Abstract
Simple SummaryThe chorion (also called egg envelope) is the primary envelope that protects the fish embryo against drying, mechanical actions, and abrupt changes in the water conditions. Alterations of the chorion during the embryo incubation are not unusual, but these are scarcely reported. Increased occurrence of chorion alterations can lead to decreased reproductive performance and important losses for fish farms. Here, we described several chorion alterations observed in samples of embryonated eggs collected from different salmon and trout farms located in southern Chile over a period of 14 years. We detected four types of chorion alterations and found soft chorion as the most prevalent alteration in the years analyzed, affecting mainly Atlantic and Coho Salmon. Eggs of Rainbow Trout displayed fewer chorion alterations among the three species analyzed. As the eggs analyzed here were produced under standard industrial conditions, we conclude that these alterations are possibly linked to changes in water conditions, which need to be further investigated.The chorion is the primary envelop that protects the fish embryo against mechanical actions, pathogens, and abrupt changes in physical and chemicals conditions of the incubation medium. During embryo development, chorion alterations are not rare, but the occurrence of these is scarcely reported. Increased frequency of chorion alterations can result in increased embryo mortality and thus decreased reproductive performance and losses for fish farms. In this study, we characterize different chorion alterations observed in samples collected over 14 years from 12 salmon and trout farms located in the region of La Araucanía in southern Chile, which sent live eyed-stage embryos (‘eyed-eggs’) for quality analysis to our laboratory. We found soft chorion as the most common alteration observed, being present in the whole 14-year series analyzed in Atlantic Salmon (Salmo salar) and affecting up to 35.0% of the samples examined in a year. This alteration also affected up to 20.0 and 5.7% of Coho Salmon (Oncorhynchus kisutch) and Rainbow Trout (Oncorhynchus mykiss) samples analyzed in a year, respectively. We also found an increase of other chorion alterations, including perforated and white-spotted chorion in Atlantic and Coho Salmon, in the last 8 years. Among the three species, Rainbow Trout exhibited fewer chorion alterations. As the embryonated eggs analyzed here were obtained from broodstocks maintained under standard industrial conditions, these alterations might be linked to changes in environmental conditions affecting the incubation water that need to be further investigated.
Highlights
The chorion is the primary envelope of the embryo, which is formed by proteins and glycoproteins synthesized in the fish liver, and by the oocyte itself in some fish species [1,2]
The mean ± SEM diameter measured in Atlantic Salmon, Coho Salmon, and Rainbow Trout was 5.9 ± 0.05 mm, 6.4 ± 0.04 mm, and 5.1 ± 0.03 mm, respectively
The different chorion alterations found exhibited similar characteristic in the three species and based on the appearance of the eyed-eggs can be described as follows: (i) soft chorion, which exhibited a soft texture and low resistance to the pressure that resulted in chorion invagination (Figure 2A); (ii) whitespotted chorion exhibited white spots on the surface of different color intensity and size, these spots remained on the chorion after removal without affecting the embryo itself (Figure 2B); (iii) dark chorion displayed an opaque appearance and embryos could not be visualized, but following chorion removal, the embryos looked normal in appearance
Summary
The chorion ( called egg envelope) is the primary envelope of the embryo, which is formed by proteins and glycoproteins synthesized in the fish liver, and by the oocyte itself in some fish species [1,2]. The chorion is organized in three laminae: the outer laminae are homogenous, while the inner lamina has alternating light and dark bands with a soft appearance [6]. The thickness of the chorion varies in different salmonid species ranging from 30 to 60 μm [7,8]; the outermost lamina (externus) exhibits a thickness of 0.3 μm, the lamina (internus) beneath the outer ranges between 25 to 50 μm, and the inner lamina (subinternus) exhibits a thickness ranging between 3 to 8 μm [7]. The main function of the chorion is the protection of the embryo against mechanical actions, drying, and abrupt changes in the physicochemical conditions of the milieu, but it is involved in nutrients uptake, fertilization, and control of pathogen agents [2,9]
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