Ultrasonographic predictors of response of European eels (Anguilla anguilla) to hormonal treatment for induction of ovarian development

Abstract

OBJECTIVE To examine ultrasonographic predictors of ovarian development in European eels (Anguilla anguilla) undergoing hormonal treatment for assisted reproduction. ANIMALS 83 female European eels. PROCEDURES Eels received weekly IM injections of salmon pituitary extract (first injection = week 1). Ultrasonography of the ovaries was performed twice during hormonal treatment (weeks 7 and 11). Eels were identified on the basis of body weight as having an adequate response by weeks 14 to 20 or an inadequate response after injections for 21 weeks. Eels were euthanized at the end of the experiment and classified by use of ovarian histologic examination. Ovarian cross-sectional area and size of eel (ie, length (3) ) were used to classify eels (fast responder, slow responder, or nonresponder) and to calculate an ultrasonographic-derived gonadosomatic index. Gray-level co-occurrence matrices were calculated from ovarian images, and 22 texture features were calculated from these matrices. RESULTS The ultrasonographic-derived gonadosomatic index differed significantly between fast responders and slow responders or nonresponders at both weeks 7 and 11. Principal component analysis revealed a pattern of separation between the groups, and partial least squares discriminant analysis revealed signals in the ovarian texture that discriminated females that responded to treatment from those that did not. CONCLUSIONS AND CLINICAL RELEVANCE Ovarian texture information in addition to morphometric variables can enhance ultrasonographic applications for assisted reproduction of eels and potentially other fish species. This was a novel, nonlethal method for classifying reproductive response of eels and the first objective texture analysis performed on ultrasonographic images of the gonads of fish.

Molecular Ontogeny of First-Feeding European Eel Larvae

“…Since then, extensive scientific inquiry has moved the field from individual efforts of reproductive failure toward a stable production of Japanese eel offspring ( Tanaka et al, 2001 ). Advances in Japanese eel culture have formed the baseline for eel research, leading to improved assisted reproduction protocols for European eel ( Pedersen, 2004 ; Palstra et al, 2005 ; Müller et al, 2016 ). However, establishment of culture technology throughout the larval stage until metamorphosis is still challenged by lack of insights on the “critical" early life history stages and dietary requirements for the unique pre-leptocephalus larvae.…”

Temperature induced variation in gene expression of thyroid hormone receptors and deiodinases of European eel (Anguilla anguilla) larvae

“…The earliest life stages of European eel have not been encountered in nature, thus research needs to focus on laboratory studies to overcome current bottlenecks and gain knowledge about the species physiology and ecology. With recent advances in assisted reproduction (Butts et al, 2014;Müller et al, 2016;da Silva et al, 2016) and larval culture of European eel (Politis et al, 2014a;Butts et al, 2016) this is now possible and warrants further investigation. In addition, the European eel genome has recently been sequenced (Henkel et al, 2012), which offers new and emerging perspectives for fundamental molecular studies in eel biology (Rozenfeld et al, 2016).…”

Salinity reduction benefits European eel larvae: Insights at the morphological and molecular level

“…To induce vitellogenesis females received weekly injections of salmon pituitary extract at 18.75 mg kg −1 body weight (Argent Chemical Laboratories, Washington, USA) [ 10 , 32 ]. To stimulate follicular maturation and induce ovulation, females received an additional injection of 17α,20ß-dihydroxy-4-pregnen-3-one (Sigma-Aldrich, St. Louis, MO, USA) at 2.0 mg kg −1 body weight [ 33 ].…”

“…After extensive efforts, breeding protocols using assisted reproductive technologies were developed for the Japanese eel, leading to the first production of eel leptocephali larvae in captivity [ 6 ]. The Japanese eel achievements formed the basis for eel research, which led to the development of artificial reproductive protocols of the American [ 7 ] and European eel [ 8 – 10 ]. Subsequent work sought to identify optimal environmental rearing conditions throughout the ELH of eels, such as temperature [ 11 – 13 ], light [ 14 ], and salinity [ 15 – 16 ].…”