Bony labyrinth morphometry reveals hidden diversity in lungless salamanders (Family Plethodontidae): Structural correlates of ecology, development, and vision in the inner ear

Abstract

Lungless salamanders (Family Plethodontidae) form a highly speciose group that has undergone spectacular adaptive radiation to colonize a multitude of habitats. Substantial morphological variation in the otic region coupled with great ecological diversity within this clade make plethodontids an excellent model for exploring the ecomorphology of the amphibian ear. We examined the influence of habitat, development, and vision on inner ear morphology in 52 plethodontid species. We collected traditional and 3D geometric morphometric measurements to characterize variation in size and shape of the otic endocast and peripheral structures of the salamander ear. Phylogenetic comparative analyses demonstrate structural convergence in the inner ear across ecologically similar species. Species that dwell in spatially complex microhabitats exhibit robust, highly curved semicircular canals suggesting enhanced vestibular sense, whereas species with reduced visual systems demonstrate reduced canal curvature indicative of relaxed selection on the vestibulo‐ocular reflex. Cave specialists show parallel enlargement of auditory‐associated structures. The morphological correlates of ecology among diverse species reveal underlying evidence of habitat specialization in the inner ear and suggest that there exists physiological variation in the function of the salamander ear even in the apparent absence of selective pressures on the auditory system to support acoustic behavior.

Ecology drives patterns of spectral transmission in the ocular lenses of frogs and salamanders

“…Further study of salamanders, including scansorial species (e.g. several plethodontids: Capshaw et al., 2019; Waldron & Humphries, 2005), is needed to make any generalizations about lens transmission in this group. Lens transmission data are unavailable for the third major lineage of extant amphibians, the mostly fossorial caecilians (order: Gymnophiona), though they have reduced visual systems and we expect their lenses to lack filtering pigments.…”

Ontogenetic variation in the crocodylian vestibular system

“…Significant differences in labyrinth morphology have been noted between secondarily fully aquatic (pelagic) amniote species and their more terrestrial or semi‐aquatic ancestors (Neenan et al, 2017; Schwab et al, 2020; Spoor et al, 2002). Other differences have also been recognised within taxonomic groups associated with different lifestyles and hunting strategies (Capshaw et al, 2019; Pfaff et al, 2015; Schwab et al, 2019). This raises the question: if labyrinth size and shape changes across phylogeny were linked to changes in habitat or behaviour, do labyrinth size and shape changes across ontogeny correlate with behavioural or biological shifts in crocodylians?…”

Semicircular canal shape diversity among modern lepidosaurs: life habit, size, allometry

“…Previous studies in different clades have suggested that aspects of inner ear morphology converge with ecology [ 5 , 14 , 17 , 18 , 24 ]. Our results from a sample of 94 species across squamates with legs do not support that hypothesis when controlling for the effects of phylogenetic relatedness and size.…”

“…Within the category of fossorial taxa, other studies have recovered a specific type of inner ear morphology associated with fossorial taxa in amphibians [ 23 , 24 ] and snakes [ 17 ]. That fossorial morphology is primarily characterized in amphibians and squamates by a notably large vestibule, increased ellipticity (described as anterior–posterior elongation), and is usually found among taxa with leg or eye reduction.…”