Reproductive physiological characteristics of tropical Celebes eels Anguilla celebesensis in relation to downstream migration and ovarian development

Abstract

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.

Morphological, ecological and physiological characteristics of downstream‐migrating and non‐migrating Pacific bicolor eels Anguilla bicolorpacifica

“…Ovarian tissue was fixed in 4% paraformaldehyde and histologically observed, and the mean diameter of the largest group of ovarian follicles was measured and calculated, as described in Hagihara et al . (2020). The transcript levels of gonadotropin β‐subunit genes ( fshb and lhb ) in the pituitary were measured as described in Hagihara et al .…”

“…The age of the eels was estimated by counting the annual rings of sagittal otoliths, the annual growth rate (mm year −1 ) of each individual was estimated according to Hagihara et al (2018c) and the annual ring formation of otoliths was validated in congeneric species distributed in this area (Hagihara et al, 2018c). To determine whether the eels had lived in the sea, estuary or freshwater habitats, sagittal otoliths were used for quantitative analysis of Sr and Ca concentrations using a wavelengthdispersive X-ray electron probe micro-analyser JXA-8230 (JEOL, Tokyo, Japan), and the mean values of otolith Sr:Ca ratios from the elver mark to the otolith edge were calculated as molar ratios (mmol mol −1 ), as described in Tsukamoto and Arai (2001) tissue was fixed in 4% paraformaldehyde and histologically observed, and the mean diameter of the largest group of ovarian follicles was measured and calculated, as described in Hagihara et al (2020). The transcript levels of gonadotropin β-subunit genes (fshb and lhb) in the pituitary were measured as described in Hagihara et al (2020), and the concentrations of sex steroids , testosterone (T) and 17β-oestradiol (E 2 )] in plasma were measured according to the methods of Yamada et al (1997).…”

“…To determine whether the eels had lived in the sea, estuary or freshwater habitats, sagittal otoliths were used for quantitative analysis of Sr and Ca concentrations using a wavelengthdispersive X-ray electron probe micro-analyser JXA-8230 (JEOL, Tokyo, Japan), and the mean values of otolith Sr:Ca ratios from the elver mark to the otolith edge were calculated as molar ratios (mmol mol −1 ), as described in Tsukamoto and Arai (2001) tissue was fixed in 4% paraformaldehyde and histologically observed, and the mean diameter of the largest group of ovarian follicles was measured and calculated, as described in Hagihara et al (2020). The transcript levels of gonadotropin β-subunit genes (fshb and lhb) in the pituitary were measured as described in Hagihara et al (2020), and the concentrations of sex steroids , testosterone (T) and 17β-oestradiol (E 2 )] in plasma were measured according to the methods of Yamada et al (1997). Fecundity of the two migrating eels was estimated according to the procedure modified from Barbin and McCleave (1997) Gaimard 1824 (97.9 ± 19.3), and these tropical eels appear to grow faster than their congeners in temperate areas (Hagihara et al, 2018c).…”

Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798

“…Such a remarkably short-distance migration for spawning to Tomini Bay close to the river mouth of the Poso River can stimulate the final maturation preparations in the lake in a short time to reach the spawning site ( Arai, 2014a ). Although the oogenesis of A. celebesensis is remarkably advanced at the timing of initiation of downstream migration ( Arai, 2014a ; Hagihara et al., 2020 ), they do not spawn immediately and require more time for spawning ( Hagihara et al., 2020 ). A. celebesensis is thought to need a few months for spawning after initiation of their downstream migration based on the oocyte developmental stage and the downstream migration season, although the timing of downstream migration was not examined monthly throughout the year ( Hagihara et al., 2012 , Hagihara et al., 2020 ).…”

“…Although the oogenesis of A. celebesensis is remarkably advanced at the timing of initiation of downstream migration ( Arai, 2014a ; Hagihara et al., 2020 ), they do not spawn immediately and require more time for spawning ( Hagihara et al., 2020 ). A. celebesensis is thought to need a few months for spawning after initiation of their downstream migration based on the oocyte developmental stage and the downstream migration season, although the timing of downstream migration was not examined monthly throughout the year ( Hagihara et al., 2012 , Hagihara et al., 2020 ). A maturing A. rostrata was completely tracked for 2400 km during its spawning migration between the Canadian east coast (Scotian Shelf) and the northern limit of the spawning site in the Sargasso Sea by means of PSATs.…”