P. Mark Lokman

Production of good quality eggs still plagues mass propagation efforts of anguillid eels. Understanding the lipid dynamics of artificially maturing eels will inevitably benefit captive rearing practices due to the importance of lipids for egg quality. Fortnightly sampling of shortfinned eels ( Anguilla australis ) subjected to hypophysation for 10 weeks allowed us to gather important information relating to multiple aspects of lipid physiology. Accordingly, expected increases in gonadosomatic (from 2.20 ± 0.14 to 23.20 ± 6.01%) and hepatic somatic indices (from 0.89 ± 0.04 to 1.93 ± 0.01%), as well as oocyte diameter (from ~ 220 to ~ 700 µm) were complemented with notable increases in hepatic and ovarian vitellogenin levels at both the gene and protein levels in treated eels compared to Ringer-injected controls. A fivefold increase in hepatic expression of oestrogen receptor-alpha, but not the -beta isoform, corroborates the role for estradiol in promoting vitellogenesis. Conversely, decreases in expression of apolipoprotein b and the peroxisome proliferator-activated receptors in the liver point to a change in the role of the liver away from the neutral lipid packaging associated with previtellogenesis towards vitellogenin loading, ultimately culminating in yolk protein sequestration by the oocytes.

Adaptation is considered essential for species facing environmental change, but our understanding of the evolution of complex ecotypes remains limited. We show that a substantial genomic cluster of co-adapted alleles has repeatedly fuelled complex life-history shifts in fishes. Specifically, the majority of strong outlier loci underpinning repeated landlocking of formerly marine-migratory Galaxias lineages are co-located on a single chromosome. Phenotypic, behavioural and genomic comparisons reveal a strong parallel basis for multi-trait ecotype shifts (osmoregulation; musculature; locomotion; reproduction) across eight independently landlocked populations. Critically, migratory populations house 'jackpot' individuals carrying intact sequences essential for rapid multi-trait life-history switches. The observed high frequencies of these migratory jackpot carriers explain the rapid ecotype shifts detected in Galaxias populations isolated by recent catastrophic landslides in New Zealand. These findings represent a complete example of complex ecotype evolution, highlighting the key role of modular genomic variation in facilitating rapid adaptive change.

The collection of perivitelline fluid (PVF) from early-stage post-activation zebrafish (Danio rerio) eggs/embryos poses a significant challenge owing to the liability of the egg/embryo to sustain damage and rupture during handling. Rupture of the blastoderm and/or yolk presents a major risk of PVF sample contamination. Previous efforts to extract PVF at such early stages have employed formalin fixation to enhance the structural integrity of the blastoderm and yolk syncytial layer, thereby reducing the likelihood of contamination. While this approach successfully mitigates blastoderm and yolk rupture, formaldehyde fixation may cause issues with downstream proteomic analyses. Recent findings indicate that zebrafish PVF contains a range of maternally inherited proteins involved in innate immune defence. However, current extraction methods compromise the reliability of downstream protein analyses, raising concerns that fixation-induced protein crosslinking may obscure the presence of maternally inherited proteins during the earliest stages of development. The micro-aspiration technique described here allows for the precise extraction of PVF from living, water-activated eggs with minimal disruption to the blastodisc and yolk. This method reduces the risk of contamination from other non-target proteinaceous egg sources and eliminates the need for formalin fixation, thereby improving the integrity of PVF samples and enhancing the reliability of subsequent downstream analyses.

Migrating freshwater eels depend on the mobilisation of stored lipids to successfully arrive at their distant spawning locations. As 11-ketotestosterone (11KT) can increase the lipid-transporting capability and enhance gonadal lipid uptake in eel, we hypothesized that this androgen would also regulate lipid mobilisation from its stores. To address this hypothesis, we first sampled residential (yellow) and migrating (silver) short-finned eels from the wild and evaluated the expression of 24 genes encoding lipolytic or lipogenic enzymes, as well as those encoding both nuclear androgen receptors, by NanoString analysis. Plasma 11KT levels in silver eels were dramatically increased, and mRNA levels of more than half of all target genes were higher in silver eel muscle; none of the target genes was significantly downregulated. Gene expression profiles in white muscle from wild-caught eels were subsequently compared with those from yellow and silver eels subjected to implantation with sustained-release implants containing 11KT. Several weeks of exposure resulted in plasma levels of 11KT that resembled those of wild-caught eels and resulted in a dose-dependent increase in gonadosomatic and hepatosomatic index; however, target gene expression profiles in muscle were barely affected. We conclude that lipid physiology in white muscle of silver eels is notably different from that in yellow eels, and that 11KT is not responsible for the differentially expressed gene profile between yellow and silver short-finned eels.

The ecological, cultural and economic importance of freshwater eels, coupled with declining wild populations, drives the need for artificial propagation of these fish. Despite the closure of the lifecycle in captivity over a decade ago, numerous bottlenecks still prevent production at a commercially viable scale. Focusing on the key event of nutrient accumulation, we employed hypophysation over a 10-week period with biweekly sampling of female New Zealand shortfinned eels (Anguilla australis). Slotblots, colorimetric lipid/cholesterol assays, fast protein liquid chromatography and radioimmunoassay were used to assess aspects of blood chemistry. The lack of any trend in triglyceride and apolipoprotein B levels in plasma over time, combined with a significant reduction in cholesterol concentrations and a 10-fold increase in vitellogenin levels, confirmed a shift from low-density lipoprotein dominance—usually associated with lipid uptake and early oogenesis—to an abundance of high-density lipoproteins linked to vitellogenesis. The ongoing presence of 11-ketotestosterone in the circulation and the increase in 17β-estradiol levels reinforced the importance of steroids in mediating the gonadotropic signal associated with hypophysation. We conclude that the accumulation of both vitellogenin and apolipoprotein-B-associated lipids is essential for nutrient accumulation in eel oocytes during artificial induction of maturation.

