What’s in this crab? MRI providing high-resolution three-dimensional insights into recent finds and historical collections of Brachyura

Abstract

Museum collections may be viewed as a unique window onto the diversity and the functional evolution of species on earth. Detailed information about the inner structure of many precious collectors' items is, however, difficult to gain without destruction of the objects of interest. Here we applied magnetic resonance imaging (MRI) to freshly fixed as well as century-old museum specimens and compared the effects of fixative (formalin, ethanol, mercury chloride) on the image quality. Three-dimensional (3D) reconstruction of MRI was exemplarily used to non-invasively visualize anatomical structures of the brachyuran species Ilia nucleus, Ozius guttatus and Austinograea williamsi. Moreover, the potential of combining MRI and micro-computed tomography (μCT) was exemplarily analyzed for O. guttatus. The best MRI quality was achieved with formalin fixation and this also applied to specimens more than 100 years old. For specimens with a straight carapace width of about 30mm, an isotropic spatial resolution of 100μm allowed for the delineation of all major organ systems such as the nervous system, the gastrointestinal tract, the reproductive system and the heart. Moreover, combining MRI and μCT revealed new insights into the interaction of the heart and surrounding skeletal structures. As examples of its potential, MRI of a specimen of O. guttatus showed a very rare double infection with bopyrid isopods and 3D reconstruction of the reproductive tract of A. williamsi revealed a remarkable size of the ovaries as well as a shape and orientation of the seminal receptacles unusual for brachyurans. Thus, MRI may open up extensive possibilities to study evolutionary and ecological questions by utilizing the immense wealth of natural historical collections without any destruction of the items.