Abstract
Sebastes schlegeliiis a commercially important fish with a special viviparous reproductive system that is cultured in near-shore seawater net cages in East Asia. In the gonadal development of the species, the gonad of males mature before those of females, which mature after mating. Mating in male/female fishes occurs in October of each year. Then, females undergoing oocyte maturation complete fertilization using stored sperm in March of the following year. The pregnancy is completed when larvae are produced in the ovary. It has been reported that embryonic nutrient supply originates entirely from the female viviparous reproductive systems. However, until now, the nutritional patterns and the processes of nutrient provision inS. schlegeliibefore parturition have not been clear. The goal of this research was to study the embryos, larvae and juveniles ofS. schlegeliiduring pregnancy. Anatomical observations, light microscopy and scanning electron microscopy were used to study the developmental characteristics of early embryos and larvae and the connecting structures between the mother and the fetus. The results showed the following: (1) Placental-like structures were found during the process of embryonic development inS. schlegelii, and these placental-like structures proliferated after fertilization. (2) The embryos ofS. schlegeliiwere encased by a saclike structure composed of blood vessels, connective tissue, and surface epithelial cells. The vessels near the embryo existed in the thecal layer. Vascularized proliferation was detected following embryonic development. (3) Starting in the gastrula stage, connections between the embryo and surrounding cells loosened, and ovarian fluid became abundant. In addition, a large number of small holes and cristae were observed on the surface of the embryo. We speculate that embryos may be able to absorb nutrients from the ovarian fluid. (4) Yolk was present throughout embryo development. (5) Two types of nutritional modes were observed, lecithotrophic and matrotrophic during embryonic development. Three forms of placental analogs may exist inS. schlegelii: (1) external epithelial absorptive surfaces; (2) trophonemata, with modifications of the ovarian epithelia for absorbing the histotroph; and (3) a follicular pseudoplacenta, with close apposition between follicle cells and embryonic absorptive epithelia.
Highlights
Embryo development requires nutrients, and sufficient nutrient supply is the main factor for normal embryonic development
We speculated that the existence of these structures is closely related to the nutrient supply during the process of embryonic growth and development, and these closely connected structures are likely to be the structural basis of maternal nutrient transfer
Placental-like structures are present during S. schlegelii embryonic development
Summary
Sufficient nutrient supply is the main factor for normal embryonic development. In viviparous species, nutrition becomes more complex because embryos develop within the female body; different sources of nutrients might be available from the pregnant female. The extant fishes that bear living young include chondrichthyans (sharks and rays) (40 families, 99 genera, and 420 species), Latimeria (1 family), and teleosts (13 families, 122 genera, 510 species) (Wourms, 1981). Different species have their own complex and specialized maternal-fetal related structures, which complicates the study of nutrient supply. There are only two sources of nutrients for viviparous fish: the embryonic yolk (lecithotrophy) and the pregnant female (matrotrophy)
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