Comparative ultra-microscopy unraveling the regressive evolutionary signals: Insights from adaptation to extreme environment

Extreme environmental conditions have profound impacts on shaping the evolutionary trajectory of organisms. Cave-dwelling fishes, known as subterranean fishes, are notable examples of regressive evolution in response to the unique ecosystem of caves. Garra longipinnis (Teleostei: Cyprinidae: Labeoninae), a freshwater cyprinid endemic to the Arabian Peninsula, resides in both caves and surface habitats in northern Oman. It showcases two distinct morphotypes: the normal surface-dwelling form (epigean) with eyes and body pigmentation, and the blind cave-dwelling form (troglomorphic) lacking both eyes and pigmentation. Two morphotypes were analyzed comparatively, focusing on body scales using light and scanning electron microscopy to unravel the possible regressive evolutionary signals in cave-dwelling G. longipinnis., The results revealed that the blind cave-dwelling form displayed a reduction in some scale features, including the reduction in scale size, smaller focal index, smaller extension index, and the absence of tubercles on the posterior portion of the scale. An additional change in the scales of the cave-dwelling population was related to the tubular part of the lateral line canal, which was longer with a wider opening (enhanced non-visual sensory systems). Furthermore, the cave population exhibited a relatively smaller body size than its surface-dwelling counterpart. These observed traits are regarded as adaptations to the constant darkness and poor nutrition conditions of the subterranean environments. By uncovering novel regressive evolutionary signals in scale morphology, this study contributes to our understanding of cave-dwelling inhabitants, shedding light on the independent influences of extreme environments on the evolution of fish populations and species.