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How do you Make Fish Electric?

 


Back to Biology for a bit:

 Do you ever wonder why two very different kinds of species share a common trait? Why do birds and bats both have wings if bats are mammals?  Whenever two organisms share the same characteristic there are two possible pathways to explain it. They are "divergent evolution" and "convergent evolution.”

 The word divergent means spreading out or radiating from a common point. Divergent evolution then is where a common ancestor gives rise to a number of species that share some of the same traits. For example, humans have hair and mice have hair. Somewhere in our ancestry there was an animal that had hair which gave rise to both lines. That does not mean we came from mice, but rather there was a common ancestor that had hair and from that animal sprung the lines of many of the creatures that have hair.

 The word convergent means approaching the same point from different directions. So convergent evolution is where two unrelated organisms independently evolve the same trait.  A good example of this would be fins. Both whales and fish have fins, and yet whales evolved from land animals that walked on legs. When the marine mammals took to water the legs eventually evolved into structures that look very much like the fins of fish. It is not that a common ancestor with fins gave rise to both whales and fish. They both have fins because in the environment where they live (water) fins are an advantage over other kinds of appendages.

 I think one of the most interesting examples of convergent evolution is the case of electric organs in fish. Organs that produce electricity have evolved no less than six separate times in completely different groups of fish. In the bony fish, most are freshwater species: the knifefish of South America, the family Gymnotidae (which includes the electric eel), the electric catfish of Africa, and the elephantfish. In the cartilaginous fish electric organs have evolved in the torpedo rays, family Torpedinidae, and the stargazers (saltwater).

 Electric organs originally evolved, not as a defense mechanism, but as a sensory device.  Most places where we find electric fish are in murky rivers and streams where visibility is very limited. In the course of evolution, some fish in these waters developed muscles that were modified enough so that they could produce a weak electromagnetic field outside of their body. Some of these fish discovered a way to read this field to "see" through the murky water. All of the groups of the electric fish probably started out like this. 

 Some of these fish then went a step further. The electric organs gradually got larger and larger and eventually reached the point that they could stun or even kill prey. The grand champion of electric fish is the electric eel (Electrophorus electricus). This fish lives in the Amazon. Electric eels can generate up to 600 volts. The electric organs make up 80% of this fish's body. Ironically, electric eels have evolved their electric organs to such a point that they can no longer generate the steady weak field needed for sensory use.

 I live in the Pacific Northwest, and we do have an electric fish off our coastline. It is the Pacific Electric Ray Torpedo californica. This member of the torpedo genus ranges from British Columbia to Baja California (note: electric rays should not be confused with Stingrays. Stingrays have a barbed tail with a "stinging" venom and are not electric). The Pacific electric ray can deliver over 200 volts and have been known to stun divers. They have a circular body with a large tail.   As with other electric fish, the electric organs consist of muscle that is layered in plates like a battery. They live in relatively shallow water and feed on fish such as herring and halibut. (The naval weapon “torpedo” was named after these fish, not the other way around.)

 When a trait evolves separately five times, that tells us that it must be a rather successful trait and one that is not insurmountable to attain. A simple change in muscle development produced a new sensory organ. If it is so successful, why are there no land animals with electric organs?   Probably because an electric field can be generated much more easily in water than in air. Sorry Magneto, your powers fall in the realm of superhero fantasy.

 

References:  The  Audubon Society Field Guide to  North  American Fishes,  Whales  and Dolphins, Alfred A. Knopf, Inc.,  New  York, 1983.


Star Liner

 

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