Seishi Hagihara

The giant mottled eel, Anguilla marmorata, is at high risk of extinction due to overfishing of glass eels and elvers to provide stock for eel farming. In Vietnam, information on the genetic diversity and population structure of this species, which is necessary for resource management, is limited. In order to address this paucity of information, sequencing of the entire mitochondrial control region (mtDNA) was carried out for 176 individuals collected from central Vietnam. The sequences were investigated using various genetic, phylogenetic and population analyses. A total of 165 distinct haplotypes were identified. The percentage of variation within and among populations was 99.26% and 0.74%, respectively. The fixation index was low (0.007) and not significant (p = 0.096). Therefore, panmixia and a lack of significant population genetic structuring seem likely for A. marmorata in central Vietnam. Most sampled eels were genetically similar to eels found in North Pacific populations (Japan, Taiwan and the Philippines), except for one sample from Quang Tri and two samples from Binh Dinh, which had high values of genetic identity (97% to 99%) with South Pacific populations (Tahiti, Fiji, New Caledonia and Papua New Guinea). Taken together, we suggest that A. marmorata from the North and South Pacific populations co-exist in central Vietnam.

Eight silver‐phase male Indo‐Pacific eels (giant mottled eels) Anguilla marmorata were collected from three small rivers on subtropical Amami‐Oshima Island, Japan, and their morphological and gonadal‐histological characteristics were examined. The total length, body mass and age of the silver eels were 543.8 ± 49.3 (496–659) mm, 376.0 ± 116.0 (282–660) g and 14.6 ± 2.3 (11–19) years old, respectively. The silvering‐related characteristics (eye index, pectoral‐fin index, gut‐somatic index) and reproductive characteristics (gonadosomatic index, histological stage) of the silver eels were significantly advanced compared to those of yellow eels, which is similar to other anguillid species.

Most cultured Japanese eels (Anguilla japonica) show male sex differentiation; however, natural gonadal sex differentiation has not been evaluated. In this study, this process was characterized in wild eels. Differentiated ovaries and testes were observed after the eels grew to 320 and 300 mm in total length, respectively. The youngest ovary and testis appeared at 3 and 4 years old, respectively; however, undifferentiated gonads were found up to 7 years, suggesting that sex differentiation was triggered by growth rather than aging. gsdf, amh, foxl2b and foxl3b were highly expressed in the testes, whereas figla, sox3, foxn5, zar1, and zp3 were highly expressed in the ovaries. The expression of cyp19a1a and foxl2a did not differ significantly between the testis and ovary. In the ovaries, the cyp19a1a and foxl2a levels were highest in the early stages, suggesting that their function is limited to early ovarian differentiation. The foxn5, zar1 and zp3 levels tended to increase in the later stages, suggesting that they function after the initiation of ovarian differentiation. In undifferentiated gonads, dimorphic gene expression was not observed, suggesting that the molecular sex differentiation phase is short and difficult to detect. These findings provide the first demonstration of the whole course of natural gonadal sex differentiation in eels at molecular and morphological levels.

Morphological, ecological and physiological characteristics of 2 downstream migrating and 2 non‐migrating female Pacific bicolor eels Anguilla bicolor pacifica are described for the first time. Total length and age of the migrating eels were 1005 mm and 10 years, and 1110 mm and 11 years old, and the non‐migrating eels were 892 mm and 8 years, and 805 mm and 9 years old. Silvering‐related characteristics (silvering index, eye index, pectoral‐fin index, gut‐somatic index, swimbladder‐somatic index) and reproductive physiological characteristics (gonadosomatic index, follicle diameter, oocyte stage, transcription of gonadotropins, concentration of sex steroids) of the migrating eels were more advanced than those of the non‐migrating eels.

Downstream‐migrating (n = 64) and non‐migrating (n = 21) female Celebes eels Anguilla celebesensis were captured from the Poso Lake–River system on Sulawesi Island, Indonesia, and their reproductive physiological characteristics were examined. A histological observation of the ovaries revealed that most non‐migrating eels were at the perinucleolus (43%) or oil‐droplet (48%) stage, whereas most migrating eels were at the early vitellogenic (36%) or midvitellogenic (61%) stage. Transcript levels of gonadotropin genes (fshb, lhb) in the pituitary gland and concentrations of sex steroids [11‐ketotestosterone (11‐KT), testosterone, 17β‐oestradiol (E₂)] in blood plasma of migrating eels were significantly higher than those of non‐migrating eels. The fshb messenger (m)RNA levels were lower in perinucleolus and oil‐droplet stages and then significantly increased in the early vitellogenic stage. The lhb mRNA levels in vitellogenic‐stage eels were significantly higher than those in perinucleolus‐ and oil‐droplet‐stage eels. The 11‐KT levels of eels at the oil‐droplet and vitellogenic stages were significantly higher than those of eels at the perinucleolus stage. The E₂ levels at the vitellogenic stage were significantly higher than those at the perinucleolus and oil‐droplet stages. These dynamics of the reproductive hormones represented the physiological background of oogenesis in A. celebesensis that has remarkably well‐developed oocytes just before downstream migration.

