Abstract Growth and silvering of cultured Japanese eels stocked in Lake Biwa, where recruitment of wild eels was inhibited by dams, were evaluated. Small and large cultured eels for stocking were sex-undifferentiated and males, respectively. Yellow eels mainly caught with long-lines in Lake Biwa were all females, suggesting sex-undifferentiated cultured eels transitioned into females following their being stocked. Considering ages of sex-undifferentiated cultured eels for stocking (1.4 ± 0.5 years) and captured eels (5.8 ± 2.8 years), stocking strategies in Lake Biwa should be designed with a 3–5-year post-release period in mind. Growth rates of yellow females in Lake Biwa (111.4 ± 38.4 mm year ⁻¹ ) were similar to or higher than those of wild individuals elsewhere. Most captured Y2 eels were classified as “typical Y2,” having ovaries with oocytes at the chromatin nucleolus or perinucleolus stage, although some were confirmed as having a fully pigmented black or dark-brown belly and ovaries at the early/late oil droplet or yolk vesicle stage, being characteristics of silver eels (“silver-like yellow”). These results demonstrated the potential role of lakes as habitats for stock enhancement of Japanese eels, providing valuable insights into the stocking strategy for this species.

Elucidating how sex ratios change as a cohort ages may aid with understanding the relative recruitment of male and female adults to a population. In sea turtles, sex is determined by incubation temperature, with males and females produced at low and high temperatures, respectively. At rookeries exposed to high incubation temperatures, the primary sex ratios are thus skewed toward females. However, because very high incubation temperatures also negatively affect the survival-related traits of female hatchlings, the female bias may be mitigated as the cohort matures and the adult sex ratio may be more male-biased than primary sex ratios. Here, we examined whether the sex ratio changes from hatchlings to breeding adults at Yakushima Island, Japan, the largest loggerhead turtle Caretta caretta rookery in the North Pacific, which is not exposed to high incubation temperatures. The mean (±SD) primary sex ratio (% female) during 1975-2022, estimated from incubation durations linked to sand temperatures, was 24.7 ± 13.6%. Based on genotype data at 3 microsatellite loci from 26 females and their 291 offspring sampled in 2020 and 2021, we detected multiple paternity in 5 clutches (19.2%) and estimated 34 unique sires, which provided a breeding sex ratio (% female) of 43.3%. Small differences between primary and breeding sex ratios suggest similar survival rates for male and female offspring. This rookery is unusual in that it has male-biased hatchling production, whereas most sea turtle rookeries have female-biased hatchling production. The Yakushima rookery is likely resilient to climate warming.

Doubled haploids (DHs) are essential founders for breeding via isogenic line production. We determined the optimal timing of the first and second (HST1 and HST2) double heat shock treatment for DH production in the willow gudgeon (Gnathopogon caerulescens). Gynogenesis was induced by fertilisation with UV-irradiated sperm (20 mJ/cm2). Each HST was performed at 40.5°C for 1 min. In experiment I, HST1 was applied 23–31 min after the induction of gynogenetic development (incubated at 20°C). HST2 was performed 15 min after HST1 treatment. When HST1 was started 25–26 min after activation, normally hatched larvae appeared relatively frequently (35.6–100%), and most were DHs. In experiment II, HST1 was started 26 min after activation, and HST2 was started 10–25 min after HST1. Starting HST2 10 min after HST1 resulted in 33% more normally hatched larvae than starting 22.5–25 min after. DHs were prevalent among hatched larvae (45.1% and 20.8%, respectively) when performing HST2 10–12.5 min after HST1. Under appropriate HST1 and HST2 timing, gynogens were at the zygote stage, early prophase, and prophase or early prometaphase of the first cell cycle. These results establish the appropriate timing of HST1 and HST2 for the induction of willow gudgeon DHs.

