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,” says 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 explains. 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 says.
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,” says 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 says.
SOURCE – Cornell University
