Structural Dynamics of Hydrated Oocytes in Captive Black Seabream, Acanthopagrus berda: Insights from Oocyte Morphology

Abstract

This study investigates the morphology, ultrastructure, and degeneration patterns of hydrated oocytes in captive broodstock of Acanthopagrus berda during the breeding seasons from 2022 to 2024. Ovarian biopsy samples consistently revealed the presence of
hydrated oocytes, identified as large, transparent, spherical cells that progressively degenerated when not spawned. Light microscopy and scanning electron microscopy (SEM) provided detailed insights into oocyte developmental stages, from early vitellogenic oocytes to hydrated oocytes (300-700 μm), with an emphasis on the degeneration of hydrated oocytes in female broodstock and hormone- induced females that failed to spawn. Notable surface changes included pore canal opening patterns and zona radiata folding, indicative of hydration-related structural modifications. Hydrated oocytes exhibited a smoother zona radiata with widely spaced pore openings compared to the densely pitted surface of mature oocytes. Progressive degeneration was characterised by membrane folding, shrinkage, rupture of the zona radiata, and eventual disintegration into irregular cellular debris. Similar patterns were confirmed in hormone induced females at an inshore hatchery, where hydrated oocytes retained within the ovarian lumen failed to ovulate and underwent similar degeneration. The degeneration process was marked by cytoplasmic collapse, persistent nuclei in early stages, and complete structural breakdown in advanced stages. The study underscores the importance of optimising hormonal treatment protocols and environmental conditions to prevent oocyte retention and to improve reproductive outcomes in marine fish broodstock management.