Search

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. • Cave-dwelling species evolve various characteristics due to life in dark extreme environments. • Regressive evolutionary signals in the blind cave-dwelling population include smaller scales, focal index, and smaller extension index. • Two morphotypes reveal that some genes are still conserved in G. longipinnis. [ABSTRACT FROM AUTHOR]

Schindleria is likely the most numerous coral reef associated fish in the world. Yet, we are far from understanding their key to success. Schindleria are tiny (<23 mm) progenetic Gobiidae with a very short generation time. Due to extreme progenesis, its ancestors were morphologically very simple, which resulted in highly similar body constructs across the taxon. Hence, very few distinctive morphologies have been identified, and species descriptions comprise few specimens; for which detailed interspecific differences in body shape and sexual dimorphisms (intraspecific) are still poorly documented. To discriminate between species, especially in sympatry, the most evident feature is the length of the dorsal fin, which has been widely used for the classification into two types: the long dorsal finned (LDF) and short dorsal finned (SDF). In Hawaii, these types are named Schindleria praematura and Schindleria pietschmanni , respectively. We investigated morphometrics of sympatric populations of these two Schindleria from an atoll of the Hawaiian Islands (23° 46′ N, 166° 11′ W) as a first step to assess putative dimorphisms between sexes and between species' types. Between types, we found significant differences in body size and among features related to propulsion, swimming, and foraging (e.g., the length of the tail or the width of the head), as well as notably different urogenital papillae, which suggests that the two species differ in habitat and in reproductive strategies. Comparing the same sex between the two types, we found that males differed in more morphological traits between the species than females (i.e., 76.2% vs. 52.4%). Within the species, sexual dimorphisms were high in both species (i.e., 71.4% off all traits in S. pietschmanni and 66.7% in S. praematura) and were also linked to swimming (e.g., the length of the dorsal and anal fins and of the tail) and to feeding (e.g., the head length and the snout length), suggesting further intraspecific microhabitat segregation between sexes. We present a rare example of interspecific and sexual morphometric divergence in two sympatric progenetic fishes. [ABSTRACT FROM AUTHOR]

The dentition of cyprinid fishes is restricted to the right and left fifth ceratobranchials, which form the lower pharyngeal jaw (LPJ). The African members of the cyprinid subfamily Danioninae are nested within the tribe Chedrini. We compare the pharyngeal dentitions of African and of Asian Chedrini (Danioninae), including Danio rerio (Danionini), considering the following morphological characters: the pattern of dentition, the number of pharyngeal tooth rows, the curving and inclination of the tooth crowns, and the ratio of the tooth-bearing area to the length of the LPJ. Similar to a previous genetic based study a close relationship is proposed among the African genera Engraulicypris , Mesobola , and Rastrineobola , and among Leptocypris , Neobola , and Chelaethiops , all of which were formerly assigned as ‘Neobolines’. Species of Rastrineobola , Engraulicypris , Mesobola , and Neobola develop an additional (fourth) row of pharyngeal teeth. The dentition of Rastrineobola argentea (Pellegrin, 1904) and the shape of the LPJ of Engraulicypris sardella (Günther, 1868) characterize these two species as specialized within the Chedrini. An additional pharyngeal tooth row also occurs infrequently in the zebrafish D. rerio (Hamilton, 1822). The ‘fourth’ row of this fish species is possibly the result of a different process for acquiring four rows of pharyngeal teeth, likely by the splitting of one tooth row. [ABSTRACT FROM AUTHOR]

Background: Quantitative evaluation of mosaics of photoreceptors and neurons is essential in studies on development, aging and degeneration of the retina. Manual counting of samples is a time consuming procedure while attempts to automatization are subject to various restrictions from biological and preparation variability leading to both over- and underestimation of cell numbers. Here we present an adaptive algorithm to overcome many of these problems.Digital micrographs were obtained from cone photoreceptor mosaics visualized by anti-opsin immuno-cytochemistry in retinal wholemounts from a variety of mammalian species including primates. Segmentation of photoreceptors (from background, debris, blood vessels, other cell types) was performed by a procedure based on Rudin-Osher-Fatemi total variation (TV) denoising. Once 3 parameters are manually adjusted based on a sample, similarly structured images can be batch processed. The module is implemented in MATLAB and fully documented online., Results: The object recognition procedure was tested on samples with a typical range of signal and background variations. We obtained results with error ratios of less than 10% in 16 of 18 samples and a mean error of less than 6% compared to manual counts., Conclusions: The presented method provides a traceable module for automated acquisition of retinal cell density data. Remaining errors, including addition of background items, splitting or merging of objects might be further reduced by introduction of additional parameters. The module may be integrated into extended environments with features such as 3D-acquisition and recognition.

