Trophic Ecology of Poecilia velifera (Regan, 1914) and Gambusia yucatana (Regan, 1914) in an Anchialine Environment in Cozumel Island, Mexico

“El Aerolito” is the most biodiverse cenote cave globally, home to Poecilia velifera and Gambusia yucatana. However, their trophic niches and interactions remain undescribed. A total of 53 P. velifera and 38 G. yucatana were analyzed for stable isotopes, along with water samples for chlorophyll, silicates, and physical–chemical parameters. Data were collected during the norths, rainy, and dry seasons in the year 2023. Isotopic niches were calculated, and a PCA analysis was used to assess environmental factors. P. velifera showed δ13C values of −25.4‰, −25.7‰, and −24.5‰, whereas G. yucatana exhibited −24.8‰, −24.0‰, and −24.2‰ across seasons. Niche size varied, with the largest during the norths and smallest during the rainy season, indicating seasonality. PCA linked environmental parameters (e.g., temperature and dissolved oxygen) to resource availability, suggesting niche shifts due to rainfall. Mangroves and insects contributed to the isotopic signals, with shifts observed during the dry and rainy seasons. The highest water parameter values occurred during the dry and rainy seasons, while the lowest was during the norths. The system, nutrient-poor and lacking chlorophyll production, relies on external sources. The study concludes that isotopic niches are dynamic and seasonally influenced by abiotic factors, especially rainfall.