After cryopreservation in a diluent (D#5-D), containing 76·2 m NaHCO 3 , 137·0 m NaCl, 1·4% soya lecithin and 10% dimethyl sulphoxide, for 30-180 days, Japanese eel Anguilla japonica sperm was 61·4-70·6% motile compared with fresh sperm. Cryopreserved sperm was used for fertilizing three batches of eggs and hatchability ranged from 20·7 to 69·1%. These results indicate that D#5-D is an useful diluent for sperm cryopreservation in Japanese eel. 2002 The Fisheries Society of the British Isles
Interspecific hybrids were obtained by artificial insemination between male European eels Anguilla anguilla and female Japanese eels Anguilla japonica. The hybrid larvae developed normally and survived up to 30 days post-hatching similarly to normal A. japonica larvae.
# 2004 The Fisheries Society of the British Isles
Silver eels Anguilla japonica (I G 1·5-3·5) had more developed rete mirabile, gas gland and submucosa than yellow eels (I G 0·4-1·5) and the development of swim bladder components increased with sexual maturity only in the early maturation process (I G <3·5). These observations indicate that the Japanese eel develops its swim bladder in either the river or shallow sea water and leaves for the open sea after the swim bladder has become adapted to a deeper-sea environment. 2001 The Fisheries Society of the British Isles
Sperm motility of Japanese eel Anguilla japonica was depressed from 59·1 to 1·1% when NaHCO 3 concentration increased from 0 to 150 m. In 25 m NaHCO 3 , when pH of the medium was 6·2, 7·2, 8·2, 9·2 and 10·2, relative sperm motility was 0, 0, 55·8, 93·7 and 136·6%, respectively to that of the control (0 m NaHCO 3 ). The remarkable effect in acid or neutral condition indicates that free-CO 2 (liberated CO 2 and H 2 CO 3 ) is a key factor for the motility inhibiting effect of NaHCO 3 .
The nucleus of the spermatozoa in both Anguilla anguilla and A. japonica species is asymmetric on the longitudinal axis, being gently curved and elongated with a hook-shaped superior end directed inside the crescent. A single spherical mitochondrion is located at the superoanterior portion of the nucleus. Nine striae extending from the basal portion of the flagellum run along the right side of nuclear surface. These are always oriented toward the mitochondrial portion and their ends are located on the mitochondrion. These features are seen not only in both Anguilla species but also in Muraenesox cinereus and in other species including other genera of Anguilliformes. It is suggested that these features be considered as common to Anguilliformes. 2000 The Fisheries Society of the British Isles
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