Microstructure of the Bonnethead Shark (Sphyrna tiburo) Olfactory Rosette

Abstract

The unusual shape of sphyrnids (hammerhead sharks) heads has led to many functional hypotheses of potential sensory advantages and enhanced olfactory performance. Recent investigations into the flow of water within the sphyrnid olfactory chamber demonstrate that this complex structure exhibits a differential pressure system between the two nares that induces flow through the chamber. This leads to differential fluid velocities in different parts of the olfactory chamber. Particularly, lamellae at the medial end of the olfactory chamber experience a near-stagnant recirculation of water. The objectives of this study were to 1) describe the microstructure of the olfactory rosette of bonnethead sharks (Sphyrna tiburo) and 2) given the variability of water flow within the sphyrnid olfactory rosette, investigate differences of individual lamellae based on their positioning within the rosette. Specifically, we investigated degree of secondary folding, percent sensory area, and relative surface along the lateral-to-medial gradient. Both degree of secondary folding and percent sensory area may serve as proxies for olfactory sensitivity, providing connectivity between area devoted to sensitivity and water flow within the olfactory organ. We found that bonnethead sharks exhibited similar lamellar morphology to other shark species. We also described the projection of the olfactory nerve layer through an individual lamella. Additionally, we found that lamellae within the medial portion of the organ, which experience slower water velocities, had less secondary lamellar folds and less sensory area. These findings imply that these areas may be less sensitive. Future work should test for sensitivity differences within the rosette along the lateral-to-medial gradient.

Ontogenetic shifts in olfactory rosette morphology of the sockeye salmon, Oncorhynchus nerka

“…Similarly, previous work in salmonids showed that the convex areas of the secondary folds lack ORNs (Kudo et al, 2009 ; Olsen, 1993 ; Yamamoto & Ueda, 1977 ). This is in contrast to many elasmobranch fishes, where the sensory epithelium covers the secondary lamellae (both troughs and peaks), with areas of nonsensory epithelium along the inner margins of the lamellae and/or as intermittent projections into the sensory regions in some species (e.g., Camilieri‐Asch, Shaw, et al, 2020 ; Schluessel et al, 2008 ; Theiss et al, 2009 ; Simonitis & Marshall, 2022 ). In adult sockeye salmon, as in other salmonids, the sensory epithelium lies only along the troughs of the secondary folds, while the nonsensory epithelium covers the secondary lamellar peaks (Figure 5c ).…”