Quantification of olfactory glomeruli in a shark: correlation with body size and receptor genes across vertebrates.

INTRODUCTION: The vertebrate olfactory system is characterized by the projection of receptor neurons toward the olfactory bulb (OB), where axons converge into synaptic structures known as glomeruli. METHODS: This study histologically investigates the OB of the small-spotted catshark (Scyliorhinus canicula), a species featuring a remarkably restricted repertoire of 49 identified receptor genes. RESULTS: Despite its limited genomic repertoire, adult S. canicula possesses about 400 glomeruli per OB, organized into distinct medial and lateral domains. These glomeruli exhibit significant size heterogeneity, with diameters ranging from 40 to 360 µm; the largest units represent some of the highest values recorded in vertebrates to date. To contextualize these findings, a comparative dataset of 16 vertebrate species was analyzed using phylogenetic generalized least squares models. CONCLUSIONS: The results demonstrate that glomerular abundance is strongly associated with both receptor gene number and body mass (R2 approx 0.94), highlighting a dual influence of genomic and allometric factors. Interestingly, the glomeruli-to-genes ratio in S. canicula (approx 8.5) aligns more closely with medium-sized mammals than with other non-mammalian taxa. This research provides the first quantification of the glomerular layer in a chondrichthyan, revealing an organizational principle that diverges from other non-mammalian vertebrates.