A recent study led by Benedict King And John Long and published in the journal Palaeontology, details how this electroreception may have evolved in placoderm fishes, the evolutionary ancestor of all modern sharks.
The sharks’ electroreceptors, also known as ampullae of Lorenzini, are jelly-filled tubes that open on the surface of sharks’ skin. Inside, each tube ends in a bulb known as the ampulla.The jelly in the tube is highly conductive, which allows the electrical potential at the pore opening to be transferred to the ampulla at the base of the tube. Voltage differences across the membrane lining each ampulla then cause nerves to be activated, sending signals to the brain.
According to the authors:
High-resolution CT scans allowed us to “digitally dissect” well-preserved fossils and reveal sensory systems preserved inside the bones. One of the best known sensory systems in fossil fishes is the lateral line system, which detects pressure changes in water. Fishes use this system to change direction as a group without crashing into each other when swimming in a shoal.
But surrounding the lateral line system in some fossil fishes was another series of small holes. CT scans revealed that their internal structure was similar to electroreceptors in living fishes, and the position of the pores matches the distribution of electroreceptors in living lungfishes.
A complex system of branching tubes (below) appears to have supplied nerves to the electroreceptors.
These ancient electroreceptor systems appear to have been particularly elaborate in fossil lungfish. Lungfish are an ancient group, which still survive in Australia, Africa and South America. Exceptionally preserved 400 million year old fossil lungfish from Australia had snouts that were covered in a dense array of these electroreceptors.
Other fossils from the same time period show that electroreceptor systems may have been quite diverse. For example, another ancient fish, related to the ray-finned fishes called Ligulalepis, has a series of large pits that widen at the base, which may represent clusters of electroreceptors.
It now appears that during the early evolution of vertebrates, electroreceptor systems were diverse and went through a period of experimentation.
Sensory systems that were like the precursor to these electrosensory systems found in modern sharks were found in lungfish species dating back to 420 million and 360 million years ago.