The Subtle Art of Electro-Flirting in Wild Knifefish

OUTSIDE JEB

The subtle art of electro- with high precision. They found that each may interfere with natural courtship. This fish emitted electric discharges at their study highlights the importance of flirting in wild knifefish own unique frequency, producing investigating physiological phenomena in ‘electrical fingerprints’ that they used to natural habitats. The team also note that identify and track individuals, as well as the insights into how weak electrical distinguish between the sexes. signals are interpreted by the fish’s brains may help us to improve the design of The team discovered that the electric bionic devices, such as retinal and discharges were closely associated with – cochlear implants for people with two types of behaviour: male female impaired vision and hearing. courting and male–male aggression. During courting, males would hound 10.1242/jeb.170084 females with short electric ‘chirps’ that Henninger, J., Krahe, R., Kirschbaum, F., Grewe, lasted less than 20 ms, until the female J. and Benda, J. (2018). Statistics of natural responded with a long chirp that lasted communication signals observed in the wild COMMUNICATION over 150 ms, signalling her intention to identify important yet neglected stimulus regimes in weakly electric fish. J. Neurosci. release eggs. These long chirps were doi:10.1523/JNEUROSCI.0350-18.2018. What do James Brown and ghost knifefish responded to with precisely timed (Apteronotus) have in common? They’ve response ‘doublet’ chirps, which Alex Evans (0000-0002-5655-012X) both harnessed the power of electric indicated the intention of the male to University of Leeds organs in order to deliver declarations of release sperm, resulting in a tightly [email protected] adoration. As close relatives of the electric synchronised fertilisation event. As well eel (Electrophorus electricus), it’s as courtship, the team found interesting probably not too shocking that ghost patterns of electro-communication How ticks put the B in knifefish also possess an electric organ between males competing for access to blood that generates an electrical discharge. As females. These events occurred when the well as emitting electricity, ghost electric discharges of a rival male knifefish are covered in electro-sensitive interfered with the signals emitting from a receptors that allow them to sense their male courting a female and tended to end surroundings, detect the presence of other with the rival male submissively individuals and communicate with other retreating from the area. fish through electric organ discharge. As ghost knifefish are nocturnal and The team was also able to demonstrate frequently hide amongst rocks and roots, how these short and long chirps were their natural behaviours are rarely ever closely linked to spawning behaviours in observed and the characteristics of their another species of brown ghost knifefish Apteronotus leptorhynchus electrical discharges have previously been ( ) in a more NUTRITION examined almost exclusively through closely monitored laboratory laboratory-based tank studies. Until now, environment. By simultaneously filming Few things wreck a picnic faster than the the natural electric conversations of ghost and recording the fish’s electric discovery of a poppy seed tucked into the knifefish and their role in wild behaviours discharges for over 5 months, collecting a warm folds behind your knee that turns out have largely remained a mystery. staggering 1.3 million individual chirps in to be a tick. If left undisturbed, it will the process, the team demonstrated that slowly gorge on your blood until it A recent study reveals the importance of the same short and long chirps recorded in resembles a flattened jelly bean – with legs. subtle bio-electric communication during the wild were closely correlated with For picnickers and parents, it’s a revolting the courting and aggression behaviours of critical mating events such as the transformation fraught with dangers, wild weakly electric fish. Lead authors of synchronised release of eggs and sperm. because ticks are major vectors of a range of the study, Jörg Henninger and Jan Benda, debilitating illnesses, including Lyme from the Eberhard Karls University of However, in contrast to previous disease; yet, for biologists, the Tübingen, Germany, set out deep into the laboratory-based tank studies, many of transformation is nothing short of rainforests of Panama to record the the EOD signals detected in the wild were remarkable. electro-communications of wild brown much weaker than expected and barely ghost knifefish (Apteronotus rostratus). activated the fish’s own electro-receptors. Ticks can endure weeks of starvation Using a grid of electrodes placed in a These subtle electric whispers had never while awaiting the chance arrival of new knifefish-inhabited creek, the team were previously been recorded in laboratory hosts, but even after tapping into a vein, able to record electric organ discharges experiments as the artificial conditions all they get is a diet of blood, upon which