Abstract Inosine monophosphate (IMP) is an important indicator of meat freshness and contributes to its umami taste. An attractive strategy for enhancing umami is to suppress the IMP-degrading activity and increase the IMP content in the skeletal muscle through genome editing technology using the CRISPR-Cas9 system. However, the molecular mechanisms underlying IMP degradation remain unclear. We cloned two ecto-5′-nucleotidase genes, designated as ecto-5′-nucleotidase-a (nt5ea) and ecto-5′-nucleotidase-b (nt5eb), from medaka (Oryzias latipes), a vertebrate model organism. Expression analysis using embryos showed that nt5ea or nt5eb overexpression remarkably upregulated IMP degradation, and that the IMP-degrading activity was higher in Nt5ea than in Nt5eb. Furthermore, we established frame-shifted or large deletion (lacking nt5ea or nt5eb locus) mutant strains and assayed the effects of gene disruptions on the amount of IMP in skeletal muscle. The nt5ea-deficient medaka showed considerable higher levels of IMP at 48 h postmortem than did the wild-type fish. The nt5eb mutants also exhibited higher IMP contents than that in the wild types, but the increase was less than that in the nt5ea mutants. Our results demonstrated that nt5e is an important regulator of IMP levels in skeletal muscle and that its loss of function was effective in maintaining IMP content.

[Synopsis] The members of the Laboratory for Aquatic Biology, Faculty of Agriculture, Kinki University collected aquatic animals at Sakatahana, Shirahama-cho, Wakayama Pref. during field practice every summer from 2007 to 2011. Here, we made a provisional list of the collected specimens of fishes, crustaceans, gastropods, and bivalves that include 58, 37, 27, and 12 species, respectively. Among them, the numbers of the species of temperate to subtropical waters were 36 in fishes, 13 in gastropods, and 4 in bivalves.

醒井養鱒場ニジマス血統集団から選抜された個体を用いた雌性発生継代によって、早期産卵形質および大型卵産出形質に関する遺伝的固定をおこなった。夜間照明による成熟調節操作を受けた1986年夏季産卵群（P）の再生産形質のうち排卵時期と産出卵のサイズの分布を調べ、産卵期間のうちで早期、中期、晩期に排卵した個体の中から大型卵を産出した個体をそれぞれ選抜し、それらの卵を用いて雌性発生で継代した。雌性発生1代目（1G）は1988年の秋に成熟し、それぞれの次世代集団は各選抜親魚の産卵期および大型卵産出に関する形質をよく受け継いでいた。そのうち特に早期に大型卵を産出したC系統の個体を用いて雌性発生継代した2代目（2G）は醒井通常系統よりも6週間も早い9月20日から排卵を開始した。1Gの卵サイズは系統ごとに広くばらついた。雌性発生C系統の1代目の平均卵径は4.90mmとP群の1.06倍であり、その実現遺伝率は0.164であった。

ニゴロブナにおける雌性発生卵の第二極体放出阻止のための低温刺激処理（CST）および高温刺激処理（HST）後の染色体の行動を観察した。受精7日後の正常仔魚生存率はCSTおよびHSTをそれぞれ媒精後7分および9分に開始した場合に最大値を示した。雌性発生は1腹卵を使って紫外線照射精子（3,000erg／mm2）で誘導した。CSTの後、第二減数分裂の紡錘体が消失し、カリオメアと半数性核、あるいは二倍性核が、雌性前核と極体の代わりに観察された。これらの構造体は処理後15分（媒精後62分）に精子星状体の中心へ移動して接合核を形成し、媒精後72分に第1卵割が起こった。一方HST後では、卵割は処理後12分に起こった。核の変化はCST後よりもHST後の方が早かったが、両処理後の核の行動は類似していた。カリオメアおよび前核の行動の違いは処理の違いではなく、卵の熟度と感受性の違いによっていると考えられた。

Human chorionic gonadotropin (HCG) was injected to one-year old captive chub mackerel (Scomber japonicus) in June 2005, a timing equivalent to the fish spawning period, to see if fertilized eggs would be formed. The sample fish were accommodated in a net cage and measured for total length, fork length, body length, head length, body depth, body width and body weight every 10 days until 100 days after hatching, then every 30-day thereafter. The sample fish were also checked for ovulation and spermiation by abdominal thrust 280 days after hatching. Their gonadal gland was then removed, weighed and measured for gonad somatic index (GSI) and, for females, KG value. Although spermiation was not confirmed in any males 280 days in age, spermiation was confirmed in half the male fish after 310 days in age and in all the male fish after 340 days in age. For the female fish, no ovulation was observed in individuals 310 days in age, but ovulation was confirmed in 8 out of 10 fish 340 days in age. Five groups of 10 male and 10 female in each received single injection of HCG (200 IU/kg BW) from 360 days in age (1st group) to 376 days in age (5th group) . The fish in four group spawned by the 3 days after the injection. In the group of eggs injected on June 27 (4th group), fertilized eggs were seen, although in a small quantity. When these fertilized eggs were cultured, they were observed to hatch normally and the hatchlings grew successfully.