Novel auditory evoked potential recordings in a holocephalan species, the spotted ratfish ( Hydrolagus colliei )

Chondrichthyans, or cartilaginous fishes, have existed for over 450 million years and possess one of the earliest-evolved auditory systems. Studying these fishes can potentially offer insights into the foundational structures and function of auditory systems across vertebrate lineages—including amphibians, reptiles, birds, and mammals. Chondrichthyans include two subclasses: Elasmobranchii (sharks, skates, and rays) and Holocephali (chimeras). While auditory studies have been conducted in elasmobranch species, no previous studies have characterized the auditory capabilities of the only extant representatives of holocephalans, chimeras. Here, we present the first auditory measurements in a holocephalan species, the spotted ratfish (Hydrolagus colliei), native to the eastern Pacific Ocean. An adaptive auditory evoked potential (AEP) acquisition technique allowed time-efficient measurement in a species that is sensitive to handling. Further, using a custom four-channel electrode array, we obtained evoked responses from multiple cranial sites. Preliminary results suggest that spotted ratfish are most sensitive to low frequency sounds, broadly similar to auditory sensitivity reported for elasmobranchs. Differences in AEP waveform morphology and spectral characteristics were observed across recording sites. These results establish an important baseline for understanding auditory function in holocephalans and contribute to broader comparative analyses of vertebrate hearing evolution.