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1 OUTSIDE JEB Journal of Experimental Biology (2018) 221, jeb170084. doi:10.1242/jeb.170084 they would surely starve. Although ticks with a B vitamin supplement and this species (ROS), a by-product of electron blood is rich in proteins and lipids, it solved everything! The ticks on drugs and movement through the respiratory chain lacks other key nutrients required for vitamins were as healthy as wild-type ticks. that supports ATP production. The dual survival. However, a fascinating new role of mitochondria intrigued Karine Salin study – by Olivier Duron from the CNRS This and countless other studies make clear and her colleagues from the University of in Montpellier, France, and his team of the numerous essential roles that bacteria Glasgow, UK, and Semmelweis colleagues from across the country – play in animal well-being. In addition, this University, Hungary, so they investigated shows how ticks overcome this nutritional study highlights the massive evolutionary whether the metabolic adjustments made deficiency. flexibility of some bacterial groups. Within by mitochondria during fasting also caused Francisella, there are both mutualists and oxidative stress in brown trout (Salmo Ticks are arachnids, but they aren’t the pathogens that are nasty enough to be used trutta), a species that naturally experiences only creepy-crawlies that suck. Several as bioweapons. Interestingly, some other periods when food is scarce. insect families are exclusive blood feeders tick species harbor nutritional symbionts and others live on a diet of plant sap; but from another genus, Coxiella,thatalso The team either fed trout fry as much as their survival doesn’t depend on this contain species used as bioweapons. they could eat or deprived them of food in alone. Typically, these beasts rely on Remarkably, both were tamed by ticks – a realistic simulation of the natural food microbial lodgers, known as mutualists, which is exciting news for biologists, but shortages experienced by these animals. to supply their missing dietary probably not enough to calm panicked After 2 weeks, the team collected the requirements. To test whether a similar parents after a day in the woods. mitochondria from the fish’s livers and symbiosis was at play in ticks, Duron and measured how much oxygen they his colleagues treated ticks with 10.1242/jeb.170068 consumed as they produced energy-rich antibiotics to kill off potential bacterial Duron, O., Morel, O., Noel, V., Buysse, M., ATP (known as state 3 respiration) and symbionts. The response was dramatic. Binetruy, F., Lancelot, R., Loire, E., Menard, while maintaining the electric gradient Male juvenile ticks fed with antibiotics C. Bouchez, O., Vavre, F. and Vial, L. (2018). that counteracts proton leak across the Tick-bactertia mutualism depends on B vitamin were 10 times less likely to reach synthesis pathways. Curr. Biol. 28, 1-7. mitochondrial inner membrane (state 4 adulthood than their untreated respiration). The team also measured the counterparts and the ticks that did survive Daniel E. Rozen (0000-0002-7772-0239) size of the electric gradient directly using Leiden University were smaller and deformed. For females, [email protected] the fluorescent probe safranin. it was even worse. Of 120 treated females, not a single one reached adulthood. So, Comparing the sizes of the livers, the team bacteria matter, but what do they provide found that those from the fasted fish were for their hosts? Fasting fish risk oxidative about one-third the size of those from the stress well-fed fish and had fewer mitochondria, To answer this question, the team leading to a 50–70% reduction in respiration dissected ticks and discovered that they rate. However, the story was very different were all colonized with a strain from the at the level of the individual mitochondria: bacterial genus Francisella. These fasted mitochondria increased state 3 bacteria are usually vertebrate pathogens, respiration and reduced state 4 respiration, but in ticks they showed the hallmarks of meaning they selectively increased their long-term mutualists. First, they were capacity to produce ATP without increasing extremely numerous and showed a the amount of energy dissipated through preference for certain tissues. In both proton leakage. A larger electric gradient males and females, the bacteria across the membranes of the mitochondria accumulated in the Malpighian tubules, a from the fasted fish supported these energy-

