Study explores link between shark nose shape, size and sensitivity of smell

Despite general similarities among elasmobranch species' (sharks, rays and skates) olfactory systems, the morphology or structure of their olfactory organ, or "rosette," differs substantially. Located in their snouts, the multi-lamellar (layered tissue) rosette is covered with both non-sensory and sensory tissue that responds to distinct odor molecules in an aquatic environment. The number, size and arrangement of lamellae differ among elasmobranch species, but the functional consequences for these differences are not fully understood.

Researchers have not yet been able to correlate organ size and complexity to odor sensitivity in teleosts (ray-finned fishes) nor elasmobranchs. Differences in sharks' olfactory systems are of particular interest not only because of their reputation for having an incredible sense of smell but also because these remarkable animals have been around since before the dinosaurs. They somehow managed to thrive in every known marine habitat for millions of years -- their sense of smell may have been key to their success.

Researchers from Florida Atlantic University are the first to quantify olfactory organ morphology by examining rosette shape and other internal structures among a diverse set of shark species using dissections, phylogenetic comparisons, and a fairly new technique, called diffusible iodine-based contrast-enhanced computed tomography (diceCT) imaging.

Results, published in the Journal of Morphology, reveal that the organs did not change in shape or number of lamellae throughout the life stages, suggesting that olfaction is a key sensory modality throughout the life of elasmobranch fishes.

Overall, researchers found more lamellae in rosettes with an elongated shape than in organs with a short and wide shape. Phylogenetic analyses showed that 2D shape and lamellar count are critical to understanding rosette variations in unrelated species.

Researchers also used in situ values of rosette volume and 2D shape from diceCT to assess the impacts of dissection on morphological measurements. Rosette measurements from dissected organs did not differ significantly compared to those from CT scans, supporting the validity of both methods.

"From our CT scans, we used raw data files that have 2D x-ray images. We then created 3D reconstructions to visualize the data. As such, we were able to examine the internal morphology from the top of the head to the tip of the snout from different angles," said Aubrey Clark, corresponding author and a doctoral student in FAU's Biomechanics Lab within the Charles E. Schmidt College of Science. "Think of this method as taking a stack of sticky notes and removing them one at a time to see the sticky note underneath. It is incredibly useful to help us understand the internal morphology in situ or as it exists naturally inside the shark because dissections can alter structures."

Among the species studied included the bonnethead sharks (S. tiburo) with rosettes that are narrow and elongated; the common thresher sharks (A. vulpinus) and shortfin mako sharks (I. oxyrinchus), which share similarities in their rosettes: a shape that is almost as wide as it is long, a thicker raphe, fewer lamellae and wider interlamellar distances; and blacktip sharks (C. limbatus) and Atlantic sharpnose sharks (R. terraenovae) with rosettes that are intermediate to the two aforementioned groups. Other members of the hammerhead shark family have long, thin olfactory organs, similar to the bonnethead shark.

"The shape and internal structure should be considered when characterizing olfactory rosettes in sharks in future studies because fineness ratio (2D shape) and lamellar number were the largest contributors to our phylogenetic analyses," said Marianne E. Porter, Ph.D., senior author and an associate professor, Department of Biological Sciences, FAU's Charles E. Schmidt College of Science. "Data from our study also should be used to further investigate rosette variation among elasmobranch species by examining the hydrodynamics of the olfactory system with 3D models and quantifying the physiological impacts of flow on odorant binding."

Study co-author is Tricia Meredith, Ph.D., director of research, FAU's A.D. Henderson University School and FAU High School, and an assistant research professor in FAU's College of Education.

Video: https://youtu.be/zyr8SMkWiu8