Isotopic variability of δ13C and δ15N signals in cave systems: insights from the blind tetra Astyanax mexicanus

Stable isotope analysis allows the study of element cycles in ecosystems and trophic ecology. δ 13 C reflects the diversity of primary productivity, while δ 15 N is a good indicator of trophic levels of organisms. Caves have limited resources due to the absence of light, reducing the trophic chains in these ecosystems. These extreme conditions impose strong selection pressures on cave-dwelling organisms, known as troglobites, which exhibit specific adaptations such as vision and pigment loss, and metabolic and physiological differences with their surface counterparts. The species Astyanax mexicanus corresponds to a model organism in the study of regressive evolution, which presents two different ecotypes, a widely distributed surface morph, and a cave-dwelling morph present in at least 34 caves in three karst regions of San Luis Potosí and Tamaulipas, Mexico. In the present study, we characterized the δ 13 C and δ 15 N of seven cave populations of A. mexicanus , corresponding to two karst regions: Sierra de El Abra and Sierra de Guatemala, representing distinct genetic cavefish lineages. We also developed a Nutrient Input Index (NI), to assess whether cave geomorphology influences resource availability. We found isotopic differences between caves and regions analyzed. Caves in the Sierra de El Abra showed higher δ 15 N values and wider trophic niche ranges compared with those in the Sierra de Guatemala, reflecting a more complex trophic network tentatively associated also with its geological history. In addition, a relationship was observed between the proximity of pools to the surface and the δ 13 C values, which could suggest differences in NI directly associated with cave geomorphology, impacting selective forces across the different cave systems.