Relationship between gonadosomatic index and spawning-capable status based on gonadal histology in the moonfish Mene maculata

In the vast ocean of fish diversity, some species stand out not just for their appearance, but for their solitary existence. The moonfish, Mene maculata, is the only living member of its entire family, a unique evolutionary lineage scattered across the warm waters of the Indo-West Pacific. For scientists studying these creatures, understanding how they reproduce is key to understanding their survival. A common tool used to track reproduction in fish is the gonadosomatic index, a simple measurement that compares the weight of a fish's reproductive organs to its total body weight. When this number rises, it usually signals that the fish is preparing to spawn. However, this number is just a snapshot of size; it does not show what is actually happening inside the cells. To truly know if a fish is ready to release eggs or sperm, researchers must look at the microscopic development of those cells, a process that reveals whether the fish is truly capable of spawning or merely carrying heavy organs.

For the moonfish, previous studies had relied on these weight-based measurements to guess when the fish were spawning, suggesting a peak season around late summer and early autumn. Some earlier observations even hinted that these fish might release all their eggs at once, a strategy known as total spawning. But without looking inside the ovaries and testes, these ideas remained unproven. A team of researchers set out to bridge this gap by examining the actual tissue of moonfish collected from the waters off Japan. They wanted to see if the weight of the reproductive organs truly matched the biological readiness of the fish, and to determine if the moonfish's unique evolutionary history had led to any unusual reproductive structures.

The scientists collected live moonfish and kept them in tanks at the Marine Science Museum in Fukushima, feeding them krill and shrimp to keep them healthy. Over time, some fish passed away naturally, while others were carefully euthanized for study. The team measured the length and weight of each fish, then carefully removed the reproductive organs. They preserved these tissues in chemical solutions, sliced them into incredibly thin sections, and stained them with dyes to make the cells visible under a microscope. This allowed them to see the exact stage of development for every egg or sperm cell inside, from the earliest, tiny beginnings to the fully mature cells ready for release.

When they looked at the male moonfish, they found a structure that was surprisingly ordinary for such a unique fish. The testes were organized into small lobes, and the stem cells that produce sperm were spread out freely throughout these lobes rather than being confined to one end. This pattern is common in many modern fish, suggesting that the moonfish has not evolved a special, unique way of making sperm. More importantly, the researchers found that the weight of the testes did not tell the whole story. They examined seven male fish and found that even those with very low reproductive organ weights still had fully mature sperm ready to be released. In fact, every single male they looked at, regardless of how heavy their testes were relative to their body, was biologically ready to spawn. This suggests that for male moonfish, a low weight measurement does not mean they are not ready to reproduce; they can hold onto mature sperm without needing their reproductive organs to swell significantly.

The story was different for the females. The researchers found that the weight of the ovaries did increase as the fish got larger, and they identified one female with a particularly high reproductive organ weight. Inside this fish, they saw a complex mix of eggs at every possible stage of development. There were tiny, immature eggs just starting to grow, medium-sized eggs filling with nutrients, and large, mature eggs ready to be released. This simultaneous presence of eggs at different stages is a hallmark of a "batch spawner," a fish that releases eggs in groups over a period of time rather than dumping everything at once. This finding directly challenges the earlier idea that the moonfish is a total spawner. The other female fish examined had mostly immature eggs, indicating they were not yet ready to spawn.

The study concludes that while the weight of the reproductive organs can be a useful clue, it is not a perfect indicator of readiness, especially for males. For the moonfish, a male can be fully capable of spawning even when his reproductive organs are small, while a female's readiness is tied to a specific, complex internal state where eggs of all sizes grow together. By looking at the actual cells rather than just the weight, the researchers have provided a clearer picture of how this unique fish reproduces. They found that the moonfish likely releases its eggs in batches throughout the season, a strategy that may help ensure the survival of its offspring in the open ocean. This detailed look inside the fish confirms that the moonfish, despite being the last of its kind, follows reproductive patterns shared by many of its distant relatives, blending the familiar with the unique.