Comparative and functional anatomy of the elasmobranch olfactory system

The sense of smell in elasmobranchs is considered to be highly developed compared to other classes of vertebrates, often referred to as "swimming nose." Numerous studies have attempted to explain this phenomenon from both behavioral and anatomical/physiological perspectives. This study aims to be one of the most comprehensive analyses of the olfactory system from an anatomical and morphological point of view. Three main areas of the olfactory system were considered: the nostrils, olfactory rosette, and olfactory bulb. Studies on the morpho dynamics of shark nostrils, organs that vary in complexity depending on the species, mainly focus on their relationship with the animal's environment, lifestyle, and water flow in and out of them, as well as the anatomical features that govern this process. Using a geometric morphometrics approach, this study investigated the shape of shark nostrils on the largest dataset used to date, correlating observed differences and similarities with the hydrodynamics of the animal and other anatomical features related to the olfactory system. Analyses of the olfactory rosette and olfactory bulb, respectively, the peripheral organ responsible for the reception of chemical molecules called odorants and the first area of the nervous system responsible for the initial processing and transmission of the signal to higher centers, were mainly histological. The primary lamellae of the olfactory rosette, extremely branched and diversified among the analyzed species, were investigated to understand both their complexity and their possible function in increasing the total surface area of the olfactory epithelium that covers them. In addition, a new variable related to the count of olfactory neurons residing in the rosette epithelium was systematically introduced. The olfactory bulb was analyzed using the isotropic fractionation technique, obtaining quantitative data on its cellular and neuronal composition as previously applied in other studies. Both types of analyses, conducted on a large number of species, demonstrate the existence of an exceptional olfactory system in elasmobranchs, surpassing that of other vertebrates like mammals. However, the system's cellular density is comparatively less developed than in other taxa. The shape of the nostrils seems to correlate with the complexity of the olfactory system's development. Although the number of olfactory receptor neurons is substantial, the ratio of these neurons to the neurons in the olfactory bulbs is similar to that of other vertebrates, and the number of olfactory receptors in the genomes is notably low. Overall, these findings, along with previous studies, suggest that elasmobranchs have developed an optimized sense of smell that is adapted to their lifestyle, rather than possessing a super olfactory system as previously thought.