Analysis of distribution differences between juvenile and adult swordtails (Xiphophorus) in natural streams in the Sierra Madre Oriental of Mexico

Ontogenetic shifts in habitat use are common among freshwater fish and play a key role in reducing intraspecific competition and optimizing survival strategies across life stages. This study investigates microhabitat selection in juvenile and adult individuals of the genus Xiphophorus (specifically X. birchmanni, X. malinche and their hybrids) in natural streams of the Sierra Madre Oriental (Hidalgo, Messico). Underwater video recordings were collected across multiple localities (Acuapa, Aguazarca, Tula e Achiquihuixtla) and analyzed through a standardized quadrat-based method to quantify the density and proportional abundance of individuals in relation to water depth and additional environmental variables. Results show a clear positive correlation between depth and proportion of adult individuals in three of the four localities sampled, supporting the hypothesis that juveniles preferentially occupy shallow zones—likely due to reduced predation risk and improved camouflage—while adults dominate deeper microhabitats. An exception was observed in Achiquihuixtla, where shallow sites were unexpectedly dominated by adults. This pattern is best explained by local environmental characteristics: steep substrate morphology and abundant algal resources, rather than water current, which was shown to be negligible. The study also highlights the potential influence of invasive species on habitat occupancy, particularly in Tula, where total densities of Xiphophorus were markedly lower. Overall, the findings confirm the ontogenetic structuring of habitat use in Xiphophorus and underscore how local microhabitat features can mediate deviations from general ecological predictions.