Living in darkness: Exploring adaptation of Proteus anguinus in 3 dimensions by X-ray imaging

Background: Lightless caves can harbour a wide range of living organisms. Cave animals have evolved a set of morphological, physiological, and behavioural adaptations known as troglomorphisms, enabling their survival in the perpetual darkness, narrow temperature and humidity ranges, and nutrient scarcity of the subterranean environment. In this study, we focused on adaptations of skull shape and sensory systems in the blind cave salamander, Proteus anguinus, also known as olm or simply proteus—the largest cave tetrapod and the only European amphibian living exclusively in subterranean environments. This extraordinary amphibian compensates for the loss of sight by enhanced non-visual sensory systems including mechanoreceptors, electroreceptors, and chemoreceptors. We compared developmental stages of P. anguinus with Ambystoma mexicanum, also known as axolotl, to make an exemplary comparison between cave- and surface-dwelling paedomorphic salamanders. Findings: We used contrast-enhanced X-ray computed microtomography for the 3D segmentation of the soft tissues in the head of P. anguinus and A. mexicanum. Sensory organs were visualized to elucidate how the animal is adapted to living in complete darkness. X-ray microCT datasets were provided along with 3D models for larval, juvenile, and adult specimens, showing the cartilage of the chondrocranium and the position, shape, and size of the brain, eyes, and olfactory epithelium. Conclusions: P. anguinus still keeps some of its secrets. Our high-resolution X-ray microCT scans together with 3D models of the anatomical structures in the head may help to elucidate the nature and origin of the mechanisms behind its adaptations to the subterranean environment, which led to a series of troglomorphisms

Ecology and web allometry of Clitaetra irenae, an arboricolous African orb-weaving spider (Araneae, Araneoidea, Nephilidae)

Volume: 36Start Page: 583End Page: 59

FIGURE 2 in Oviparity, viviparity or plasticity in reproductive mode of the olm Proteus anguinus: an epic misunderstanding caused by prey regurgitation?

FIGURE 2 Regurgitated salamander (Salamandra salamandra) larvae. The first image A) shows the larva (3.1 cm of total length) still alive after regurgitation with no obvious damage visible; B) shows the second larva (3.2 cm of total length), with heavier damage around the gills. A close-up of the respective damaged area of the C) neck and gills of the larva is shown.Published as part of Recknagel, Hans, Premate, Ester, Zakšek, Valerija, Aljančič, Gregor, Kostanjšek, Rok & Trontelj, Peter, 2022, Oviparity, viviparity or plasticity in reproductive mode of the olm Proteus anguinus: an epic misunderstanding caused by prey regurgitation?, pp. 153-165 in Contributions to Zoology 91 (3) on page 159, DOI: 10.1163/18759866-BJA10029, http://zenodo.org/record/834341

FIGURE 3 in Oviparity, viviparity or plasticity in reproductive mode of the olm Proteus anguinus: an epic misunderstanding caused by prey regurgitation?

FIGURE 3 Scanning electron micrographs of the olm's teeth marks on regurgitated salamander (Salamandra salamandra) larvae. Heads of all larvae are facing towards the left. A) Teeth marks (arrowheads) on the dorsal side of the head corresponding to the position of the olm's jaw and B) parallel teeth marks on the left side of the head above the gills (g) of the larvae shown in fig. 2A. C) Teeth marks on the anterior edge of laceration above the left gills (g) and D) above the right eye (e) of the head of the larva shown in fig. 2B and C. (Scale bars represents 500 µm).Published as part of Recknagel, Hans, Premate, Ester, Zakšek, Valerija, Aljančič, Gregor, Kostanjšek, Rok & Trontelj, Peter, 2022, Oviparity, viviparity or plasticity in reproductive mode of the olm Proteus anguinus: an epic misunderstanding caused by prey regurgitation?, pp. 153-165 in Contributions to Zoology 91 (3) on page 160, DOI: 10.1163/18759866-BJA10029, http://zenodo.org/record/834341

Environmental DNA in subterranean biology update: from “Where?” to “How many?”

Recent records of Proteus anguinus outside its historically known range (Gorički et al. 2017), discovered through detection of its DNA dissolved in groundwater (environmental DNA or eDNA), mark the beginning of a new era in the study and conservation of cryptic subterranean biodiversity. An upgraded technology, droplet digital PCR (ddPCR), initially developed for studies of gene expression, detection of genetically modified organisms and in medical diagnostics, is being tested for improved detection of the much smaller and rare stygobiont, the cave clam Congeria jalzici. In parallel to eDNA assay development for various stygobiotic species of the Dinaric Karst, a groundwater-sample library is being created. The samples will be available for future analysis of their species composition and will also serve as a source of information on any changes in species distribution over time. In another line of eDNA research, the utility of ddPCR for direct quantification of eDNA molecules in groundwater is being explored by using the large, accessible and well-characterized (Zakšek and Trontelj 2017) natural Proteus population in the Planina Cave (Slovenia) as a model. The eDNA methodology may in the future be applied in estimation and monitoring of Proteus population sizes without having to see, mark or otherwise disturb the animals themselves