Axolotl: Most vertebrates descended from ancestor with sixth sense

Using the Mexican axolotl as a model to represent the evolutionary lineage leading to land animals, and paddlefish as a model for the branch leading to ray-finned fishes, the researchers found that electrosensors develop in precisely the same pattern from the same embryonic tissue in the developing skin, confirming that this is an ancient sensory system.

People experience the world through five senses but sharks, paddlefishes and certain other aquatic vertebrates have a sixth sense: They can detect weak electrical fields in the water and use this information to detect prey, communicate and orient themselves.

A study in Nature Communications that caps more than 25 years of work finds that the vast majority of vertebrates - some 30,000 species of land animals (including humans) and a roughly equal number of ray-finned fishes - descended from a common ancestor that had a well-developed electroreceptive system.

This ancestor was probably a predatory marine fish with good eyesight, jaws and teeth and a lateral line system for detecting water movements, visible as a stripe along the flank of most fishes. It lived around 500 million years ago. The vast majority of the approximately 65,000 living vertebrate species are its descendants.

"This study caps questions in developmental and evolutionary biology, popularly called 'evo-devo,' that I've been interested in for 35 years," said Willy Bemis, Cornell professor of ecology and evolutionary biology and a senior author of the paper. Melinda Modrell, a neuroscientist at the University of Cambridge who did the molecular analysis, is the paper's lead author.

Hundreds of millions of years ago, there was a major split in the evolutionary tree of vertebrates. One lineage led to the ray-finned fishes, or actinopterygians, and the other to lobe-finned fishes, or sarcopterygians; the latter gave rise to land vertebrates, Bemis explained.

Some land vertebrates, including such salamanders as the Mexican axolotl, have electroreception and, until now, offered the best-studied model for early development of this sensory system. As part of changes related to terrestrial life, the lineage leading to reptiles, birds and mammals lost electrosense as well as the lateral line.

Some ray-finned fishes - including paddlefishes and sturgeons - retained these receptors in the skin of their heads. With as many as 70,000 electroreceptors in its paddle-shaped snout and skin of the head, the North American paddlefish has the most extensive electrosensory array of any living animal, Bemis said.

Until now, it was unclear whether these organs in different groups were evolutionarily and developmentally the same.

Using the Mexican axolotl as a model to represent the evolutionary lineage leading to land animals, and paddlefish as a model for the branch leading to ray-finned fishes, the researchers found that electrosensors develop in precisely the same pattern from the same embryonic tissue in the developing skin, confirming that this is an ancient sensory system.

The researchers also found that the electrosensory organs develop immediately adjacent to the lateral line, providing compelling evidence "that these two sensory systems share a common evolutionary heritage," said Bemis.

Researchers can now build a picture of what the common ancestor of these two lineages looked like and better link the sensory worlds of living and fossil animals, Bemis said.

Fish-like sensors mimics lateral line

A PRESSURE sensor that mimics the way a fish's lateral line works could help submersible craft navigate.

The technology could improve underwater robots' ability to detect hazards, such as deep sea vents and shipwrecks, when the water is too murky for a camera to work effectively, or the object is too close for sonar, says Douglas Jones at the University of Illinois at Urbana-Champaign.

The lateral line is a sense organ that runs along the sides of most fish and enables them to detect changes in water pressure. This allows fish to sense depth and the direction of the current, and also means they can swim in synchronised schools even in darkness.

The line is peppered with hair cells called neuromasts that fire in response to pressure waves. The pattern of signals generated along the line allows the fish to pick out the tiniest flick of a nearby tail.

Jones, alongside Chang Liu at Northwestern University in Evanston, Illinois, made an artificial neuromast by adding boron to a 500-micrometre-long silicon hair to create a stress-sensitive resistor. As the hair bends in response to water motion, its resistance changes, allowing the force of the water's movement to be calculated.

The researchers tested the sensor by fixing an array around a plastic pipe and measuring the response to various objects placed in the water, including a live crayfish. The animal's wiggling legs generated a pattern of pressure signals that enabled the team to calculate its precise location in relation to the pipe. Because the hairs in the array are oriented at right angles to each other, the direction of the water motion can be easily determined (see diagram).

Finally, the distance to the object can be worked out in the same way a fish would, by moving water in the direction of the object and timing how long it takes to receive an echo (Bioinspiration and Biomimetics, DOI: 10.1088/1748-3182/5/1/016001). "We're hopeful it will be useful for making sure any underwater vehicle doesn't get trapped by unseen obstacles," says Jones.

"Anything that fortifies an undersea robot's sense of its environment, and its ability to avoid obstacles, is to be welcomed," says Andy Bowen of Woods Hole Oceanographic Institution in Massachusetts. Bowen designed Jason Junior, the remote-controlled robot sub that famously imaged the wreck of the Titanic in 1986. "In the search for new deep sea vents, I can imagine this artificial lateral line autonomously sensing a hydrothermal plume and using an on-board algorithm to home in on the source of it. It's a fascinating technology," he says.