A liquid crystal programmable phase modulator (PPM) is used as correcting device in an adaptive optics system for three-dimensional ultrahigh-resolution optical coherence tomography (UHR OCT). The feasibility of the PPM to correct high order aberrations even when using polychromatic light is studied, showing potential for future clinical use. Volumetric UHR OCT of the living retina, obtained with up 25,000A-scans/s and high resolution enables visualization of retinal features that might correspond to groups of terminal bars of photoreceptors at the external limiting membrane.

The tuatara Sphenodon punctatus, restricted to a few New Zealand offshore islands and now strictly protected, belongs to the Rhynchocephalia, the smallest order of extant reptiles. Earlier light microscopical studies on the retina of this species described photoreceptors with both rod- and cone-like features and the presence of a fovea. A limited amount of retinal material from S. punctatus has now allowed us to prepare the first-ever electron microscopic observations on the eye of this reptile. We were able to distinguish three types of photoreceptor, all with fine structural features characteristic of cone cells. Large single cones as well as double cones had open discs in their outer segments and straight axons with pedicle-type terminals. An additional cone type, characterized by somewhat more slender inner and outer segments, vitreally-displaced cell bodies and oblique axons, resembled short-wavelength cones known from other sauropsids. No cells with rod characteristics could be confirmed in the samples, although they might occur in retinal regions not available for this study. We conclude that the tuatara has cone-like photoreceptors, which-as in other crepuscular or nocturnal reptiles-have acquired rod-like features. The phenotypic adaptations notwithstanding, the set of photoreceptor types is quite typical of the reptilian eye and in some respects reminiscent of those seen in lizards and turtles.

The topography and spectral characteristics of mammalian photoreceptors correlate with both, the present ecological demands and the evolutionary history. The South American Opossum is a marsupial mammal with unspecialized habitus and crepuscular lifestyle. A sparse population of cones (max. = 3000/mm2) can be differentiated into four subtypes by morphological, topographical and immunocytochemical criteria. In spite of this unusual diversity the cone types can be split into two functional groups: The population of single cones labeled by antibody OS-2 for short wavelength sensitive pigments was ubiquitous but at very low densities (200/mm2). The single cones labeled by antibody (COS-1) against long wavelength sensitive pigments constitute the dominant population in the area centralis (2300/mm2). These two single cone types correlate with the pair typically present in placental mammals. Discrimination of spatial and color contrast may be provided by this "modern" set. The COS-1 labeled double and single cones bearing an oil droplet, display a different pattern by being restricted to the inferior (non-tapetal) half of the retina (max = 800/mm2). This additional set of cones with oil droplets and long wavelength pigments is a conservative feature of the opossum retina and other marsupials. As an accessory cone system it is possibly providing enhanced sensitivity at mesopic conditions. During the early evolution of nocturnal mammals with its prominent expansion of rod vision these cone types were conserved but then were lost in placental mammals. Thus the unique features of mammalian retinas are the result of two evolutionary steps: first a reduction of cone based vision, followed by a secondary differentiation of photopic vision and behaviour relying on the remaining set of cones.

The retinas of placental mammals appear to lack the large number and morphological diversity of cone subtypes found in diurnal reptiles. We have now studied the photoreceptor layer of a South American marsupial (Didelphis marsupialis aurita) by peanut agglutinin labeling of the cone sheath and by labeling of cone outer segments with monoclonal anti-visual pigment antibodies that have been proven to consistently label middle-to-long wavelength (COS-1) and short-wavelength (OS-2) cone subpopulations in placental mammals. Besides a dominant rod population (max. = 400,000/mm2) four subtypes of cones (max. = 3000/mm2) were identified. The outer segments of three cone subtypes were labeled by COS-1: a double cone with a principal cone containing a colorless oil droplet, a single cone with oil droplet, and another single cone. A second group of single cones lacking oil droplets was labeled by OS-2 antibody. The topography of these cone subtypes showed striking anisotropies. The COS-1 labeled single cones without oil droplets were found all over the retina and constituted the dominant population in the area centralis located in the temporal quadrant of the upper, tapetal hemisphere. The population of OS-2 labeled cones was also ubiquitous although slightly higher in the upper hemisphere (200/mm2). The COS-1 labeled cones bearing an oil droplet, including the principal member of double cones, were concentrated (800/mm2) in the inferior, non-tapetal half of the retina. The two spectral types of single cones resemble those of dichromatic photopic systems in most placental mammals. The additional set of COS-1 labeled cones is a distinct marsupial feature. The presence of oil droplets in this cone subpopulation, its absence in the area centralis, and the correlation with the non-tapetal inferior hemisphere suggest a functional specialization, possibly for mesopic conditions. Thus, sauropsid features have been retained but probably with a modified function.