Porcelain crabs (Porcellanidae) are one of three taxa within anomuran crustaceans (Anomala) which possess a crab-like body form. Curiously, these three lineages evolved this shape independently from true crabs (Brachyura) in the course of the evolutionary process termed carcinization. The entire pleon in porcelain crabs is flexed under the cephalothorax and the carapace is approximately as broad as long. Despite their crab-like habitus, porcelain crabs are phylogenetically nested within squat lobsters (Munidopsidae, Munididae, Galatheidae). With a pleon which is only partly flexed under the cephalothorax and a cephalothorax which is longer than it is broad, squat lobsters represent morphologically intermediate forms between lobster-like and crab-like body shapes. Carcinization has so far mostly been studied with respect to outer morphology; however, it is evident that internal anatomical features are influenced through this change of body shape too. In this paper, the situation in Galatheoidea is elucidated by adding more taxa to existing descriptions of the hemolymph vascular systems and associated structures and organs. Micro-computer tomography and 3D reconstruction provide new insights. Autapomorphic states of various internal anatomical characters are present in nearly all the studied species, also reflecting some degree of anatomical disparity found within Galatheoidea. The ventral vessel system of porcelain crabs differs distinctly from that of squat lobsters. The differences in question are coherent (i.e. structural dependent) with morphological transformations in the integument, such as the shortening of the sternal plastron, which evolved in the course of carcinization. Shifts in the gonads and the pleonal neuromeres are coherent with the loss of the caridoid escape reaction, which in turn is a consequence of carcinization. The arterial transformations, however, are minor compared to other instances of carcinization in anomuran crustaceans since the last common ancestor of squat lobsters and porcelain crabs was already "half carcinized".

Background: Micro-computed tomography (micro-CT) provides detailed 3-dimensional (3D) visualization of anatomical structures and encourages morphological reinvestigation of organs with delicate features. The low radiodensity of soft tissues necessitates preceding sample preparation to conduct X-ray imaging with decent contrast between different tissues. In this study, we demonstrate the preparation with three radiopaque agents in combination with elimination of liquids by critical point drying (CPD) introduced for ocular samples.Methods: Enucleated porcine eyes were prepared with ethanolic iodine (EI), aqueous iodine-potassium iodide, or ethanolic phosphotungstic acid (EPTA). Micro-CT scans of the samples were conducted in a moist environment with an isotropic resolution of 9.2-12.5 µm voxel size. Subsequently, samples were chemically dehydrated and critical point (CP) dried to conduct a second scan in a dry environment with a resolution up to 4.7-5.4 µm in voxel size. The visualization effects were qualitatively and semi-quantitatively evaluated with regard to the generated contrast between different ocular tissues.Results: All three contrast agents accumulated well in most of the investigated ocular tissues and lead to an increased X-ray attenuation which allowed for differentiated visualization of ocular structures. Problematic agent penetration into the lens was obvious for iodine-potassium iodide and EPTA. Artificial damages of the lens and thickness reduction for the cornea and sclera due to CPD were noticed. The effects of the different contrasting treatments are described and compared with regard to the effects of CPD. Exclusively CP dried samples that were not treated with contrast agents could also be visualized excellently with a good distinction of different ocular structures from each other.Conclusions: All ocular structures can be visualized by micro-CT. To contrast moist samples, the best results were achieved with iodine potassium iodide (IPI). CPD improved the scan quality in all cases. Even without pretreatment with contrasting agents, the CP dried samples showed a contrast similar to the IPI treated samples.

Cover illustration. The morphological transformation of hermit crabs into king crabs represents one of the evolutionary most fascinating scenarios in decapod evolution. It is certainly the most remarkable case of carcinization, or evolutionary shaping into a crab‐like form. In this issue of the Journal of Morphology Keiler and coauthors (pp. 759–778) investigate the hemolymph vascular system and adjacent anatomical structures of several recent hermit crab (Paguridae) and king crab (Lithodidae) species. The cover image shows a dorsal view of a corrosion cast of the circulatory system in Paralomis granulosa catched in the Pacific off the coast of Chile (Photograph by Jonas Keiler).

The morphological transformation of hermit crabs into crab-like king crabs in the evolution of decapod crustaceans represents a remarkable case of carcinization or evolutionary shaping into a crab-like form. In this study, we focus on internal organs such as the hemolymph vascular system and adjacent anatomical structures of several Recent hermit crab (Paguridae) and king crab (Lithodidae) species. There are various correspondences in the morphology of the arterial systems in the dorsal cephalothorax of the two taxa, especially with regard to the anterior aorta, anterior lateral arteries, and hepatic arteries. In the pleon, the posterior aorta in both taxa displays a proximal bifurcation and follows an asymmetrical course. The ventral vessel system, on the other hand, which mainly supplies the limbs, differs significantly between the taxa, with pagurids displaying the plesiomorphic condition. The pattern of the ventral vessel system in Lithodidae is influenced by morphological transformations of integumental structures during carcinization. One of these transformations was the broadening of the sternites, which resulted in a widening of the space between the endosternites. In addition, changes in the morphology of the endophragmal skeleton in Lithodidae led to an increase in the potential for intraspecific variability and interspecific variation in the arterial branching pattern.

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