The survival of all animals is based on their ability to detect and process a range of environmental stimuli, which ultimately gives rise to complex behavioral responses. Given the diversity of chordates and the range of ecological niches they inhabit, it is possibly not surprising that vertebrate sensory systems have evolved a range of sophisticated means of processing environmental information, such as light and sound. However, for species that inhabit complex and/or extreme sensory environments, there are heightened selection pressures for increasing sensitivity to detect and extract subtle features of environmental signals. The effectiveness of this sensory arms race ultimately dictates each species’ strategies for finding food and mates, avoiding predators, socially communicating and successfully navigating their environment (Collin, 2012; Elmer et al., 2021).

The aim of this Research Topic was to provide a current update on the breadth and depth of comparative sensory research to improve the understanding of the diversity of sensory systems in chordates and to complement an earlier Research Topic that was focussed on aquatic vertebrates.

The three articles that were accepted under this topic provided excellent examples of how light and sound are sampled by the eyes (phototransduction) and the inner ear (mechanotransduction), respectively. Further, the authors discuss how the receptor arrays in each sensory modality are organized to optimize sensitivity within both visual and auditory (three dimensional) spaces.

In Wagner et al. (2022), the tubular eyes of deep-sea opisthoproctid spookfishes are revealed to contain image-forming retinal diverticula, often combining both reflective and refractive optics, which appear to be more anatomically and optically complex than all other vertebrate eyes discovered to date. Together with eye and body movements, these ocular adaptations direct ventrolateral illumination into the dorsally-directed tubular eyes to extend each species’ visual field, presumably to detect the approach of predators in the photon-limited mesopelagic zone of the open ocean. In this detailed investigation of six species of “barrel-eyed” spookfishes, the authors also present a credible case regarding the evolution of such complex eyes in which tubular eyes with image-forming diverticula could have evolved from conventional laterally-positioned eyes via a series of intermediate steps (Nilsson and Pelger, 1994). This article exemplifies the intense selection pressures on the visual systems in animals that inhabit extreme environments in order to maximise sensitivity to both dim downwelling sunlight and bioluminescent emissions that are frequently present in the mesopelagic zone (Locket, 1977).

Along similar lines, Clarke and Taylor (2023) employed multiple hidden Markov models (HMMs) to uncover the rich diversity of visual photopigments (or opsins) from the UniProt Reference Proteomes database that is constructed from translated genome sequences and represents nearly 1,400 eukaryote species and 24 million sequences. The authors concentrated their efforts on opsins or G-protein-coupled coupled receptors (GPCRs) that have roles in phototropism, entrainment of circadian rhythms and vision. Their model retrieved over 2,000 opsin sequences from 262 species, thereby establishing a consensus classification system and allowing for broader taxonomic comparisons. In recent years, there has been a large number of opsin types discovered, e.g., melanopsin, pinopsin, neuropsin, teleost multi-tissue opsin, peropsin and gluopsin (a GPRC mainly found in various dragonfly and butterfly species that is closely related to retinal G protein coupled receptors (RGRs) and retinochromes), in addition to the more classical retinal photoreceptor opsin. Many of these were named based on the tissue or cell-type in which they were first found, while others have been classified according to their phylogenetic history, function, or even after the signal transduction pathway through which they operate. HMMs is a powerful tool that describes existing amino acid sequence alignments to which all other sequences can be subsequently scored and aligned in a consistent and reproducible way. Interestingly, the actinopterygian fishes have the highest fraction of opsins among the different classes of chordates (6.9% of all seven transmembrane receptor sequences), while the arthropodan possess had the highest percentage of opsins (11.4%) within the invertebrates, which reflects the high levels of diversity within their species and habitats. The need for a new nomenclature has also recently been recognized for chordate photoreceptor types based on their evolutionary history, where a naming system that refers to orthologous cell types across a diversity of species has been lacking (Baden et al., 2025). As conducted previously for other retinal cell classes, the classification is informed by functional, anatomical, developmental and molecular identities of the whole neuron, including the opsin gene that each photoreceptor type expresses.

The last article in this series is also an evolutionary investigation but, in contrast to the previous articles that concentrate on the diversity and evolution of maximizing visual sensitivity in a range of light environments, this study provides insight into how sound is detected and localized in a representative of the earliest jawed vertebrates, i.e., the New Zealand carpet shark Cephaloscyllium isabellum (Elasmobranchii). Sauer et al. (2022) reveal that the size of the maculae and the number of hair cells increased during development in the saccular, lagenar and utricular maculae (up to a three-fold), but not in the macular neglecta. However, the orientation of the hair cells remained consistent during ontogeny with all maculae exhibiting a bi-directional orientation pattern, where the hair cells were split into two groups with opposing (180° difference) orientations. Increases in hair cell number is thought to enhance auditory (and vestibular) sensitivity, while the orientation (polarity) of the hair cells provides insight into how this species localizes sound. Although there are only a few ontogenetic studies of the auditory and vestibular systems in this group of cartilaginous fishes, it appears that the hair cell orientation patterns vary across the Chondrichthyes (i.e., in other sharks, but also in rays, skates and chimerids), especially within the macular neglecta, a (non-otolithic) organ that serves an important role in sound detection (Corwin, 1981). This level of diversity has been borne out in a more recent study on the macula neglecta, that found it to be relatively small, with low hair cell density in the Port Jackson shark Heterodontus portusjacksoni. Further, this is consistent with this species’ benthic lifestyle, which suggests that suggesting that this species relies more on substrate-borne vibrations and spatial stability than on acute directional hearing, thus aligning with its ecology in wave-exposed, rocky reef habitats (Robins et al., 2025).

In summary, the Research Topic ‘Biodiversity of Sensory Systems in Chordates’ is a broad and growing theme of research focus. A comparative approach will also be necessary to investigate the sensory vulnerabilities of chordates in response to potentially severe future anthropogenic pressures.