Abstract
The olfactory organ of Chondrichthyes is characterized by a central support with several lamellae covered by a sensory olfactory epithelium. Although secondary folds are present on the lamellae in all the chondrichthyan species analyzed to date, their shape and size have not been described. We here analyze the olfactory organ of 13 elasmobranch and 1 holocephalan species, describe the shape of the secondary folds and evaluate how they contribute to the epithelial surface area. The secondary folds vary in shape and size, and they should always be considered when comparing the epithelial surface area among species; in fact, the increase of the area, due to the presence of the secondary folds, spans from 70 to 495% in the considered species. Because of the complexity of the shapes, we approach the description of the secondary folds by analyzing histological sections of the olfactory lamellae to obtain illustrative silhouettes. We introduce two indexes in order to describe a 2D-sectioned shape of the secondary folds. Considering the different numerical parameters which describe the morphology of the olfactory organ (secondary folds included), a principal component analysis elucidates the possible ecological role and phylogenetic relationship of the chondrichthyan olfactory organ.
Highlights
Olfaction plays important roles in feeding (Gardiner et al, 2014), mating (e.g., Kajiura et al, 2000), and homing (Gardiner et al, 2015) of Chondrichthyes
The olfactory organ size, as well as the surface area of the largest lamellae in the mid part of each olfactory organ were measured according to Figures 1C,D and are reported in Table 2, together with the lamellar number (Ferrando et al, 2017b), and the corresponding gross surface area
The non-sensory epithelium covered the edge of the primary lamellae, as described by Schluessel et al (2010), while the sensory epithelium was observed, in all the specimens, to cover the face of the primary lamellae, including the secondary folds
Summary
Olfaction plays important roles in feeding (Gardiner et al, 2014), mating (e.g., Kajiura et al, 2000), and homing (Gardiner et al, 2015) of Chondrichthyes. Chondrichthyes share complex olfactory organs, yet they are characterized by a noteworthy morphological variability among species (e.g., Schluessel et al, 2008, 2010; Meredith and Kajiura, 2010). The number of primary olfactory lamellae does not vary ontogenetically (Theiss et al, 2009) nor with body size, but ranges between about 30 to 700 lamellae among species (Ferrando et al, 2017b). Anatomical features, such as lamellar number and lamellar surface area, have been used to investigate the relationship between form and function of the olfactory organ. The lamellar number and surface area have been correlated to life history traits and habitat use, e.g., bentho-pelagic or pelagic life modes (Schluessel et al, 2008; Meredith and Kajiura, 2010)
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