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

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Led by Jagan Jayachandran, assistant professor of aerospace engineering, and Adam Powell, associate professor of mechanical and materials engineering, the work helps address an urgent climate change problem. Aviation accounts for approximately 2.5% of all global greenhouse emissions, according to the International Council on Clean Transportation (ICCT), and that number is only expected to increase.

“As aviation continues to grow, so will the industry’s emissions, says Powell. “We need to think out of the box and look at sustainable materials that will contribute to a long-term solution toward reducing the transportation sector’s carbon footprint.”

Through modeling and computation analysis, Jayachandran and Powell developed a formula for a fuel that consists of magnesium, a mineral that is found all over the globe, most abundantly in the world’s oceans. A slurry of magnesium hydride — a chemical compound made up of magnesium and hydrogen — mixed with hydrocarbon fuel would burn to produce CO₂, water vapor and magnesium oxide (MgO) nanoparticles. The magnesium hydride fuel would also give planes the range for long-haul flights — e.g., from Boston to Tokyo — something that has been a challenge for other sustainable aviation fuels to provide. That longer range is achieved, in part, due to the chemical properties of the slurry — a lower volume of it is needed for combustion than a typical aviation fuel.

“We found this fuel would have up to 8% more range than other today’s jet fuel, and more than two to three times longer range than liquid hydrogen or ammonia which other researchers have proposed as sustainable fuels,” said Jayachandran.

The Department of Energy describes a sustainable aviation fuel as a “biofuel used to power aircraft that has similar properties to conventional jet fuel but with a smaller carbon footprint.” These biofuels have been made from resources including corn grain, algae, forestry, and agricultural residues, among others. Using a biofuel as the hydrocarbon in this slurry with magnesium hydride could potentially lead to net negative emissions.

The research was supported by a WPI TRIAD Seed Grant, a university award intended to encourage and promote interdisciplinary collaboration and innovation.

Jayachandran and Powell plan to further their research through physical experiments with samples of the fuel, and are also pursuing potential funding from a federal agency. In addition, they hope this work can inspire others and contribute to a more sustainable world.

Noting the promise of research to mitigate emissions and other climate threats, Powell said, “we hope our work, which opens up a new category of sustainable aviation fuel will spark the imagination of other researchers. The sky’s the limit.”

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Materials provided by Worcester Polytechnic Institute. Note: Content may be edited for style and length.