Naomi Mikawa

A silvering index for the Japanese eel Anguilla japonica

To establish a simple and reliable index for determining silvering stages of the Japanese eel, Anguilla japonica, we observed the colorations of various body parts and biological characteristics of the eels collected in a coastal area of Japan (Mikawa Bay). The four silvering stages are characterized by the colorations of pectoral fins and ventral skin as follows: (1) Y1, yellow eel without a metallic hue at the base of pectoral fins, (2) Y2, late yellow eel with a metallic hue at the base of the pectoral fins but without melanization at the tip of pectoral fins, (3) S1, silver eel with complete melanization at the tip of pectoral fins but without full pigmented belly in black or dark brown, and (4) S2, late silver eel with black or dark brown belly. The body size, eye diameter and sexual maturity of each stage increased in the order of Y1, Y2, S1 and S2 stages, whereas the digestive tract degenerated in the same order, suggesting a sequential development of these ontogenetic stages identified in the study. The Y1, Y2 and S1 stages could be also distinguished by canonical discriminant function analysis using three internal (gonad-somatic index, GSI; hepato-somatic index, HIS; and gut index) and two morphometric (condition factor and eye index) parameters, supporting the significance of these stages. This method of staging for the silvering process of the Japanese eel appeared to be applicable to all specimens of this species, since this index used only simple external characteristics that would be easy to observe during field surveys.

Effects of water temperature on early development of Japanese eel Anguilla japonica

To determine an optimal temperature range for efficient production of healthy eel larvae Anguilla japonica, the effect of water temperature on hatching, survival, and deformity rates was examined. The early ontogeny of morphological features in this species by incubating eggs at five different temperatures (19, 22, 25, 28 and 31 °C) was examined. Hatching occurred at 24, 28, 46 and 58 h after fertilization when incubated at 28, 25, 22 and 19°C, respectively; no eggs hatched at 31°C The growth rate of prefeeding larvae increased as water temperature was elevated and acquisition of feeding ability was also accelerated at higher temperature. Significantly high hatching rates (76–86°, P<0.05) and survival rates (61–86%, P<0.05) were observed at relatively high temperatures (22–28°C). The deformities were ‘open lower jaw’, ‘pericardial edema’, and ‘notochordal bending’, in which the mouth was deformed with a downward projecting lower jaw, the pericardial cavity was swollen, and the notochord was bent or twisted to various degrees, respectively. Open lower jaw and pericardial edema were especially predominant at 19°C, with rates of 68 and 92%, respectively, compared with 31 and 10% at 25 and 28°C, respectively. The occurrence of notochordal bending was not affected by temperature. The optimal temperature for incubation and rearing A. japonica eggs and prefeeding larvae is approximately 25–28°C.

Molecular cloning of fresh water and deep-sea rod opsin genes from Japanese eel Anguilla japonica and expressional analyses during sexual maturation1

We have determined the complete cDNA sequences of fresh water rod opsin gene (fwo) and deep‐sea rod opsin gene (dso) from Japanese eel Anguilla japonica. The cDNA clones of fwo and dso consisted of 1437 and 1497 nucleotides, respectively. The predicted opsins of both genes consisted of 352 amino acid residues. Southern blot and PCR analyses of genomic DNA indicated that the Japanese eel genome contains only one fwo and one dso and they are intronless. Quantitative RT‐PCR analyses revealed that the expression of fwo decreases with sexual maturation while that of dso increases.

Annual changes in ovarian development and plasma estradiol-17β level in reared female common Japanese conger, Conger myriaster

Annual changes in the gonadosomatic index (GSI), oocyte diameter, gonadal histology and plasma estradiol-17β (E2) levels were investigated in female common Japanese conger Conger myriaster in captivity. Juveniles were caught in September 1999 and reared in seawater at temperatures ranging from 10–20°C for 3 years. All fish were immature when captured in September 1999. GSI and oocyte diameter increased from October 2000, peaked between summer and autumn 2001, and bottomed-out in winter 2001. Plasma E2 level also increased from October 2000, but remained high. The ovarian developmental stage was at the peri-nucle olus stage or the oil droplet stage until September 2000. Vitellogenesis started in October 2000 and oocytes progressed to the tertiary yolk globule stage by summer 2001. However, vitellogenic oocytes regressed in all females after autumn 2001. The neogenetic oocytes began to increase after November 2001 and ovarian development progressed in 2002 as they did in 2001, although maximum GSI in 2002 was half its 2001 value. These data indicate that ovarian development in the common Japanese conger has an annual periodicity, and that these congers may be able to spawn in multiple years under rearing condition.

Effects of silvering state on induced maturation and spawning in wild female Japanese eel Anguilla japonica

The effects of silvering state of wild female Japanese eels Anguilla japonica on the success of induced maturation and the following spawning were examined. Thirty-eight females, collected in Mikawa Bay, were divided into four stages based on their silvering state: yellow (Y1), late-yellow (Y2), silver (S1) and late silver eels (S2). Despite injections of salmon pituitary extract (SPE) through the standard technique, Y1 and Y2 eels did not respond to the treatment with undeveloped gonad (gonad-somatic index [GSI]: 0.3–0.9), and all these females died by 5 weeks, probably due to an abnormal physiological condition. Most S1 (81%) and S2 eels (100%) matured completely (GSI: 17.8–51.4), and finally spawned successfully (69% for S1, 89% for S2). S2 eels fully matured with oocytes of over 750 μm in diameter by significantly smaller number of injections of SPE (5–6 times) than the case of S1 eels (6–8 times). The amount of eggs released by S2 eels (0.65±0.11 g/fish per body weight [BW]) was significantly larger than those by S1 eels (0.54±0.09 g/fish per BW). There was no difference in fertilization and hatching rates between eggs released by S1 eels and those of S2 eels. These results indicate that the success of induced maturation and spawning in wild female Japanese eels depends on their silvering state, and matured eggs can be obtained efficiently through the use of S2 eels rather than other stages.

Monthly changes in the swim bladder morphology of the female Japanese eel Anguilla japonica in the coastal waters of Mikawa Bay, Japan

In the coastal area of Mikawa Bay, central Japan, specimens of the female Japanese eel Anguilla japonica could be divided into two groups according to the degree of swim bladder development. In one case, they were undeveloped, and in the other, they were highly developed with large rete mirabile, thick submucosa, and a well-developed gas gland. The morphology of the swim bladder in the latter group was comparable with that in the artificially fully maturated eel. The specimens with an undeveloped swim bladder were caught during all months, although their number was small. The specimens with a highly developed swim bladder were most abundant in November and December. During these months, catch of the specimens also increased sharply, by more than 10 fold compared to that in other months. These observations indicate that most of the eel appearing in coastal Mikawa Bay from October to January have a highly developed swim bladder adapted for a deep-sea environment. It was also conjectured that these specimens most likely migrated from rivers feeding into Mikawa Bay, toward spawning grounds in the open sea, and that this occurred after development of their swim bladders was completed. Actually, the specimens caught in the upstream of the Toyo River feeding into Mikawa Bay from late August to early October already had highly developed swim bladders.