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Cetacean Inner Ear

Periodic Reporting for period 1 - Cetacean Inner Ear (Cetacean Inner Ear)

Reporting period: 2017-06-01 to 2019-05-31

The morphological study of cetacean cochlea, as well as the possible alterations associated to sound exposure, represent a key conservation issue to assess the effects of noise pollution on marine ecosystems. Noise pollution can affect cetaceans by causing lesions in their inner ear, which can be severe enough to induce the individual to strand and die. In cases of severe permanent hearing loss, supporting cells of the organ of Corti (hearing organ) replace apoptotic sensory cells forming a “scar”. Sensory cells (or hair cells) of the inner ear are responsible for transducing the mechanical sound wave into an electric signal allowing the transmission of the auditory information to the brain. Sensory cells are also in charge of enhancing auditory sensitivity and frequency selectivity. The objectives of this study consist in conducting a comparative morphological analysis of the ultrastructure of the cochlea from stranded cetaceans 1) to assess their species specific hearing sensitivities by creating cochlear frequency maps and 2) to investigate possible lesions as a consequence of sound exposure. Once the cochlear frequency map for a species is resolved, it will be possible to estimate the acoustic characteristics of a source that may have caused these lesions. In addition, this study 3) investigated the process of hair cell apoptosis and the subsequent ‘scar’ formation to be able to distinguish newly formed lesions from old ones. This is particularly interesting to be able to assess if a hearing impairment could have been related to the stranding. Toothed whales have the possibility to attenuate their hearing sensitivity when a loud sound is played after a warning sound. Our research also focused on 4) examining the potential elements that might play a role in this protective mechanism. This research filled some gaps on hearing in cetaceans, brings tools to the decision makers to better regulate marine activities and contributes with essential knowledge for assessing the effects of noise pollution on cetacean hearing.
Dr Morell has analyzed the ears of 32 individuals from 10 cetacean species during the timeframe of the Cetacean Inner Ear project. Some ears from harbor porpoise and beluga whale were processed for scanning electron microscopy. We used geometric morphometrics measurements from scanning electron micrographs of the organ of Corti from 10 locations of the cochlear spiral in harbor porpoises, beluga whales, mustached bats, rats, mice and gerbils. We used the data of echolocating bats and rodents (species of known frequency maps) to train machine learning techniques to further predict the cochlear frequency map for harbor porpoise and beluga whale.

The main research results of the project are: 1) the predictions of cochlear frequency maps for cetacean species such as harbor porpoise and beluga whale based on the morphometrics of the organ of Corti, as well as 2) the optimization of a protocol to analyze inner ears allowing the detection of potential cases of noise-induced hearing loss. Immuofluorescence protocol allows us to combine several antibodies to label the cells of the organ of Corti and type I afferent innervation, which transmits the auditory information. The new protocol using immunofluorescence developed in this project will allow us to distinguish between newly formed lesions from old ones.

We disseminated the results of this project through different platforms, ranging from public lectures, Facebook and online media publications, and newspapers addressed to general public, to presentations in workshops and international conferences for scientific community, industry and regulators. During the duration of the project Cetacean Inner Ear, Dr Morell published 5 peer-reviewed articles (1 is currently submitted, and 3 in preparation from this project), 1 book chapter, 3 case reports, 8 invited talks, 5 oral and 4 poster presentations in international conferences.
We have contributed substantially to the state of the art in this field since we have shown that there is a relationship between shape of the cells of the organ of Corti and encoding frequency in rodents. In addition, the relationship shape-frequency is consistent among several species of terrestrial mammals. Finally, we can use the predictive model created in the Cetacean Inner Ear project to predict the cochlear frequency map of potentially any species of mammal. Thus, we will possibly be able to predict the hearing range of other mammalian species whose audiograms have not been measured. This is the first time that we have been able to predict the cochlear frequency map for harbor porpoise and beluga whale using morphometrics of the cells of the organ of Corti. This discovery will fill a gap on the field of hearing in marine mammals and effects of underwater noise.

If a lesion is found in a cochlea from harbor porpoise or beluga whale using the methodology optimized in the current project, we will be able to determine which are the frequencies that are impaired. In cases of noise-induced hearing loss, we will be able to then identify the potential sources that have trigged this lesion since damage due to noise exposure is frequency dependent. Thus, we are now able to provide tools to other researchers and policy makers to improve our understanding on the effects of noise pollution in the hearing system of cetaceans.