How is a Bird Like a Fish? Rare Organ to Detect Spatial Position Suggests a Link

Skimming the treetops and slicing through the ocean, birds and fish are worlds away from each other. But recent attention to a sharks’ developmental genes suggests that this group of bony fishes shares a common evolutionary ancestor with birds. The common link? An obscure sense organ.

This organ, known as the spiracular organ, sits behind the jaw in sharks and skates and may provide sensory information about the jaw position and movement. Its many hair cells line the wall of the organ’s spongy connective tissue, perking up each time the jaw opens.

Birds, too, have a sensory organ that signals information about their location as they soar and dive through the air. This paratympanic organ is associated with middle ear activity and acts as a barometer: Its many hair cells detect changing atmospheric pressure, providing real-time updates about the bird’s position in its environment.

Andrew Gillis, associate scientist at the MBL, recently published new findings linking the shark’s spiracular organ to the bird’s paratympanic organ. According to the research team, there are shared developmental origins between these two sensory organs. 

During embryonic development, the bird’s paratympanic organ emerges from a distinct, thickened patch of skin, known as a placode. In their report, published in Development, the team added to these previous findings, showing that the spiracular organ also develops from a corresponding placode. What’s more, researchers found that the shark’s spiracular organ shares precise patterns of gene expression and innervation with the bird’s paratympanic organ, Gillis said. 

According to Gillis, these findings imply that there was an additional sense organ in the last common ancestor of all jawed vertebrates – one that was lost in most, but conserved in sharks, skates, and birds.  

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This research was funded by the National Science Foundation, Royal Society University Research Fellowship, the University of Cambridge Isaac Newton Trust, and the Plum Foundation John E. Dowling Fellowship and the Laura and Arthur Colwin Research Fellowship at the Marine Biological Laboratory.