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Migration is a complex phenotypic trait with some species containing migratory and nonmigratory individuals. Such life history variation may be attributed in part to plasticity, epigenetics, or genetics. Although considered semianadromous, recent studies using otolith geochemistry have revealed life history variation within the critically endangered Delta Smelt. Broadly categorizable as migratory or freshwater residents, we examined Restriction site Associated DNA sequencing data to test for a relationship between genetic variation and migratory behaviors. As previously shown, we found no evidence for neutral population genetic structure within Delta Smelt; however, we found significant evidence for associations between genetic variants and life history phenotypes. Furthermore, discriminant analysis of principal components, hierarchical clustering, and machine learning resulted in accurate assignment of fish into the freshwater resident or migratory classes based on their genotypes. These results suggest the presence of adaptive genetic variants relating to life history variation within a panmictic population. Mechanisms that may lead to this observation are genotype dependent habitat choice and spatially variable selection, both of which could operate each generation and are not exclusive. Given that the population of cultured Delta Smelt are being used as a refugial population for conservation, as a supply for wild population supplementation, and currently represent the majority of all living individuals of this species, we recommend that the hatchery management strategy consider the frequencies of life history-associated alleles and how to maintain this important aspect of Delta Smelt biological variation while under captive propagation., (© The Author(s) 2022. Published by Oxford University Press on behalf of Genetics Society of America.)

The rapid decline of longfin smelt Spirinchus thaleichthys , a threatened euryhaline forage fish in California, is a serious concern for scientists and resource managers. To recover and conserve this species, a captive culture program was initiated, focusing on the collection, captive rearing and breeding of wild broodstock, and the rearing of their offspring. Although progress has been made in the collection of broodstock and the production and culturing of larvae, no studies have evaluated the rearing of juvenile life stages in captivity. The present study examines methodological considerations for culturing F1 juvenile longfin smelt, specifically, the first efforts toward weaning juveniles to a dry commercial pellet feed. Cultured juvenile longfin smelt were fed live Artemia only or co-fed Artemia and dry feed for 62 days, and the effects of feed type on juvenile survival, growth, body condition, and fatty acid profiles were examined. No significant differences were observed between feeding treatments, despite an 80% reduction in Artemia in the co-feeding treatment. Furthermore, examination of fish stomach contents at the end of the trial confirmed the transition to dry feed. This is the first study to indicate successful feeding by longfin smelt on dry commercial pellets, and suggests that juvenile longfin smelt can be fully weaned onto dry feeds. Results of this study are critical for closing the lifecycle of longfin smelt in captivity and developing a successful conservation culture program for this imperiled species.

The complete mitochondrial genome was sequenced in two individuals of the surf smelt Hypomesus japonicus . The genome sequences are 16,762 and 16,771 bp in size, and the gene arrangement, composition, and size are very similar to the other smelt mitochondrial genomes published previously. The difference between two H. japonicus genomes studied is 0.37%, which is noticeably higher in comparison with other osmerid fishes. The level of sequence divergence between H. japonicus and related osmerids belonging to genera Hypomeus , Osmerus , and Mallotus varies within a very narrow range (12.31-13.72%) indicating poor phylogenetic resolution of this complex fish group., Competing Interests: The authors acknowledge no financial interest or benefit from the direct applications of this research. The authors report that they have no conflicts of interest. The research on mitochondrial genome sequencing was conducted at the Department of Ecology and Evolutionary Biology, University of California, Irvine, USA. The data analysis and manuscript preparation were conducted at the National Scientific Center of Marine Biology, Vladivostok, Russia., (© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.)

The complete mitochondrial genome was sequenced in two individuals of the Arctic rainbow smelt Osmerus dentex . The genome sequences are 16,615 and 16,616 bp in size, and the gene arrangement, composition, and size are very similar to the other smelt mitochondrial genomes published previously. The difference between the two O. dentex genomes studied is 0.25%, that is, 5.0 times higher in comparison with close species, the European smelt O. eperlanus studied previously. The level of mitochondrial genome divergence between O. dentex and close osmerid fishes, O. eperlanus , and O. mordax is high enough (6.86-7.54%) to consider all of them as separate biological species., Competing Interests: No potential conflict of interest was reported by the authors., (© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.)

The complete mitochondrial (mt) genome was sequenced in two individuals of the European smelt Osmerus eperlanus . The genome sequences are 16,608 and 16,609 bp in size, and the gene arrangement, composition, and size are very similar to the other smelt mt genomes previously published. The difference between the two genomes studied is low, 0.05%. The difference is significantly higher (6.95%) between the O. eperlanus genomes studied here and the genome of the congeneric species, O. mordax (HM106493.1) available in GenBank. The distribution of divergence is non-uniform along the genomes. There is a continuous segment (≈2.5 kb) encompassing tRNA-Phe, complete 12S rRNA gene, tRNA-Val, and partial 16S rRNA gene, which demonstrates significantly lower levels of divergence than on average for the whole genome. The 12S rRNA and 16S rRNA genes are frequently used for phylogenetic and molecular taxonomy analyses and could underestimate the level of divergence between osmerid fishes., Competing Interests: The funders had no role in the study design, data collection, data analysis, decision to publish, or preparation of the manuscript. The authors alone are responsible for the content and writing of the paper. The authors acknowledge no financial interest or benefit from the direct applications of this research. The authors report that they have no conflict of interest., (© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.)

Reproduction of external fertilizing vertebrates is typically constrained to either fresh or salt water, not both. For all studied amphibians and fishes, this constraint includes immotile sperm that are activated after ejaculation only by the specific chemistry of the fertilizing medium in which the species evolved (fresh, brackish, or salt water). No amphibians can reproduce in the sea. Although diadromous fishes may migrate between salt and fresh water, they are shackled to their natal environment for spawning in part because of sperm activation. Here, we report for the first time among all documented external fertilizing vertebrates, that in the absence of any external media, sperm are motile at ejaculation in a marine spawning fish (Osmeridae, capelin, Mallotus villosus ). To illuminate why, we evaluated sperm behavior at different salinities in M. villosus as well as the related freshwater spawning anadromous rainbow smelt ( Osmerus mordax ). Surprisingly, sperm performance was superior in fresh water for both species. M. villosus spend their entire life at sea but our results show that their sperm are deactivated by sea water, suggesting a freshwater ancestry. By circumventing constraining water chemistry, we interpret the unique pre-ejaculatory sperm activation in this species as a novel adaptation that enables fertilization in the marine environment. These findings also contribute to understanding the persistence of anadromy, despite great energetic costs to adult fishes.

Pond smelt Hypomesus olidus is a small-sized economic species of freshwater fish with delicious meat and high output. In this study, the complete mitochondrial genome of H. olidus sequenced to be 16,786 bp in length, including 13 protein-coding genes, 2 ribosomal RNAs, 22 transfer RNAs and a control region. The overall base composition of H. olidus is 24.3% for A, 27.3% for T, 29.2% for C and 19.2% for G, with a slight A + T bias. The mitogenome sequence data may provide useful information to the population genetics analysis of H. olidus and the elucidation of evolutionary mechanisms in Hypomesus fish., Competing Interests: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article., (© 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.)