Abstract
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.
Highlights
Troglobites, source-populations restricted to subterranean habitats, usually present autapomorphies expressed as morphological, physiological, behavioral and ecological characteristics that can be directly or indirectly related to the underground environment, and are known as troglomorphisms (Trajano, 2001)
There were significant differences (p < 0.05) between average aggregated melanosome areas in P. kronei and P. spelaea caudal fins compared with P. transitoria
No significant differences were observed between P. kronei and P. spelaea
Summary
Troglobites, source-populations restricted to subterranean habitats, usually present autapomorphies expressed as morphological, physiological, behavioral and ecological characteristics that can be directly or indirectly related to the underground environment, and are known as troglomorphisms (Trajano, 2001). In evolutionary studies, troglobitic organisms are an excellent source of data on both constructive and regressive mechanisms, since the selective pressures they undergo are relatively easy to identify (Trajano, Bockmann, 1999; Trajano, 2001). The large Brazilian diversity in subterranean fishes, one of the richest in the world and which includes species at different stages of eye and pigmentation regression (Trajano, Bichuette, 2010), offers an excellent opportunity for comparative studies on these specializations (Felice et al, 2008). Studies range from morphology, ontogeny and classical genetics to ultra-structure and molecular biology, mostly focusing on the Mexican tetra characins, genus Astyanax Baird, Girard, 1854 (Jeffery, 2006; Protas et al, 2006, 2007; Wilkens, Strecker, 2017)
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