Saturday, May 27, 2017

Is the Pirate Perch Really a Ghost? A Look into the Mechanism of Chemical Camouflage, by Kevin Eliason

The pirate perch isn’t a pirate and it isn’t a perch, but it may be a ghost. The pirate perch (Aphredoderus sayanus) is a fish that lives throughout much of the southern United States and Mississippi drainage. The pirate perch is a monotypic family, sayanus being the only species. Page and Burr describe the species having: a short deep body often 3-4 in long, large head, large mouth, anus and urogenital opening between branchiostegal membranes(Page and Burr 2010)(fig 1). The pirate perch lives in backwaters and swampy areas, often around vegetation and over mud bottom. Pirate perch are prey for birds, fish, and water snakes. In order to avoid predation pirate perch often stay close to woody debris for protection and are nocturnal to help avoid site based predators. The pirate perch in turn feed on primarily small freshwater shrimp and insect larvae but are generalist feeders (McCallum 2012). Pirate perch also use a form of chemical camouflage/ chemical deception to potentially avoid predation and enhance predation on chemical sensitive prey (Resetarits and Binckley 2013).
You may be asking yourself, “What is this chemical camouflage?”  In the case of pirate perch, the mechanism of camouflage isn’t exactly known, however, it is believed that pirate perch have evolved it as a novel way of hiding from prey. Dr. Resetarits explored the capabilities of chemical camouflage in pirate perch using artificial ponds in which pirate perch were put in adjacent ponds with sunfish and ponds without fish as a control. Dr. Resetarits found that beetles and tree frogs both actively avoided ponds with sunfish, but no significant difference between pirate perch ponds and controls were observed (Binckley and Resetarits Jr. 2003; Resetarits and Pintar 2016). Resetarists proposed three possible mechanisms for this camouflage: Distorted or mixed signals, mimicry of a non-threatening organism, or simple cloaking and lack of signal. I will explore each of these methods, and together we will try and uncover the ghost of the fish.
Figure 1. Aphredoderus sayanaus Image Credit: Ellen Edmonson and Hugh Chrisp
First, distorted and mixed chemical signals; a mechanism would imply that the pirate perch still emits a chemical signal but it doesn’t register the same as other fish. This method is the most probable, in my opinion, based on Resetarists’s findings as the number of beetles and tree frogs were generally lower then controls, just not at a significant level. This apparent trend may imply that some sense of predation was still occurring at the ponds with pirate perch. This would lead us to believe that it is still emitting a chemical signal, but not easily detected.
Second, mimicry of a nonthreatening organism. This method is used often in nature as a defense against predators. Some familiar mimics may include coral snake/milk snakes and stick bugs. This mimicry is used as a defense against predators to either hide in plain site or imitate a dangerous organism. There are also aggressive forms of mimicry like freshwater mussels whom lure fish in with their mantles shaped like small fish and insects in order to inoculate them with young. Parasites are also well known to use mimicry to trick hosts into eating them. If pirate perch utilized this method it would be likely that they are mimicking something that is either not a fish or as a nonthreatening fish, like a sucker or non-insectivorous fish. I believe mimicry to be the least likely of the three proposed but also one of the hardest to prove. Some potential issues are if mimicry is the mechanism used, what are the pirate perch mimicking and how does that mimicry deceive different types of prey items.
The third and final mechanisms is cloaking of the chemical signal. This mechanism would insinuate that the pirate perch has evolved to not exhibit the chemical markers that other fish have. This method is also very probable as the true mechanism of pirate perch’s chemical camouflage. This is probable due to evolution being easier for pirate perch to lose their chemical signature or produce a chemical signature that that doesn’t bind or respond to the normal receptors. Also, being a monotypic species it is possible that lacking chemical signatures has been an ancestral trait that only survives in the pirate perch.
In conclusion, I believe that out of the three methods it is most likely that the pirate perch achieves chemical camouflage using distorted or mixed signals. I support this belief based on Resetarits’s findings with beetles and tree frogs. Also, It is unlikely that beetles and tree frogs receive signals in the exact same way, this makes me believe that a distorted signal is more likely then cloaking which may not be effective on all types of organisms. In addition, it is also possible that the mechanism used by pirate perch may not fit neatly into one of these three categories. Rather, the mechanism is a little bit of two or all three mechanisms. This could be achieved by distortion of the chemical signal that may be sensed more like some other organism. Alternatively, it also could be distortion to the point that the sense is overwhelmed and essentially cloaking has occurred. These questions cannot be answered with the current knowledge of the pirate perch. This mechanism, which potentially represents a new evolution in the predator-prey arms race. The pirate perch will no doubt be a subject of returning research as the ichthyology community begins to unravel the mysteries of the pirate perch and its chemical camouflage.
Binckley, Christopher A., and William J. Resetarits Jr. 2003. “Functional Equivalence of Non-Lethal Effects: Generalized Fish Avoidance Determines Distribution of Gray Treefrog, Hyla Chrysoscelis, Larvae.” Oikos 102(3): 623–29.
McCallum, Malcolm L. 2012. “Notes on the Diet and Egg Clutches of the Pirate Perch (Aphredoderus Sayanus) from Central Arkansas.” Southeastern Naturalist 11(3): 543–45.
Page, Lawrence M., and Brooks M. Burr. 2010. A Field Guide to Freshwater Fishes: North America, North of Mexico. Second. Boston; New York: Houghton Mifflin.Print.
Resetarits, William J., and Christopher A. Binckley. 2013. “Is the Pirate Really a Ghost? Evidence for Generalized Chemical Camouflage in an Aquatic Predator, Pirate Perch Aphredoderus Sayanus.” The American Naturalist 181(5): 690–99.
Resetarits, William J., and Matthew R. Pintar. 2016. “Functional Diversity of Non-Lethal Effects, Chemical Camouflage, and Variation in Fish Avoidance in Colonizing Beetles.” Ecology 97(12): 3517–29.

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