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The Astyanax genus represents an extraordinary example of phenotypic evolution, being their most extreme examples the blind and depigmented morphs, which have evolved from independent surface-dwelling lineages. Among cave organisms, Astyanax cavefish is a prominent model system to study regressive evolution. Before this study, 34 cave populations were known for the Astyanax genus to be inhabited by the cave morph. The majority of those cave populations are distributed in Northeast México, at the Sierra Madre Oriental (32 cavefish), in three main areas: Sierra de Guatemala, Sierra de El Abra, and Micos, and two in the Balsas basin in the state of Guerrero, Mexico. In the present study, we describe a new cave population found 4.5 km Southward of Pachón cave, the most northern cave population known for the Sierra de El Abra limestone. El Refugio cave is a resurgence with a mixed population of fish with different levels of troglomorphism, and surface fish, resembling other hybrid populations within the Sierra de El Abra. Based on a mitochondrial DNA characterization of the 16S ribosomal DNA sequence, we could identify the mitochondrial lineage of this population, which was placed closely related to the “New Lineage”, sharing haplotypes with the surface (i.e. Arroyo Lagartos) and Pachón populations, instead of with the cave populations from Central Sierra de El Abra (e.g. Tinaja cave). El Refugio cave population gives additional evidence of the intricate history of this system, where migration, drift, and selection have shaped the evolution of the cave morphs through the independent episodes of the Astyanax mexicanus history.

The karstic cave, la Cueva de Los Sabinos, located in the Sierra de El Abra in the state of San Luis Potosí, Mexico, is mostly known for hosting a population of blind, depigmented Astyanax mexicanus cavefish. Herein, we report the discovery of a non-pigmented sponge (Porifera) in the final sump of this cave. No genus or species name could be attributed because we did not collect any specimen. Up to now, the sponge distribution seems restricted to a single pool in la Cueva de Los Sabinos, but further careful exploration of other pools of the cave as well as closely related cavities is warranted. To our knowledge, this observation constitutes the fourth report of a freshwater, white, cave-adapted sponge in the world and the first for Mexico and North America. It is also the eleventh troglobite species encountered in Los Sabinos. Our discovery confirms the exceptionally rich biodiversity of this cave ecosystem.