Gill chloride cell morphology and Na+,K+-ATPase activity were examined in cultured Japanese eels (Anguilla japonica) acclimated to fresh water (FW) or seawater (SW), and in yellow and silver eels caught in wild stocks. Gill Na+,K+-ATPase activity was higher in SW-acclimated cultured eels than in FW eels. Chloride cells were detected in both filament and lamellar epithelia by immunocytochemical staining using anti-Na+,K+-ATPase serum. The filament chloride cells were more abundant and larger in SW eels than in FW eels, whereas there was no apparent difference in lamellar chloride cells. In wild-caught eels, gill Na+,K+-ATPase activity increased as they developed from yellow to silver eels. Filament chloride cells in silver eels were more active than those in yellow eels. In contrast, lamellar chloride cells, which were frequently observed in yellow eels, had decreased in number or disappeared in silver eels. These findings suggest that chloride cells in the filament are responsible for excretion of excess salt in hyperosmotic environments, and that lamellar chloride cells may play a significant role in hypoosmotic environments, presumably acting as sites of ion uptake. The excellent euryhalinity of the Japanese eel may be due, at least in part, to the presence of functionally different types of chloride cells. The activation of filament chloride cells in silver eels in FW, together with the increase in Na+,K+-ATPase activity, could be interpreted as a preadaptive response to forthcoming entry into SW.

The burrow morphology of the Japanese eel Anguilla japonica was studied using in situ resincasting in a mud bottomed tidal drainage channel adjacent to the Fukui River in Tokushima, Japan. Two eels (62Á5 and 56Á3 cm total length) were initially fished from the burrows to verify that they were being used by A. japonica. Casts were made of 10 burrows that were found to have from one to three openings and main tunnels that were parallel to the axis of water flow in the channel. The maximum depths of the tunnels in the mud were 17Á8-30Á0 cm. The diameters of main tunnels ranged from 1Á2 to 7Á9 cm, were almost always wider than the bodies of the Japanese eels examined, and were more variable in the horizontal axis than in the vertical axis. There were no other animals capable of constructing a long and thin burrow in this channel, so these observations indicate that anguillids are able to construct their own burrows in soft mud sediments that may be used for extended periods of time. # 2005 The Fisheries Society of the British Isles

The seasonal occurrence of 72 silver phase Japanese eels Anguilla japonica collected in the East China Sea is described and their morphological features analyzed. These silver eels were captured with dip nets under lights around fishing boats at night near islands in the northeast region of the East China Sea over 7 yr. The eels had the typical morphological characteristics of silver eels, such as a black metallic body color and enlarged eyes. All females had started ovarian maturation with gonad somatic indices of 1.3 to 3.5. No eels smaller than 400 mm or yellow phase eels were collected or observed. Considering the limited seasonal occurrence of these eels, and that their morphological features were characteristic of silver phase migrating eels, these eels appeared to be passing through sampling areas on their spawning migration to the region just west of the Mariana Islands in the Philippine Sea.

ABSTRACT. Five horrnone-treated fernale Japanese silver eels Anguilla japonica were tagged with ultrasonic transmitters and released by submersible in the West Pacific at seamounts of the West Mariana Ridge, their supposed spawning grounds. Four eels were tracked for 60 to 423 min in the vicinity of the searnounts. They did not settle at the seamounts but swam at a mean speed of 0.37 m s-' into Open water above deep ground. Their mean swimrning depth ranged from 81 to 172 m. Experiments suggest that pre-matured A. japonica rnigrate to their spawning grounds in temperate warm water and at shallow depths.

No abstract