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How do fish ovaries differ in shape and internal structure?

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Detailed cross-sectional illustration of a human ovary showing the stages of follicle development, ovulation, and corpus luteum formation, with labeled structures including primordial, primary, secondary, and mature follicles, a ruptured follicle releasing an ovum, and corpus luteum and albicans. An inset at the bottom shows the ovary's location in the female reproductive system. — ovary The steps of ovulation, beginning with a dormant primordial follicle that grows and matures and is eventually released from the ovary into the fallopian tube.

In most fish, ovaries are elongated structures that extend nearly the length of the body cavity. While many fish possess paired ovaries, some species have only one, such as certain teleosts and viviparous elasmobranchs.12

Regarding their internal structure, fish ovaries are categorized based on the presence or absence of cavities:

Saccular or Lacunate Ovaries: Many teleosts, particularly viviparous ones, possess a permanent ovarian cavity formed during development by an invagination of the surface that traps a portion of the coelom. This cavity is lined with germinal epithelium and features ovigerous folds that increase the surface area for egg proliferation. In other teleosts, a temporary cavity may form after ovulation as the cortex shrinks, which is later obliterated as new eggs enlarge.1
Compact Ovaries: Ovaries that lack these internal cavities are characterized as compact.1

These cavities are continuous with the oviduct, allowing eggs to be shed directly into them. In contrast to mammals, which have compact ovaries without medullary cavities, the structural variations in fish are closely tied to their reproductive strategies and the development of the ovarian cortex and medulla.1