Convergent evolution of albinism in cave animals is driven by shared mechanisms and adaptive advantages

Loss of pigmentation is a hallmark of cave-dwelling animals, yet the mechanisms and evolutionary forces driving this convergence remain unclear. Here, we investigate the cellular and biochemical basis of albinism across phylogenetically diverse cave species, including annelids, mollusks, and vertebrates. Across all taxa, melanin loss consistently results from disruption of the first step in the biosynthetic pathway. Despite this metabolic block, cells capable of melanin synthesis are retained showing distributions similar to pigmented surface relatives and contributing to innate immune responses during tissue repair. These conserved features raise the question of what evolutionary forces drive pigmentation loss. To test whether albinism reflects neutral drift or adaptive evolution, we use the model cavefish Astyanax mexicanus and find that melanin synthesis imposes a significant energetic cost, and that independently evolved albino populations show elevated dopamine levels. Thus, the convergent disruption of melanin biosynthesis may confer both direct benefits by energy savings under resource-limited cave conditions, and indirect advantages through a trade-off between melanin and catecholamine synthesis from their shared precursor, L-tyrosine. Our findings support an adaptive hypothesis for the repeated evolution of albinism in cave animals.