Comparative Morphology of Plethodontid Olfactory and Vomeronasal Organs: How Snouts Are Packed

Abstract:  Size and shape of sensory organs are features, among others, that can affect sensory functioning. Natural selection can act on such features to optimize performance in particular environments and for particular life histories and behavioral repertoires, although selection is not the only force influencing a particular structure. I examined olfactory and vomeronasal organ morphology in several species of salamanders in the Family Plethodontidae ranging from completely aquatic, semiaquatic, terrestrial, to arboreal. The Plethodontidae is the most speciose and diverse urodele group, making it a particularly interesting subject of evolutionary study. I examined microanatomy with the use of serial sections of complete snouts of species from the Appalachians and Neotropics, the two centers of plethodontid diversity. All species, except the aquatic Desmognathus marmoratus, had thick olfactory epithelia very anteriorly in the snout, especially mid-point in the main olfactory chamber, which may optimize odorant delivery along the path of airflow in through the external nares and out through the internal nares. Desmognathus marmoratus had greatly reduced olfactory epithelia. All species had well developed vomeronasal organs, particularly Desmognathus species, indicating the importance of this system for plethodontids. Small species had nasal chambers organized to optimize space for both olfactory and vomeronasal epithelia, and all Neotropical species had particularly thick sensory epithelia; these may be compensatory mechanisms to ensure adequate sensory receptor cell numbers in small snouts and/or with large cell sizes. A number of different gland types, including nasal, intermaxillary, nasochoanal, nasolabial, vomeronasal, and orbital, occupy a large and variable space within rostra, and their functions warrant further investigation. Chemoreception and vision are the two most important sensory modalities for plethodontids, and the simpler organization of chemosensory systems compared to vision, and compared to chemoreception in other vertebrates, make plethodontids attractive subjects in the study of the evolution of chemosensory systems.