The ability of fishes to adapt to any aquatic environment seems limitless. It is enthralling how new species keep appearing at the deep sea or in subterranean environments. There are close to 230 known species of cavefishes, still today the best-known cavefish is Astyanax mexicanus, a Characid that has become a model organism, and has been studied and scrutinized since 1936. There are two morphotypes for A. mexicanus, a surface fish and a cavefish. The surface fish lives in central and northeastern Mexico and south of the United States, while the cavefish is endemic to the "Sierra del Abra-Tanchipa region" in northeast Mexico. The extensive genetic and genomic analysis depicts a complex origin for Astyanax cavefish, with multiple cave invasions and persistent gene flow among cave populations. The surface founder population prevails in the same region where the caves are. In this review, we focus on both morphotype's main morphological and physiological differences, but mainly in recent discoveries about behavioral and metabolic adaptations for subterranean life. These traits may not be as obvious as the troglomorphic characteristics, but are key to understand how Astyanax cavefish thrives in this environment of perpetual darkness.

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MCTPs (Multiple C2 domain proteins with two transmembrane regions) are evolutionarily and structurally related to other C2 proteins which play fundamental roles in exocytosis and membrane trafficking, however their specific role has been little studied. This work points out possible functional implications of MCTPs by comparing their primary amino acid sequence and functional domains. MCTP amino acid sequences were identified in non-chordates and chordates. The primary sequences grouped in three classes: MCTP, MCTP-1 and MCTP-2. MCTP is present only in non-chordates, while MCTP-1 and MCTP-2 are present in chordates. MCTP genes emerged early in metazoan evolution and are well conserved across species including humans. Genomic analysis of diverse species of representative phyla showed that the three C2 domains (C2A-C2C) and transmembrane regions (TMR) are well conserved. The C2 domains have eight β strands as well as aspartate residues known to bind calcium. Interestingly, we identified a lysine-rich cluster, also known as polybasic cluster in C2A and C2B, which is known to bind lipids in other proteins. We also describe the phylogenetic distribution of MCTPs and analyze conserved domains and their predicted secondary structure in metazoans. We highlight important motifs that have not been previously described in MCTPs C2A and C2B domains that suggest MCTPs potentially bind phospholipids. Our observations show MCTPs are proteins widely distributed in eukaryotic organisms and may play an important role in membrane fusion or exocytosis.