X-ray imaging of a water bear offers a new look at tardigrade internal anatomy

Abstract

Background Tardigrades (water bears) are microscopic invertebrates of which the anatomy has been well studied using traditional techniques, but a comprehensive three-dimensional reconstruction has never been performed. In order to close this gap, we employed X-ray computed tomography (CT), a technique that is becoming increasingly popular in zoology for producing high-resolution, three-dimensional (3D) scans of whole specimens. While CT has long been used to scan larger samples, its use in some microscopic animals can be problematic, as they are often too small for conventional CT yet too large for high-resolution, optics-based soft X-ray microscopy. This size gap continues to be narrowed with advancements in technology, with high-resolution imaging now being possible using both large synchrotron devices and, more recently, laboratory-based instruments. Results Here we use a recently developed prototype lab-based nano-computed tomography device to image a 152 μm-long tardigrade at high resolution (200–270 nm pixel size). The resulting dataset allowed us to visualize the anatomy of the tardigrade in 3D and analyze the spatial relationships of the internal structures. Segmentation of the major structures of the body enabled the direct measurement of their respective volumes. Furthermore, we segmented every storage cell individually and quantified their volume distribution. We compare our measurements to those from published studies in which other techniques were used. Conclusions The data presented herein demonstrate the utility of CT imaging as a powerful supplementary tool for studies of tardigrade anatomy, especially for quantitative volume measurements. This nanoCT study represents the smallest complete animal ever imaged using CT, and offers new 3D insights into the spatial relationships of the internal organs of water bears. Electronic supplementary material The online version of this article (10.1186/s40851-019-0130-6) contains supplementary material, which is available to authorized users.

Functional morphology of a lobopod: case study of an onychophoran leg

“…Detailed comparison of our findings with those from the other two extant panarthropod groups, the arthropods and tardigrades, is also problematic. The marine and limnoterrestrial tardigrades are the only other extant panarthropod group that shows unjointed limbs [21,22]. However, miniaturization in these animals seems to have led to a reduction in the individual muscles to single myocytes [21,51].…”

Nanoscopic X-ray tomography for correlative microscopy of a small meiofaunal sea-cucumber

“…Hence, microCT has been integrated increasingly in correlative microscopy applications, to define VOIs and as a reference for sectioning-based microscopy 4,13,31 . But to genuinely gain information about the microstructure of tiny biological specimens, the resolution of conventional microCT does not suffice 8 .…”

“…CLEM is a collective term for all different combinations of light and electron microscopy, ranging from fluorescence super-resolution microscopy to conventional bright field light microscopy and from focused-ion-beam scanning electron microscopy (FIB-SEM) over serial-block-face SEM (SBF-SEM) to transmission electron microscopy (TEM) 1,4-6 . Since light and electron microscopy techniques conventionally feature limited penetration depths through matter, they typically rely on sectioning or complete destruction of the sample to create 3D information of entire small biological specimens 1,7,8 . This often prevents isotropic resolution, and the resulting data are affected by deficiencies such as sectioning and alignment artefacts 1,4,5 .X-ray computed tomography (CT) is a widely used tool for non-destructive 3D imaging with isotropic resolution 3,9,10 .…”

“…Previous studies have shown that our NanoCT setup is capable of analysing the proper morphology of critical-point dried (CPD) and agarose-embedded biological samples 8,10,19 . It was further shown that osmium tetroxide staining combined with CPD creates satisfactory contrast in our NanoCT data 10 .…”

Mitochondrial alternative oxidase contributes to successful tardigrade anhydrobiosis

“…Although searching on the mechanisms has revealed their differentiation between tardigrade species (e.g. Hengherr et al, 2008; Rebecchi et al, 2013; Bemm et al, 2017; Gross et al, 2019; Kamilari et al, 2019), available data appears to indicate common and important role of proper mitochondria functioning. Accordingly, it has been shown that mitochondria uncoupling abolishes tun formation (Halberg et al, 2013) and reactive oxygen species (ROS) generated in majority by mitochondria (e.g.…”