An improved and simplified terminology for reproductive classification in fishes

Abstract

As the number of fish reproductive studies has proliferated, so has the number of gonadal classification schemes and terms. This has made it difficult for managers and scientists to communicate and for comparisons to be made between studies. We propose the adoption of a simple, universal terminology for the phases in the reproductive cycle that can be used with all male and female elasmobranch and teleost fishes. These phases were chosen because they define key milestones in the reproductive cycle representing critical parameters such as size at maturity, duration of spawning season, location and diel periodicity of spawning, and fecundity. The phases we propose include: Immature, Developing, Spawning Capable, Actively Spawning, Regressing and Regenerating. Although the histological criteria identifying each phase may vary for different species and phases may not always occur sequentially, each phase is conceptually universal. The Immature phase can only occur once. The Developing phase signals entry into the gonadotropindependent stage of oogenesis and spermatogenesis and gonadal growth. The Spawning Capable phase indicates fish that will spawn this season because development within ovaries (fully grown vitellogenic oocytes) or testes (spermatozoa in lumens/ducts) is sufficiently advanced. Actively Spawning females are those that show recent evidence of spawning (i.e., hydrated or ovulated oocytes). Females of many species cycle between the Spawning Capable and Actively Spawning phases during the reproductive season and these phases are necessary to determine fecundity, spawning frequency, location and diel periodicity. Spawning Capable and Actively Spawning phases are difficult to differentiate histologically in males. The Regressing phase indicates fish that are completing the spawning season. Fish in the Regenerating phase are sexually mature but reproductively inactive. We show how researchers can incorporate species-specific histological criteria or classes within each of the universal phases, allowing more specific divisions yet preserving the overall reproductive terminology for comparative purposes.