Population structure and reproductive traits of Octospiniferoides incognita (Acanthocephala: Neoechinorhynchidae) in Rhytiodus microlepis (Characiformes: Anostomidae) across seasonal water level fluctuations in a Neotropical Lake.

Abstract

This study investigated the population dynamics and reproductive biology of the acanthocephalan Octospiniferoides incognita in the intestine of the herbivorous fish Rhytiodus microlepis throughout the seasonal water level fluctuations of a floodplain lake in Amazonas State, Brazil. Between July 2022 and November 2023, hosts were sampled during the rising water, high water, receding water, and low water phases. The results show that the water level fluctuations significantly influence infection levels. Prevalence was 73% during rising water, reached 100% during high water, and remained high during receding water (89%), whereas no parasites were recorded during low water. Abundance and mean intensity were higher during high water and receding water. The population of O. incognita showed sexual dimorphism, with females attaining larger body sizes than males, and both sexes occurring preferentially in the middle portion of the intestine. Three reproductive morphotypes were recognized for each sex. Copulatory activity began during rising water, whereas the highest egg production was concentrated during high water and early receding water. Size at first sexual maturity (L50) was estimated at 0.12 cm, indicating an early maturation strategy. These findings clearly show that the infection dynamics and reproductive processes of O. incognita are closely synchronized with the flood pulse, which regulates host availability and promotes the success of trophic transmission in the Amazonian floodplain ecosystem.