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

The closed Poecilia sphenops lineage examined in this study showed progressive phenotypic variation across approximately nine to ten generations maintained under stable aquarium conditions. Two chromatic lines, black and orange mottled, remained stable throughout the lineage, while recurrent dwarf morphotypes were identified mainly in recent male cohorts. These forms included reduced somatic mass, peduncle elongation and occasional structural abnormalities. Reproductive records collected between 2021 and 2025 suggested a recurrent seasonal spawning pattern dominated by December breeding events, with less frequent activity observed in April and June. Although no genetic analyses were performed, the observed patterns may be compatible with phenotypic divergence within a small isolated population maintained under long-term environmental stability. These observations suggest that confined P. sphenops populations may provide useful systems for documenting divergent morphotypes across successive generations.