If the molecules essential for life had social circles, adenosine triphosphate (ATP) would be high society. It is a highly conserved, highly important molecule involved in a wide range of both intracellular and extracellular activities. But what about the role of ATP once it has made it’s way out of an organism and into the external environment? For terrestrial life, such “ambient ATP” may not be that consequential but for marine life (particularly carnivorous marine life), environmental ATP can have a profound effect on behavior.
In the 1980’s and 90’s, researcher Richard Zimmer (among others) became interested in the chemical mediation of feeding activity in marine decapods, specifically California Spiny Lobsters (Panuliris interruptus.). For those of you unfamiliar with Panularis sp., they are nocturnal predators/facultative scavengers that are easily distinguished from “true lobsters” by their lack of chelae (claws) among other characteristics. Anyhow, while Zimmer et. al. were busy conducting their investigations, another researcher, William E.S. Carr, demonstrated that a relative of P. interruptus known as P. argus possessed chemoreceptors in their olfactory organs that were stimulated by ATP; however, he stopped short of exploring the role those receptors might play in the overall behavior of the lobster.
Zimmer thus took the next logical step and after a series of meticulous experiments involving P. interruptus, determined that ATP was a potent chemical mediator of foraging behavior in these crustaceans, specifically by eliciting locomotive behavior associated with recognizing and finding food. Essentially, the lobsters interpreted ambient concentrations of ATP as a “molecular dinner bell” and came running.
So what made ATP such an effective stimulant of feeding behavior? Zimmer proposed that the power behind ATP’s potency lay in it’s ability to maintain a high “signal to noise” ratio in the environment. In order for a chemical to serve as a good “signal” in a marine environment, it needs to maintain a high concentration relative to the background “noise”. In other words, it has to be able to persist in concentrations greater than those commonly occurring in the surrounding environment. It is this difference in concentration that helps an organism determine when a signal is a true indicator of a given event and not just “business as usual.” ATP exists in very high concentrations within living or freshly killed organisms and in very low concentrations in the external environment. Thus, if P. interruptus detected any elevated concentrations of ATP in the surrounding seawater, chances were that a desirable food item was nearby and it began to look for it.
But why ATP specifically; why not another compound? After all, animals release all sorts of chemicals into the environment. Why not use urea or perhaps ammonium as an indicator of a nearby foodstuff. Well, consider what detecting ATP specifically in the environment would mean. Since ATP is rapidly converted to adenine monophosphate (AMP) once tissue begins to degrade, it serves as a reliable indicator of live or freshly killed prey items. Such live (or very recently deceased) items are, from a carnivore’s standpoint, the most nutritious and generally worth the effort it takes to find and consume them. Compounds like urea and ammonium on the other hand, while also indicators of a potential food source, are produced in large quantities through nitrogen catabolism and the activities of the anaerobic and aerobic bacteria which decompose dead tissue. From the lobster’s perspective, such signals would likely serve to indicate an animal which may have been dead for a while and thus lower-quality nutrition. Furthermore, from a “chemoreception mechanics” perspective, ammonium and urea are omnipresent in the marine environment and thus would be hard-pressed to maintain the high “signal to noise” ratio that is needed for effective chemoreception.
So there you have it! It turns out that ATP is much more than just the “universal energy currency” you learn about in Biology 101. It’s also a powerful environmental mediator of foraging activity in at least one genus of marine carnivore and I wouldn’t be surprised if you were to find it to have a similar effect in others. So the next time you happen to be pondering why the Spiny Lobster at your local Aquarium is suddenly scrambling around like crazy, just look for the ATP.
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