Subterranean Fishes of the World

6887

2025

bioRxiv preprint

bioRxiv preprint, this version posted February 8, 2025. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without perm

The Mexican tetra (Astyanax mexicanus) has emerged as a leading model for evolutionary biology and the study of adaptation to extreme subterranean environments. The river-dwelling morphs of the species are distributed in Mexico and Texas, while the blind and cave-adapted morphs inhabit the karstic caves of the Sierra Madre Oriental in northeastern Mexico. Although the molecular, cellular and genetic underpinnings of Astyanax cavefish evolution are being elucidated, our understanding of its habitat and environment remains limited, thereby constraining the interpretations of its morphological, physiological, and behavioral adaptations. In particular, the physico-chemical parameters of the water and the hydrological regimes to which cavefish are subjected remain largely unexplored. From 2009 to 2024, we have recorded the physico-chemical parameters of the water in which Astyanax mexicanus cavefish and surface fish are found in the El Abra and La Colmena regions of the states of San Luis Potosí and Tamaulipas, Mexico. Data were collected using a variety of devices and probes, including both point and longitudinal measurements of water quality. A total of 13 caves out of the 33 known Astyanax caves and 27 surface stations (rivers, springs, ponds) were sampled. The comparison of epigean and hypogean waters showed strong signatures of the two macro-habitats. As compared to surface, cave water was fresher, much less conductive and highly anoxic. Moreover, at the meso-habitat level, a comparison between caves revealed significant differences in both specific water parameters and hydrological regimes. One- or two-year longitudinal recordings demonstrated that some caves exhibited relatively stable hydrological regimes, while others experienced multiple, sudden and significant fluctuations. Finally, distinct pools within a single cave showed notable differences, displaying a highly reproducible increase in water temperature as a function of depth or distance from the cave entrance, and revealing specificities at the micro-habitat level. Our comprehensive dataset on cave water quality and hydrodynamics is interpreted in the context of an integrated view of cave biology and the evolution of cave organisms. Our findings have significant implications for understanding the evolution of cavefishes and the biology of the species in general

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