Phenotypic Divergence and Dwarf Morphotypes in a Closed Poecilia sphenops Lineage.

The closed Poecilia sphenops lineage examined in this study shows progressive phenotypic divergence across approximately nine to ten generations under stable aquarium conditions. The population originated from a reduced founder group and currently consists of nineteen individuals distributed across generations G8–G10. Two chromatic lines, black and orange mottled, remained stable throughout the lineage, while the orange phenotype developed progressively stronger mottling patterns in recent generations. Male individuals displayed marked variation in body size and morphology, including three recurrent dwarf morphotypes classified as mild, intermediate and extreme. These forms were concentrated in the most recent cohorts and included traits such as peduncle elongation, reduced somatic mass and occasional structural or functional abnormalities. In contrast, females maintained consistently higher viability and showed comparatively limited morphological variation beyond colouration. Reproductive records collected between 2021 and 2025 revealed a stable seasonal pattern dominated by December spawning events, with occasional activity in April and June. Although no genetic analyses were performed, the observed patterns are consistent with phenotypic divergence occurring in a small, isolated population maintained under long-term environmental stability. Together, these observations suggest that confined P. sphenops populations may provide useful systems for documenting the emergence and persistence of divergent morphotypes across successive generations.