The skate spiracular organ develops from a unique neurogenic placode that is distinct from lateral line placodes

The spiracular organ is an epithelial pouch or tube lined with mechanosensory hair cells that is found embedded in the wall of the spiracle in many non-teleost jawed fishes. It is innervated via a branch of the anterior lateral line nerve and usually considered a specialised lateral line organ, despite its presumed function as a proprioceptor for jaw movement. It is homologous to the paratympanic organ: a hair cell-lined epithelial pouch embedded in the wall of the middle ear of birds, alligators and Sphenodon. A previous study showed that the chicken paratympanic organ and its afferent neurons originate from a molecularly distinct placode immediately dorsal to the geniculate placode. Here, fate mapping in a cartilaginous fish (little skate, Leucoraja erinacea) shows that the spiracular organ derives from a previously unrecognised neurogenic placode immediately dorsal to the geniculate placode that is spatially and molecularly distinct from lateral line placodes. Retrograde labelling of the spiracular organ identified afferent neurons located within the geniculate ganglion, as reported previously for paratympanic organ afferents. These findings support the independence of this unique jawed-vertebrate mechanosensory organ from the lateral line system.