Morphometric analysis of the elasmobranch olfactory rosette.

The olfactory rosettes of elasmobranchs vary in shape and structure among species, but the functional consequences of this diversity are unresolved. Our goal was to quantify rosette morphology on dissected as well as diffusible iodine-based contrast-enhanced computed tomography (diceCT)-imaged specimens to analyze the drivers of observed trends in a phylogenetic context and compare the methodologies. We hypothesized that lamellar count and rosette shape (fineness ratio) would not scale with animal size, but other rosette size variables would scale positively. We dissected rosettes from 14 elasmobranch species and collected morphometric data (fineness ratio, lamellar count, interlamellar distance, lamellar thickness, and raphe width). A subset of rosettes (five species) was used to analyze the effects of body size, while all 14 species were used for a phylogenetic principal component analysis (pPCA). We found that fineness ratio and lamellar counts varied significantly among species, and were positively correlated. The first two principal components of the pPCA explained 82% of the variation, with fineness ratio and lamellar count contributing most to the loadings, respectively. DiceCT was used for in situ imaging of four species of Carcharhiniformes. There were no significant differences between rosette structure or volume when comparing values from dissected specimens to values from in situ specimens obtained using diceCT. We also quantified the volume of the excurrent channel in the olfactory capsule. These data add to our understanding of how olfactory organ shape varies among species and can be used to create three-dimensional models for future olfactory hydrodynamic studies.