Oocyte and egg quality indicators in European eel: Lipid droplet coalescence and fatty acid composition

Abstract

During European eel assisted reproduction, timely administration of hormones that induce oocyte maturation and ovulation is a major factor influencing subsequent egg quality. This treatment commonly comprises one injection of fish pituitary extract (PE) as a primer followed by a 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) injection. In this context, the present study aimed at optimizing timing of the dual hormone administration by applying a lipid droplet-based oocyte maturation scale, previously developed for Japanese eel to determine the maturational status of each female. Using wild-caught female eels, the potential effect of female size, egg fatty acid composition and dry weight on egg quality was also analyzed. Larval survival at 3 days post hatch was used to differentiate High-and Low-quality egg batches. Results showed that lipid droplet diameter was significantly smaller in High-quality eggs than in Low-quality egg batches, indicating that females producing High-quality eggs received the PE primer and DHP generally at an earlier developmental stage than those producing Low-quality batches. These results confirm that oocyte lipid droplet diameter is a useful indicator of female maturational status for optimization of induction of oocyte maturation and ovulation in European eel. Additional parameters, including female size, egg fatty acid composition and dry weight, were similar between high and low quality egg batches. This insight regarding the fatty acid composition of eggs obtained from wild-caught female eels may help advancing the development of tailored diets for increased reproductive success of farmed broodstock.

Differential impacts of carp and salmon pituitary extracts on induced oogenesis, egg quality, molecular ontogeny and embryonic developmental competence in European eel

“…It is notable that SPE is derived from Pacific salmon ( Oncorhynchus keta ), a semelparous species with synchronous oocyte development, while CPE is derived from common carp, an iteroparous species with asynchronous oocyte development. In the case of anguillid eels, evidence is accumulating that they are batch spawners with asynchronous or group synchronous oocyte development [ 41 , 60 , 81 ]. In Japanese eel, sequential expression of FSH and LH at the brain pituitary level has been documented throughout induced ovarian development [ 82 , 83 ] and a similar pattern was also found in European eel with increasing levels of LH and decreasing FSH levels throughout experimental maturation [ 84 ].…”

“…Salmon and carp pituitary extracts were obtained from Argent Chemical Laboratories, Washington, USA, diluted in NaCl 0.9 g/L, ground, and centrifuged at 3600 RPM for 20 minutes, following [52,59], and supernatants stored at -20˚C until use. Dependent on body-weight increase and oocyte developmental stage, monitored by biopsies, an additional injection of the respective hormone was given to each female as a primer [49,60]. After 12-24 hours the female received an injection of 17α,20ß-dihydroxy-4-pregnen-3-one (DHP) (Sigma-Aldrich, St. Louis, MO, USA) at 2 mg kg -1 current BW to induce follicular maturation and ovulation [59].…”

Changes in Cadmium Concentration in Muscles, Ovaries, and Eggs of Silver European Eel (Anguilla anguilla) during Maturation under Controlled Conditions

“…Developing methods for artificial reproduction [ 23 , 24 , 25 , 26 ] and regular stocking [ 27 ] is among the ways of preserving the European eel in the natural environment. As the maturation process in the eel is much longer (e.g., [ 26 , 28 ]) than in other fish species (e.g., [ 29 , 30 ]), lasting up to several dozen weeks in eel [ 31 , 32 , 33 ], it requires comprehensive studies [ 26 , 32 , 33 , 34 , 35 ]. This can be attributed to the fact that the eels’ environment changes from freshwater to saltwater during their spawning journey [ 31 ].…”

What Goes Wrong during Early Development of Artificially Reproduced European Eel Anguilla anguilla? Clues from the Larval Transcriptome and Gene Expression Patterns

“…European male eels were first matured by injection of urine from pregnant women ( [1]; containing human chorionic gonadotropin-hCG), females by hypophysation (i.e., weekly injection of pituitary extracts) in the 1960s [2], and eggs were first fertilized in 1980 [3], after which the first larvae were produced in the early 1980s [4]. Although several groups can now produce larvae of European eel on a regular basis [5][6][7][8], massive mortality often occurs [9,10], particularly during the first week after hatching. Survival rates during the first week vary widely from 0 to 90% in European eels [10].…”