No abstract

Despite the absence of rainfall or of any rise in river water level, the seaward migration of Japanese silver eels frequently occurs after the passage of a 'dry depression', suggesting that the migratory behavior of the eels is influenced by the atmospheric depression rather than by rainfall or elevated water level, as is usually thought. Our analyses showed that seaward migration of silver eels takes place mainly during the new moon in autumn to winter as water temperature decreases, suggesting the possibility that their migratory behavior depends on at least 3 independent environmental factors: water temperature, lunar phase and the passage of an atmospheric depression. However, water temperature and lunar phase were insufficient to cause the eels to start their migration. The passage of a depression was required as the final event to begin the seaward migration. These findings strongly suggest that the passage of a depression may be the only trigger that begins the seaward migration of silver eels.

Development of embryos and larvae in the common Japanese conger Conger myriaster was observed after artificial fertilization. Eggs were obtained from females matured artificially by hormone injections and milt was obtained from males matured naturally. Fertilized eggs were kept in seawater at 12–14°C. The first cleavage occurred at 4 h, epiboly began at 24 h, the embryonic body was formed at 38 h and hatching occurred at 84 h after insemination. Newly hatched larvae were approximately 2.5 mm (total length) and similar to those of Anguilla japonica in terms of external features. The mouth and anus opened on the 7th day after hatching. Pigments began to appear at the tip of the tail on the 10th day. The total length of the larvae reached approximately 8 mm on the 11th day. Eye pigmentation began on the 14th day. One larva lived for 19 days without food.

Annual changes in ovarian development and plasma estradiol-17 b b b b level in reared female common Japanese conger, Conger myriaster ABSTRACT: Annual changes in the gonadosomatic index (GSI), oocyte diameter, gonadal histology and plasma estradiol-17 b (E 2 ) 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

No abstract

We investigated the process and characteristics of oogenesis in the common Japanese conger Conger myriaster. Young fish caught in November 1996 were reared for use in this experiment. Fish were sampled monthly from December 1997 to August 1998. Some were injected with human chorionic gonadotropin to stimulate ovarian maturation from May to August 1998. Oocytes from the chromatin nucleolus stage to the secondary yolk globule stage were obtained from non‐hormone‐treated fish; those of more advanced stages were obtained from hormone‐treated ones. We divided oocyte development into eight stages from the chromatin nucleolus stage to the maturation stage. The yolk vesicle stage was not separated because yolk vesicles began to appear just after appearance of yolk globules. Oocyte, oil droplet, yolk globule and nucleus diameters all increased concomitant with oocyte development. Oil droplet and yolk globule diameters increased remarkably at the maturation stage. However, zona radiata thickness peaked at the secondary yolk globule stage, decreasing gradually thereafter. Increased gonadosomatic index was related to oocyte development as found in European and Japanese eels receiving hormone treatment to mature. The present study is the first report describing oogenesis characteristics in congrid eels. It indicates that oogenesis is almost identical to that of other anguillid eels.

Annual changes in gonadal maturation of female Japanese eel Anguilla japonica in sea water were investigated histologically over 5 years in the Mikawa Bay, Japan, where they occurred throughout the year except in March. Almost all immature Japanese eels (yellow eels) occurred mainly from April to September, and they were rare after November. In contrast, maturing Japanese eels (silver eels) occurred from October to February. The gonado‐somatic index (IG) and oocyte diameters of yellow eels were <1·0 and 150 μm, respectively, and oocytes were at the peri‐nucleolus or the oil droplet stages. The IG and oocyte diameters of silver eels were greater than those of yellow eels and most oocytes developed to the primary yolk globule stage. The numbers of silver eels lacking oocytes at the primary yolk globule stage increased after January in Mikawa Bay, although IG and oocyte diameters remained unchanged. In contrast, silver eels caught at the mouth of the bay in January possessed oocytes that had advanced to the secondary yolk globule stage. These observations indicate that oocyte development changes seasonally, especially after winter in Mikawa Bay.

In order to find out the role of sodium bicarbonate (NaHCO 3 ) on the initiation of sperm motility in the Japanese eel Anguilla japonica , interactions were investigated between NaHCO 3 and various reagents (K + channel blocker 4-aminopyridine [4-AP], ammonium chloride [NH 4 Cl], sodium acetate and calcium chloride [CaCl 2 ]) that could regulate internal factors (intracellular K + , intracellular pH [[pH] i ] and intracellular Ca 2 + ) in sperm motility. Contradictory effects of NaHCO 3 were observed (i.e. an inhibitory effect when 4-AP was absent and a promoting effect when 4-AP was present). Sodium bicarbonate inhibited the initiation of sperm motility in the Japanese eel. However, NaHCO 3 restored the motility of immotile sperm that 4-AP inhibited. The inhibitory effect of NaHCO 3 disappeared with the addition of NH 4 Cl, which raised [pH] i , but the promoting effect was not affected by [pH] i . Although NaHCO 3 recovered motility in the presence of 4-AP, this recovery was also observed with the addition of CaCl 2 instead of NaHCO 3 . In the initiation of sperm motility in the Japanese eel, two roles for NaHCO 3 are suggested: an inhibitory role relating to the regulation of [pH] i and a promoting role relating to the uptake of another initiation factor, which could be Ca 2 + .