elephantfish
August 5, 2015

Move over Garmin: African fish species have their own built-in fish finders

A family of African fish known as mormyrids communicate through a unique series of electric discharges generated by an organ in their tails, and some of them can gather detailed information about the creature from which the signal originated from its waveform.

In fact, some of the 200 species that make up this fish family are able to determine the species, sex, age, relative dominance status, emotional state and perhaps even identity, while others lack this ability to perceive differences in waveform variations, experts from Washington University in St. Louis explained in new research published Tuesday in the journal eLife.

While the fish with this ability have been the subject of previous research, those without the enhanced communication system are less well understood – which is what led Dr. Bruce Carlson and graduate student Christa Baker to analyze these creatures in an attempt to discover why they perceive things differently than the more receptive members of their family.

Receptors in the skin are distributed differently

Dr. Carlson and Baker analyzed the sensory receptors and found that the less-perceptive types of mormyrids encode signals far differently than their more discriminating counterparts. In addition the receptors of the less discriminating fish tended to be more tuned to the collective signals from schools of shoals of fish rather than those given up by a single creature.

fish finders

“There had to be a neural correlate for the perceptual differences,” Dr. Carlson explained in a statement, “so we looked to see if something was happening 'out in the periphery' where the signals are originally detected and encoded for processing in the brain... As far as we know, this is the first time anyone has found a receptor tuned to group communication signals rather than those coming from individuals.”

These weakly electric fish possess sensory receptors known as knollenorgans in their skin, which they use to detect pulses given off by the fish surrounding them. These receptors can be found all over the bodies of the more discriminating fish, but only in three clustered areas on both sides of the head in the less-perceptive ones. The receptors that were broadly distributed produced spikes, the authors said, while the clustered ones produced oscillations at a constant frequency.

Oscillating receptors detect lower frequencies than spiking ones

Baker explained that when the oscillating receptors received a signal, they reset their oscillations to a specific point in the cycle, which briefly synchronized the oscillations of different receptors. Dr. Carlson added that this is first sensory receptor known to encode stimuli by basically hitting the reset button on its ongoing oscillation phase. Further investigation revealed that the two types of receptors do not encode the same information.

The spiking receptors, the researchers explained, fire a nerve impulse when there is an upward or downward excursion in the signal and are good when it comes to encoding precise timing cues in signaling waveforms. Oscillating receptors, however, only encode the onset of the signal and its point of origin. Spiking receptors are more sensitive to frequencies close to those that comprise the signals of other members of their species, while oscillating ones pick up lower frequencies, which are used to help fish detect and locate large groups of fish.

So why are the two groups of mormyrids so different?

Dr. Carson explained that their “best guess” is that “differences in social behavior, in social organization, selected for differences in sensory capacities. Based on what I've seen in the field, the fish able to perceive small differences in the communication signals tend to be solitary and territorial, whereas the fish whose receptors are tuned to group signals tend to be more gregarious.”

“My guess is that these two different lifestyles place very different selective pressures on communication,” he added. “If you're solitary and territorial and you detect another fish in the area, you want to know exactly who that fish is. Is it a potential competitor, a potential mate, or a different species you're not worried about? On the other hand, if you're living in a shoal or school of fish, it's not so important to identify individuals. Just sticking with the group is going to be a successful strategy.”

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Feature Image: Gnathonemus petersii - Elephant nose fish - Tapirfish. (Credit: GnathoMichel/Wikipedia)

Story Image: African fish called mormyrids communicate by means of electric signals (white lines following fish). Research has shown that fish in one group (top five) can glean detailed information from a signal's waveform, but fish in another group (bottom five) are insensitive to waveform variations. What is the neurological basis for this difference in perception? (Credit: Bruce Carlson)