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It has been suggested that changes in oceanic conditions4 are contributing to the recent drastic decline of anguillid eels worldwide5, by disrupting their spawning areas and the transport of their larvae (leptocephali)4. But little is known about such spawning areas. On the basis of data collected on leptocephali over almost 50 years and analysis of their hatching dates, we have proposed that the Japanese eel spawns near seamounts west of the Mariana Islands (14–17° N, 142–143° E), close to the time of the new moon6. We have now verified the location and timing of spawning by Japanese eels after collection and analysis of the newly hatched pre-leptocephali.

During research cruise KH-05-1 aboard RV Hakuho Maru, we collected and genetically identified 130 pre-leptocephali (size, 4.2–6.5 mm) and 60 leptocephali (11.7–18.4mm) of the Japanese eel in the region of the North Equatorial Current in June 2005. The pre-leptocephali were collected around 14° N, 142° E, to the west of the Suruga Seamount in the southern part of the West Mariana Ridge (Fig.1).

Figure 1: Collection sites of small eel larvae.
figure 1

Left, catches of the Japanese eel, Anguilla japonica, in the western North Pacific Ocean between 1961 and 2002 (n=2,418; ref. 6); yellow circles, sites where leptocephali larger than 7 mm were collected. Metamorphosing leptocephali (blue triangles) and oceanic glass eels (green squares) were also found. Black dots, sites where no larvae were found. Right, eel catches in the western North Atlantic Ocean (1913–85; ref. 2). Shaded areas, sites where leptocephali (⩽10 mm in size) of each species were collected2. Red squares mark collection sites of pre-leptocephali (⩽7 mm) of the Japanese eel (left) or of both Atlantic eel species2 (right). NEC, North Equatorial Current. Inset, distribution and number of Japanese eel pre-leptocephali (⩽7 mm; green circles) and leptocephali (>7 mm; yellow circles) collected during the time of the new moon in June 2005. Red triangles, seamounts. White circles, stations where no Japanese eel leptocephali were found.

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Some pre-leptocephali were identified as A. japonica on board by real-time polymerase chain reactions7; this identification was later confirmed by DNA sequencing of additional specimens8. (For details, see supplementary information.) The larvae were collected on the day of the new moon and for two days afterwards. They were at various stages of development: some had unpigmented eyes and no teeth, others had pigmented eyes, early teeth and jaws (Fig. 2). Daily growth rings of calcium mineralization in ‘ear stones’ known as otoliths showed that the eels had hatched 2 to 5 days previously, indicating that spawning had occurred about 4 days before the new moon.

Figure 2: Japanese eel pre-leptocephali at different stages of development.
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a, Head regions of early-stage larvae with no teeth, jaws or eye pigmentation; b, some larvae show early eye pigmentation; c, some have early teeth but no jaws; d, others have teeth that are more developed and early jaws. Larva lengths in a–d are 5.0, 4.7, 5.2 and 4.2 mm, respectively. Scale bar, 1 mm.

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We have identified a precise spawning location by collecting newly hatched pre-leptocephali. The area seems to be much smaller than the spawning area estimated for the Atlantic eels (Fig. 1). It is located at an optimum latitude for the leptocephali to enter the Kuroshio Current that flows north towards the eels' habitat areas in east Asia. It seems that spawning occurs in a narrow range of latitudes because otherwise leptocephali would not find the northward flow and might instead enter the Mindanao Current, which flows southwards to places where there are no Japanese eels4,6.