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

## TL;DR

This study shows that the spiracular organ in cartilaginous fish develops from a unique placode, distinct from the lateral line system, and is related to the paratympanic organ in birds and reptiles.

## Contribution

The paper identifies a new neurogenic placode in cartilaginous fish and establishes its evolutionary relationship to the paratympanic organ in amniotes.

## Key findings

- The spiracular organ in little skate develops from a neurogenic placode dorsal to the geniculate placode.
- The spiracular organ is spatially and molecularly distinct from lateral line placodes.
- Afferent neurons of the spiracular organ are located in the geniculate ganglion, similar to the paratympanic organ.

## Abstract

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.

Summary: Fate mapping in a cartilaginous fish supports the evolutionary independence from the lateral line system of the mechanosensory spiracular organ, and its homology with the amniote paratympanic organ.