Sulawesi Island of north-central Indonesia is located in a region where at least 6 species of tropical anguillid eels are present, but the reproductive ecology and biodiversity of these eels in each area of the Indonesian archipelago remains poorly understood. Some information about these species was obtained from collections of their leptocephalus larvae made during several times of the year and from year-round collections of their recruitment-stage glass eels at a few locations. A sampling survey of anguillid leptocephali was conducted in March 2010 in both the Celebes Sea and Tomini Bay of Sulawesi Island to learn about the biodiversity and reproductive ecology of the eels in the region. Twenty-eight anguillid leptocephali were collected at 13 different stations, with genetic identification indicating that 3 species of eels had spawned in the two areas. Larvae were more abundant in the Celebes Sea (N = 21; 16.0-52.1 mm TL) than in Tomini Bay (N = 7; 9.6-54.8 mm). The abundant 16-21 mm size-class of Anguilla bornensis in the Celebes Sea indicated that species had recently spawned there, and spawning had also occurred in Tomini Bay by A. celebesensis (17.4 mm). These data and previous life history information suggest that A. celebesensis may have two spawning seasons in the Celebes Sea, but only one main spawning season in Tomini Bay. Anguilla borneensis may spawn at several times of the year in the Celebes Sea. Anguilla marmorata and A. biocolor pacifica spawn outside the Indonesian Seas, with A. marmorata recruiting in large numbers in the Sulawesi Island region during much of the year. Other spawning locations of A. celebesensis and A. interioris likely exist in Indonesian waters. Therefore, further research is needed to understand the reproductive ecologies and biodiversity of the tropical anguillid eels in each region of Indonesia in relation to geographic and climatic factors.

Downstream‐migrating Anguilla celebesensis eels were predominant relative to Anguilla marmorata in October, November, December, January and February (75.9–92%), while no A. celebesensis occurred and A. marmorata were predominant in May and July (96–100%), at the outlet of Poso Lake, Sulawesi Island, Indonesia. Merging these results with those from published data suggests that most A. celebesensis start downstream migration during the early to middle rainy season, and A. marmorata migrate almost year‐round with a peak from the late rainy to middle dry season.

A total of 261 individuals of the four tropical eel species, Anguilla celebesensis, Anguilla marmorata, Anguilla bicolor pacifica and Anguilla interioris, were collected from 12 locations around Sulawesi Island, Indonesia, to gain knowledge about the riverine distribution of tropical eels. Anguilla marmorata was predominant in the lower reaches of Poso River (94·4% of total eel catch in the sampling area), Poso Lake (93·3%), three small inlet rivers of Tomini Bay (100%) and Laa River (92·3%). Anguilla celebesensis occurred frequently in the inlet rivers of Poso Lake (63·5%). Anguilla bicolor pacifica and Anguilla interioris were rare (1.5 and 0.4%, respectively). Otolith Sr:Ca ratio electron‐probe micro analysis (EPMA) for individual migratory histories revealed that 15 A. celebesensis caught in Poso Lake and its inlet rivers were categorized into 14 river eels (Sr:Ca < 2·5) showing upstream migration seemingly at their elver stage and only one sea eel (Sr:Ca ≥ 6·0) that stayed in the marine habitat for the majority of its life after recruiting to Sulawesi Island before its late upstream migration. In A. marmorata, 19 examined eels from Poso Lake and its inlet rivers were all river eels, while 17 eels from the lower reaches of Poso River were two river eels, six sea eels and nine estuarine eels (2·5 ≤ Sr:Ca < 6·0) that mostly lived in the brackish water. The sex ratio of A. celebesensis was highly skewed towards a dominance of females (99%). In A. marmorata, females were predominant in Poso Lake (95·2%), its inlet rivers (94·7%) and Laa River (100%), while males were more frequent in the lower reaches of Poso River (76·5%) and small inlet rivers of Tomini Bay (94·1%). These results indicate that the riverine distribution pattern of tropical eels differs among species and between sexes.

The age and growth of migrating tropical eels, Anguilla celebesensis and Anguilla marmorata from central Sulawesi, Indonesia, were examined. Migrating eels (63 A. celebesensis and 38 A. marmorata) were obtained from weirs near the Poso Lake outlet and non‐migrating eels (35 A. celebesensis and 119 A. marmorata) were captured by baited hooks, eel pots, scoop net and electro‐fishing in the Poso River system, Laa River system, Baluga River, Tongku River and Padapu River from February 2009 to October 2010. In both species, the proportion of eels with opaque otolith edges showed a single peak in July, suggesting that one annulus (a pair of translucent and opaque zones) was formed each year in their otoliths. Mean ± s.d. and range of total length (L_T) and age was 785·2 ± 114·9 (585–1083) mm and 7·5 ± 1·6 (5–11) years in migrating female A. celebesensis and 1132·2 ± 173·7 (800–1630) mm and 11·6 ± 3·3 (7–23) years in A. marmorata. The age of migrating female eels was negatively correlated with annual growth rate, 100·7 ± 17·2 (68·1–145·0) mm year⁻¹ in A. celebesensis and 97·9 ± 19·3 (66·6–131·6) mm year⁻¹ in A. marmorata, but there was no significant correlation between the L_T and annual growth rate in either species. The annual growth rates of these female tropical eels were typically higher than those of temperate anguillid species, suggesting a latitudinal cline in growth rate in the genus Anguilla reflecting the environmental conditions of their growth habitat.