we present a gap-free, telomere-to-telomere (T2T) genome assembly of L. argentimaculatus, generated using a hybrid approach combining PacBio HiFi, Oxford Nanopore ultra-long reads and Hi-C technology. The resulting assembly comprises exactly 24 scaffolds spanning 1.03 Gb, perfectly matching the haploid chromosome number with a contig N50 of 46.17 Mb. Notably, this assembly resolves all physical gaps present in previous versions, achieving a BUSCO completeness score of 98.2%. Comprehensive genome annotation successfully predicted 23,167 protein-coding genes. Among these, 22,067 genes (95.25%) were functionally annotated across major public databases, including eggNOG, InterPro, and Swiss-Prot. Furthermore, structural analysis successfully identified 19 telomeres and 20 centromeres, validating the chromosomal integrity. This high-fidelity, gap-free reference genome provides a robust foundation for comparative genomics, population genetics, and the genetic improvement of Lutjanidae species.

Animals

The hedonic impact of cleaner-client fish interactions is mediated by the opioid system.

According to the 'incentive salience hypothesis' reward processing involves two main components, including the motivation to obtain a reward (i.e. incentive salience or 'wanting') and the hedonic pleasure felt during its consumption (i.e. hedonic impact or 'liking'), which are dissociable. The processing of these hedonic mechanisms is suggested to be mediated by opioid neurotransmission; however, most evidence comes from humans and other mammals. Here we argue that, in mutualistic associations, client fish seek to interact with cleaner fish not only due to the immediate benefits of being cleaned but also because of the hedonic impact of tactile stimulation, modulated by the opioid system. We used a conditioned place preference (CPP) paradigm to test the hedonic dimension of motivation to be cleaned and found that de-parasitized client fish (the butterflyfish Chaetodon auriga) preferred the compartment paired with cleaner fish (Labroides dimidiatus). Treatment with the μ-opioid receptor agonist The µ-opioid receptor agonist 2-Ala-4-mephe-5-gly-enkephalin (DAMGO) dose-dependently increased preference, while treatment with the antagonist naloxone decreased it. To test for fish incentive salience, we used a detour task, with barriers being added after each trial to demand higher response strength to reach the target and found that client fish can circumvent barriers to reach a cleaner-adjacent compartment. Response strength (number of barriers added before 'giving up') was not affected by treatment with either naloxone or DAMGO. Our results show that cleaner-client interactions are hedonically positive for clients, an effect that is mediated by the opiodergic system. Visual contact with cleaners is sufficient to activate incentive salience programmes, but no evidence of participation of the opioidergic system in these programmes was found. Overall, we provide the first evidence confirming the participation of the opioidergic system in motivation to be cleaned in mutualistic associations.

Animals

In Situ Hybridization and RT-PCR Detection of Nervous Necrosis Virus in Fourfinger Threadfin, Eleutheronema tetradactylum, in Taiwan.

Between April and July 2020, suspected outbreaks of nervous necrosis virus (NNV) infection were observed in fourfinger threadfin (Eleutheronema tetradactylum) fingerling hatcheries in Pingtung County, southern Taiwan. Affected fish exhibited spiral swimming behaviour and abdominal distension, resulting in mortality rates between 50% and 100%. Histopathological examination showed severe vacuolation in the brain and ocular tissues, with large oval and/or irregular basophilic cytoplasmic inclusion bodies in the brain. Phylogenetic analysis of the viral replicase (RNA1) and capsid protein (RNA2) genes revealed high nucleotide sequence identities among the isolates in this study, with sequence similarity rates of 96.9%-99% for RNA1 and 98.2%-99.0% for RNA2 compared to RGNNV reference strains available in the NCBI GenBank database. This is the first detection of betanodavirus in fourfinger threadfin in Taiwan, using RT-PCR and ISH. A positive correlation between elevated water temperatures and disease severity indicates the need for year-round surveillance and genomic analysis to clarify the epidemiology of FTNNV. The data suggest that infected eggs may facilitate the vertical transmission of Betanodavirus. Crucially, utilising virus-free broodstock, alongside routine health screening and environmental control, is essential for mitigating NNV risks in fourfinger threadfin aquaculture.

Animals

Habitat Specialisation Impacts Clownfish Demographic Resilience to Pleistocene Sea-Level Fluctuations.

Habitat fragmentation and loss are key threats to biodiversity, yet their impacts on marine species remain poorly understood. Clownfishes, which rely on sea anemones for shelter and reproduction, provide an interesting model to explore how ecological specialisation mediates species responses to habitat perturbations. We used whole-genome data from 382 individuals across 10 species with varying host specialisations to reconstruct demographic histories and infer spatial genetic structure to assess the impact of Pleistocene sea-level fluctuations. Generalist species, associated with multiple hosts, maintained stable effective population sizes () and population connectivity during habitat fragmentation, reflecting resilience to environmental instability. In contrast, specialists experienced severedeclines and genetic structuring, driven by their dependence on specific hosts, without signs of population recovery following habitat reconnection. Spatial genomic analyses identified the Indonesian Through-Flow as a key dispersal corridor and the Coral Triangle as a critical hub of genetic diversity, while continental shelves and extensive open ocean regions appeared as barriers to gene flow. Our findings reveal how host specialisation shapes clownfish population dynamics, emphasising the importance of incorporating ecological dependencies into conservation assessments and deepening our understanding of species responses to ecological constraints and environmental changes over evolutionary timescales.

Animals

Genetic investigation of population structure in Atlantic chub mackerel, Scomber colias Gmelin, 1789 along the West African coast.

Sustainable management of transboundary fish stocks hinges on accurate delineation of population structure. Genetic analysis offers a powerful tool to identify potential subpopulations within a seemingly homogenous stock, facilitating the development of effective, coordinated management strategies across international borders. Along the West African coast, the Atlantic chub mackerel (Scomber colias) is a commercially important and ecologically significant species, yet little is known about its genetic population structure and connectivity. Currently, the stock is managed as a single unit in West African waters despite new research suggesting morphological and adaptive differences. Here, eight microsatellite loci were genotyped on 1,169 individuals distributed across 33 sampling sites from Morocco (27.39°N) to Namibia (22.21°S). Bayesian clustering analysis depicts one homogeneous population across the studied area with null overall differentiation (F ST = 0.0001ns), which suggests panmixia and aligns with the migratory potential of this species. This finding has significant implications for the effective conservation and management of S. colias within a wide scope of its distribution across West African waters from the South of Morocco to the North-Centre of Namibia and underscores the need for increased regional cooperation in fisheries management and conservation.

Animals

Complete mitochondrial genomes and phylogenetic analysis of three species of Indo-Pacific freshwater gobies, Stiphodon (Gobiiformes,Oxudercidae).

Stiphodon is a genus of gobioid fishes found primarily in freshwater streams on islands throughout the Indo-Pacific region. It is the most speciose genus of Sicydiinae, with more than three dozen named species. Complete mitochondrial genome (mtDNA) sequences were determined for three species, namely Stapledon atropurpureus, Stapledon elegans, and Stapledon semoni. Maximum likelihood and Bayes inference inferences from 13 protein-coding genes (11,433 bp) or the COXI gene (1,551 bp) alone were made on these three together with four mitogenomes previously available. S. percnopterygionus and S. tuivi formed an outgroup to the other five Stiphodon species, meanwhile Stapledon atropurpureus and S. semoni were recovered as sister species. The ratios of the non-synonymous to the synonymous ubstitution rates for all PCGs were in the range 0\ (Ka/Ks) \1, which suggests they have been subject to purifying selection.

Animals