Sharks' Electro-Sensing Organs Linked to Human Features?

Maryann Mott
for National Geographic News

February 14, 2006

Hunting sharks have the remarkable ability to sense the electric energy
generated by prey. Using special head organs, the predators can detect
even the slightest muscle twitch of a flounder buried in sand.

But the cellular origin of sharks' electrical "ESP" has remained a mystery—until now.

Scientists at the University of Florida and the University of Louisiana have identified specialized cells as the sources of sharks' powers.

Embryonic neural crest cells have the ability to become many different structures. (Read an excerpt from a National Geographic magazine article on embryonic stem cells.)

In humans, for example, these cells give rise to head and facial features.

Using molecular tests of shark embryos, the researchers found genetic evidence of neural crest cells in the animals' electricity-sensing organs.

The findings support the idea that the common ancestor of all vertebrates (animals with spinal columns) also detected electric fields. But over time mammals, reptiles, and birds lost the ability, the theory goes.

"Our work is really the first demonstration of the embryonic origin of these organs, and it gives us some insight into how they arose during evolution," said Martin Cohn, a developmental biologist at the University of Florida Genetics Institute in Gainesville.

The discovery is reported in the current edition of the journal Evolution and Development.

Good Sense

Cohn, along with lead study author Renata Freitas, looked for genetic evidence of neural crest cells in embryos of the lesser spotted catshark, a species that mostly hunts at night. The team found indications of the cells in the embryos' electro-sensory organs.

The scientists believe that during development neural crest cells migrate from the sharks' brains into various regions of the head. There the cells create the framework for the electro-sensory system.

The process is similar to the development of the lateral line, a sensory organ in fish. The lateral line allows the animals to sense environmental conditions, such as temperature.

Scientists suspect that as ancient vertebrates emerged from the sea, they eventually lost their lateral lines as well as their ability to sense electric fields.

Today only a few species, such as sharks, sturgeons, and lampreys, have electro-sensing capabilities.

"You can imagine how valuable this system would be if you were aquatic, because water is so [electrically] conductive," said James Albert, a study co-author.

But sensing electric signals doesn't work as well on land, since air isn't a good conductor of electricity.

"When it happens [on land], it's called a lightning bolt, and you don't need special receptors to sense it," Albert said.

Dyeing to Know

Sebastian Shimeld, a zoology professor at the University of Oxford in England, is excited by the findings.

"It adds electro-sensation to the list of neural crest-derived novelties that appear to have been of fundamental importance for the early evolution of the vertebrates," said Shimeld, who was not involved in the study.

But Glenn Northcutt, a professor of neuroscience at the University of California, San Diego, is skeptical about the claim that neural crest cells give rise to electro-receptors.

More tests are needed, he said.

"It still requires a definitive experiment, where the developing neural crest cells are marked with dye, the embryo develops, and the dye clearly shows up in the electro-receptors," Northcutt said.

Dye tests are a classic method for mapping cell movements. The tests have been used to examine the origins of limbs and brain cells and the activities of cancer cells.

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