Īnanga ( Galaxias maculatus ) is a widespread freshwater fish, found around coastal New Zealand, Chatham Islands, southern Australia and southern South America, exhibiting both diadromous and land‐locked life history patterns. We assessed population size structure, size at maturity, timing of reproduction, and post‐reproductive survival in the Waikouaiti and Kakanui rivers, two regionally important G. maculatus populations in southern New Zealand. Size at maturity varied between rivers and sexes, with males generally maturing at smaller sizes than females. The timing of reproduction and recruitment was observed to vary across a relatively small spatial scale. Population length‐frequencies indicated that both populations exhibited annual population dynamics, with most individuals spawning in their first year, then apparently dying soon afterwards (as indicated by a marked drop in post‐spawning abundance). However, a few individuals with recently spent or resting gonads were present over the winter months, indicating some post‐spawning survival. There was a lower frequency of exceptionally large (putatively 1+) individuals than reported for other populations around New Zealand and elsewhere, suggesting over‐winter survival following spawning varies. Based on the variation in life history characters over small spatial scales, we suggest that management of regionally important G. maculatus populations should be based on catchment level information.

Lipoprotein receptors, including low-density lipoprotein receptor (LDLr) relatives (Lrs) and LDLr-related proteins (Lrps), belong to the LDLr supergene family and participate in diverse physiological functions. In this study, novel sequences of lr and lrp genes expressed in the ovary of the short-finned eel, Anguilla australis, during early gonadal development are presented. The genes encoding the LDLr-like, Lrp1-like, Lrp1b-like, Lrp3, Lrp4-like, Lrp5-like, Lrp6, Lrp10, Lrp11, Lrp12-like, and Lr11-like proteins were found and identified by sequence and structure analysis, in addition to phylogenetic analysis. Genes encoding proteins previously implicated in follicle development and vitellogenin (Vtg) uptake in oviparous vertebrates were also identified, i.e. lr8 (including lr8 + and lr8- variants) and lrp13; their identification was reinforced by conserved synteny with orthologues in other teleost fish. Compared to other lr/lrp genes, the genes encoding Lr8 + , Lr8-, and Lrp13 were highly expressed in ovary during early development, decreasing as oocyte development advanced when induced by hypophysation. Furthermore, lr8 + , lr8-, and lrp13 were dominantly expressed in the ovary when compared with 17 other tissues. Finally, this study successfully detected the expression of both lr8 variants, which showed different expression patterns to those reported in other oviparous vertebrates and provided the first characterisation of Lrp13 in Anguilla sp. We propose that lr8 + , lr8-, and lrp13 encode putative Vtg receptors in anguillid eels.

The giant mottled eel, Anguilla marmorata, is at high risk of extinction due to overfishing of glass eels and elvers to provide stock for eel farming. In Vietnam, information on the genetic diversity and population structure of this species, which is necessary for resource management, is limited. In order to address this paucity of information, sequencing of the entire mitochondrial control region (mtDNA) was carried out for 176 individuals collected from central Vietnam. The sequences were investigated using various genetic, phylogenetic and population analyses. A total of 165 distinct haplotypes were identified. The percentage of variation within and among populations was 99.26% and 0.74%, respectively. The fixation index was low (0.007) and not significant (p = 0.096). Therefore, panmixia and a lack of significant population genetic structuring seem likely for A. marmorata in central Vietnam. Most sampled eels were genetically similar to eels found in North Pacific populations (Japan, Taiwan and the Philippines), except for one sample from Quang Tri and two samples from Binh Dinh, which had high values of genetic identity (97% to 99%) with South Pacific populations (Tahiti, Fiji, New Caledonia and Papua New Guinea). Taken together, we suggest that A. marmorata from the North and South Pacific populations co-exist in central Vietnam.

Background Acquisition of high quality sperm is key to the artificial propagation of eels in captivity, but fertility drugs are expensive and repeated handling is stressful to the fish. An interrupted treatment regime (an initial hormone injection to stimulate spermatogenesis, followed several weeks later by weekly booster injections to induce sperm maturation) for acquisition of sperm in captive male eels has promise for high sperm quality on the one hand, and animal welfare benefits on the other. To further develop this approach for shortfinned eel, Anguilla australis, we evaluated the efficacy of (i) different initial doses of human chorionic gonadotropin (hCG) and (ii) route of administration. Methods Male eels were artificially induced to mature with a single injection of 0, 250, 500 or 1,000 IU/fish of hCG, administered either intramuscularly (IM) or intraperitoneally (IP). Sperm maturation was induced with 150 IU hCG/fish from week 5 onwards and sperm collected for evaluation of quality by computer-assisted sperm analysis. Results Control males did not mature and hence, sperm could not be retrieved and analysed, but all other treatments were effective in inducing testicular maturation. Milt volume tended to be higher for fish injected IM compared to those injected IP, whereas hCG dose had no effect. Conversely, the concentration of spermatozoa tended to be higher for several sperm collection time points in IP-injected than in IM-injected fish. Sperm quality, represented by percent motility, percent progressive motility and curvilinear velocity, was equal in fish given an initial dose of 250 IU hCG to those given higher initial doses of hCG. Conclusions We recommend that an initial dose of 250 IU hCG/fish be administered to induce spermatogenesis in male A. australis, and, after a period of 4–5 weeks, weekly booster injections of ∼150 IU hCG/fish be administered in the day prior to sperm collection; both routes of administration (IM or IP) are equally effective. We contend that an interrupted treatment regime has notable benefits for induced maturation in male anguillids, as it reduces fish handling and manipulation and reduces the resources required to produce high quality sperm.