Mechanisms of pigmentation loss in subterranean fishes

Reproduction, development, asymmetry and late eye regression in the Brazilian cave catfish Ituglanis passensis (Siluriformes, Trichomycteridae): evidence contributing to the neutral mutation theory

The troglobitic (exclusively subterranean source population) catfish Ituglanis passensis (Siluriformes, Trichomycteridae) is endemic to the Passa Três Cave, São Domingos karst area, Rio Tocantins basin, Central Brazil. This unique population presents variably reduced eyes and melanic pigmentation. We describe reproduction and early development in this species based on a spontaneous (non-induced) reproductive-event that occurred in the laboratory in January–February, 2009, while simultaneously comparing with data from the cave-habitat and a previous reproductive event. Egg laying was parceled. Egg-size and number were within variations observed in epigean congeners. Larvae behavior and growth is described. A single surviving specimen was monitored over two years. Eye-regression started late, one year after birth, and followed a pattern of stasis phases intercalated with slow growth and fluctuating asymmetric rates. Late eye regression, associated with asymmetry in eye development and intra-population variability of troglomorphic traits, as shown by several Brazilian subterranean fishes, provide support for the Neutral Mutation Theory.

Regression of dark color in subterranean fishes involves multiple mechanisms: response to hormones and neurotransmitters

ABSTRACT Organisms with source-populations restricted to the subterranean biotope (troglobites) are excellent models for comparative evolutionary studies, due to their specialization to permanent absence of light. Eye and dark pigment regression are characteristics of most troglobites. In spite of the advance in knowledge on the mechanisms behind eye regression in cave fishes, very little is known about pigmentation changes. Studies were focused on three species of the genus Pimelodella. Exemplars of the troglobitic P. spelaea and P. kronei were compared with the epigean (surface) P. transitoria, putative sister-species of the latter. Melanophore areas and densities are significantly lower in the troglobitic species. Evaluating the in vitro response of these cells to adrenaline, acetylcholine and MCH, we observed a reduced response in both troglobites to adrenaline. The same trend was observed with MCH, but not statistically significant. No response to acetilcholine was detected in all the three. Contrary to expectations, even though eye-regression in P. spelaea was much lower than in P. kronei, pigmentation regression was more advanced. Multiple mechanisms of loss showing a mosaic of traits in troglobitic fishes are discussed here.

A case of complete albinism in the catfish Cambeva guareiensis (Siluriformes: Trichomycteridae)

Abstract: Albinism has been recorded in Neotropical freshwater fishes, mostly for nocturnal or cryptobiotic species. We report herein a case of albinism in the catfish Cambeva guareiensis (Trichomycteridae) from the Guareí River basin, Upper Paraná River basin, southeastern Brazil. The albino fish was caught with seven individuals with typical color pattern of the species. The features of the albino fish in life and shortly after preservation are described and illustrated.

Extreme Adaptation in Caves

Cave adaptation leads to unique anatomical specializations in many taxonomic groups. As the role of vision is reduced or disappears in a subterranean environment, other specializations arise to allow the organism to successfully detect and interact with their environment. A suite of unique, convergent phenotypes associated with subterranean adaptation has emerged (termed troglomorphy), with reduction or loss of pigmentation and eyes being the most conspicuous. Two vertebrate groups that have successfully colonized and adapted to subterranean environments are cavefishes and cave salamanders. There are many shared troglomorphic anatomical characters shared between these two groups, and we describe herein the morphological traits that are unique to fishes and salamanders that are adapted to caves and other subterranean habitats. Troglobionts, animals strictly bound and adapted to underground habitats, are outcomes of not just regressive evolution, but also constructive adaptation. There are skeletal changes, such as broadening and flattening of the head, as well as hypertrophy of non-visual modalities. Cavefishes and salamanders have lost eyes and pigmentation, but also enhanced mechanosenzation, chemosenzation and, in some cases, electroreception. Both cavefishes and cave salamanders have become important models in the study of the ecology, behavior, and evolution of subterranean colonization and adaptation. However, our knowledge is primarily limited to a few taxa and many questions remain to be studied. Anat Rec, 2018. © 2018 American Association for Anatomy.

A potential benefit of albinism in Astyanax cavefish: downregulation of the oca2 gene increases tyrosine and catecholamine levels as an alternative to melanin synthesis

Albinism, the loss of melanin pigmentation, has evolved in a diverse variety of cave animals but the responsible evolutionary mechanisms are unknown. In Astyanax mexicanus, which has a pigmented surface dwelling form (surface fish) and several albino cave-dwelling forms (cavefish), albinism is caused by loss of function mutations in the oca2 gene, which operates during the first step of the melanin synthesis pathway. In addition to albinism, cavefish have evolved differences in behavior, including feeding and sleep, which are under the control of the catecholamine system. The catecholamine and melanin synthesis pathways diverge after beginning with the same substrate, L-tyrosine. Here we describe a novel relationship between the catecholamine and melanin synthesis pathways in Astyanax. Our results show significant increases in L-tyrosine, dopamine, and norepinephrine in pre-feeding larvae and adult brains of Pachón cavefish relative to surface fish. In addition, norepinephrine is elevated in cavefish adult kidneys, which contain the teleost homologs of catecholamine synthesizing adrenal cells. We further show that the oca2 gene is expressed during surface fish development but is downregulated in cavefish embryos. A key finding is that knockdown of oca2 expression in surface fish embryos delays the development of pigmented melanophores and simultaneously increases L-tyrosine and dopamine. We conclude that a potential evolutionary benefit of albinism in Astyanax cavefish may be to provide surplus L-tyrosine as a precursor for the elevated catecholamine synthesis pathway, which could be important for adaptation to the challenging cave environment.