Hydrodynamic flow in the peripheral olfactory system of pelagic fishes: A comparison between southern bluefin tuna, Thunnus maccoyii and black marlin, Istiompax indica

The peripheral olfactory system of teleosts reveals great morphological diversity, which directly affects the delivery of water-borne odorants in an aquatic environment. In pelagic teleosts, hydrodynamic conditions also play an important role in olfaction, as they continuously experience a flow of water across their tapered head. In this study, the structure of the olfactory chamber, olfactory rosette, and nostrils of two species of pelagic teleosts, the southern bluefin tuna Thunnus maccoyii , and the black marlin Istiompax indica , are examined in order to understand the morphology of their peripheral olfactory system. The inter-nostril distance and the volume of the ventilatory sac are also assessed using light microscopy and diffusible iodine-based contrast-enhanced computed tomography to quantitatively assess the theoretical hydrodynamics of olfaction. The results show that both species have their olfactory chambers distant from the tip of the snout, possess a thick boundary layer above the nostrils, a circular olfactory rosette, and a radiating arrangement of olfactory lamellae. A comparison between the two species reveals that T. maccoyii has a long inter-nostril distance (20.05 mm), small nostril openings (incurrent: 0.13 mm 2 , excurrent: 4.88 mm 2 ), a horizontally long olfactory chamber, and a relatively small ventilatory sac (209.81 mm 3 ), while I. indica has a short inter-nostril distance (3.60 mm), large nostril openings (incurrent: 4.50 mm 2 , excurrent: 8.15 mm 2 ), a vertically long olfactory chamber, and a large ventilatory sac (308.14 mm 3 ). The morphological similarities between the two species suggest olfactory adaptations for a pelagic lifestyle, while the differences highlight the adaptations suitable for the unique hydrodynamic conditions associated with different swimming speeds. • Thunnus maccoyii have a thick, turbulent boundary layer at their anterior nostrils • The olfactory chamber of Istiompax indica is structurally distinct from T. maccoyii • Both species have circular olfactory rosettes with a radial arrangement of lamellae • Juvenile and mature T. maccoyii have the same average number of olfactory lamellae • Morphological adaptations in both species are optimised for hydrodynamic conditions