Olfactory Morphology and Sensitivity
Elasmobranchs are widely regarded to possess superior olfactory capabilities compared to teleost fishes. We employed an underwater electro-olfactogram (EOG) technique to determine the sensitivity of various species to biologically relevant odorants. Surprisingly, we found that elasmobranchs are generally similar to other fishes and do not appear to possess any greater olfactory sensitivity.
We also quantified the surface area of the lamellae that comprise the olfactory rosette and determined that size is not correlated with sensitivity. Although sphyrnids have a greater number of individual lamellae, the total surface area does not differ from the carcharhinids. The dorso-ventrally compressed cephalofoil of sphyrnids constrains the size of the rosette although the greater head width enables them to possess a greater number of smaller lamellae.
The widely spaced nares of sphyrnid sharks have also been suggested to provide them with better directional localization of odors. We compared the olfactory system morphology of sphyrnid sharks to their closely related carcharhinid relatives to test whether this hypothesis is supported. Most sphyrnid sharks possess a distinct prenarial groove on the anterior margin of the cephalofoil that serves to channel water from along the length of the groove into the nares. This effectively increases the swath of water that is sampled by the head of the sphyrnid shark and provides them with a greater probability of encountering an odor molecule. However, because the odor is channeled from anywhere along the length of the prenarial groove, the effective separation distance between the left and right sides is no different than in a carcharhinid shark.