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Otolith microstructure and microchemistry were examined in Anguilla japonica final stage leptocephali and glass eels. Otolith increment width, relatively constant and narrow in the leptocephalus, increased sharply from age 80 to 160 d. Sr:Ca ratios in the otolith, which increased during the leptocephalus stage, thereafter showed a rapid drop, coinciding with the increase in increment width. A comparison of the ontogemc change patterns in otolith increment width and Sr:Ca ratios between leptocephali and glass eels strongly suggested that these coincidental changes between 80 and 160 d indicate the onset of metamorphosis in the leptocephali. Metamorphosis appeared to have been completed before the increment width had reached its maximum, suggesting a duration of the former of some 20 to 40 d.KEY WORDS E e l . Angu~lla japonica . Metamorphosis. Otol~th Daily growth increments Sr Ca rat~os 'E-mail-arait0ori.u-tokyo.ac.jp

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The apparent use of marine and freshwater habitats by Anguilla australis and A. dieffenbachii was examined by analyzing the strontium (Sr) and calcium (Ca) concentrations in otoliths of silver eels collected from Lake Ellesmere, which is a shallow brackish-water coastal lagoon in New Zealand. The age and growth of these eels was also examined using their otolith annuli. Size and ages of females were greater than those of males for both species. Growth rates were similar among sex and species, but the highest growth rates were observed in eels that experienced saline environments. Line analyses of Sr:Ca ratios along a life-history transect in each otolith showed peaks (ca. 15 to 21 × 10 -3 in A. australis, 14 to 20 × 10 -3 in A. dieffenbachii) between the core and elver mark, which corresponded to the period of their leptocephalus and early glass eel stage in the ocean. Outside the elver mark, the Sr:Ca ratios indicated that eels had remained in different habitats that included freshwater (average Sr:Ca ratios, 1.8 to 2.4 × 10 -3 ), areas with relatively high salinities (average Sr:Ca ratios, 3.0 to 7.4 × 10 -3 ), and in some cases individuals showed clear evidence of shifts in the salinity of their environments. These shifts either indicated movements between different locations, or changes in the salinity of the lake. There were more individuals of A. australis that used areas with intermediate or high salinities, at least for a short time (85% of individuals), than A. dieffenbachii (30%). These findings suggest that these 2 southern temperate species may have the same behavioral plasticity regarding whether or not to enter freshwater or remain in marine environments, as has been recently documented in several northern temperate anguillid species.KEY WORDS: Anguilla dieffenbachii · Anguilla australis · Age · Growth · Otolith microchemistry · Habitat use · Lake Ellesmere · New ZealandResale or republication not permitted without written consent of the publisher

No significant differences were found in mean ages among months within any one species, whereas significant differences were found among species. Hatching was estimated to occur throughout almost the entire year in A. celebesensis and A. marmorata compared with intermittently over about 6 mo in A. bicolor pacifica. Based on year-round recruitment of A. bicolor pacifica at the mouth of the Poigar River in an earlier study and age at recruitment being constant throughout the year, the spawning season of this species may also occur throughout the year. In all species examined, positive linear relationships were found between age at metamorphosis and age at recruitment, suggesting that early metamorphosing larvae were recruited to freshwater habitats at an early age. Year-round recruitment of tropical glass eels to the river mouth would necessarily follow year-round spawning and stable recruitment age. Such a recruitment mechanism differs from that of temperate eels, the latter having a limited spawning season followed by a limited period of recruitment.