site from which they could alter tick STRESS OXIDATIVE saving modifications to improve the nutrition. More importantly, the bacteria efficiency of energy transduction in the also accumulated in the ovaries of During lean times of food scarcity, mitochondria further. pregnant females, thereby ensuring a animals go all out to get the biggest bang stable route of transmission from mothers for their energetic buck, including So, fasted mitochondria were more to offspring. Indeed, all tested tick eggs adjusting energy metabolism and efficient, but how did this relate to the risk carried Francisella. Second, consistent downsizing non-essential processes such of oxidative stress caused by the oxygen with other intracellular bacterial as growth and reproduction. This strategy by-products of metabolism? The team symbionts, the Francisella genomes were has the immediate benefit of stretching estimated the amount of oxidative stress so degraded that these bacteria could limited energy stores, but may have hidden that the mitochondria were experiencing never live outside of their hosts. However, costs not revealed until later in life. One by injecting a compound into the fish that despite these losses, the genes for potential trade-off of the metabolic changes measured the presence of one of the toxic biosynthesis of B vitamins were made during fasting is increased oxidative oxygen by-products: hydrogen peroxide. conspicuously intact. stress due to modifications to the The fasted mitochondria had nearly twice mitochondria. These complicated as much hydrogen peroxide as fed To test whether Francisella provides ticks organelles are critical for metabolism, but mitochondria, supporting the notion that with the B vitamins essential for their are also a major source of potentially toxic reduced metabolism can potentially lead survival, the team fed the antibiotic-treated oxidants in the form of reactive oxygen to oxidative stress. How this increase in Journal of Experimental Biology

2 OUTSIDE JEB Journal of Experimental Biology (2018) 221, jeb170084. doi:10.1242/jeb.170084 stress occurred is not clear, but it could be hibernation and emergence from the active frogs, suggesting that gut bacteria due to the increase in the electric gradient hibernation. composition changes in these animals as (larger gradients are sometimes associated they begin their hibernation. with high oxidative stress) or other aspects One strategy that wood frogs use to of the liver function, such as cutbacks to preserve water and salts during Frogs have a relatively low-energy the system that mops up toxic oxygen by- hibernation is to produce urea when they lifestyle, which allows them to tolerate products as an energy-saving measure. break down nitrogen-rich proteins. Urea is low-nutrient environments, so retaining as less toxic than other forms of nitrogen many nutrients as possible is key to Regardless of its root cause, increased waste, such as ammonia, which has to be survival, particularly during periods of oxidative stress could lead to long-term life- diluted and leads to the loss of water and fasting, such as hibernation. Through this history challenges for fasted fish, such as salts. However, the amphibians lack the work, Wiebler and his group have provided DNA damage and ageing. This study also ability to recuperate the valuable nitrogen the first evidence of nitrogen recycling provides mechanistic insight into the trade- from urea. Might the frog enlist some from urea in wood frogs and have off between short-term gains from reducing ‘helpers’–such as their intestinal flora – highlighted the important role that friendly metabolism during fasting and long-term to retain the nitrogen? A team of gut bacteria may play in all amphibians. costs to subsequent life-history traits. researchers from Miami and Pittsburgh Talk about waste recycling at its best! 10.1242/jeb.170332 Universities, led by James Wiebler, ’ 10.1242/jeb.170142 Salin, K., Villasevil, E. M., Anderson, G. J., Auer, examined the frog s gut to look for S. K., Selman, C., Hartley, R. C., Mullen, W., bacteria that can produce the enzyme Wiebler, J. M., Kohl, K. D., Lee, R. E., and Chinopoulos, C. and Metcalfe, N. B. (2018). urease, which breaks down urea to Costanzo, J. P. (2018). Urea hydrolysis by gut Decreased mitochondrial metabolic requirements produce nitrogen. bacteria in a hibernating frog: evidence for urea- in fasting animals carry an oxidative cost. Funct. nitrogen recycling in Amphibia. Proc. R. Soc. B Ecol., 1-9. 285, 20180241. First, the researchers isolated the gut from Brittney G. Borowiec (0000-0003- hibernating adult male frogs and Oana Birceanu (0000-0002-3345-8769) 0087-7577) Wilfrid Laurier University separated out three segments: the foregut, McMaster University [email protected] [email protected] midgut and hindgut. In each of these segments, the group measured the size (length and mass) of the segment, the bacterial load and type, and urease A fish out of saltwater activity. They found that, even though the Waste recycling by gut hindgut was the smallest segment, it symbionts contained the largest number of bacteria and also had the largest urease activity out of the three. Next, the authors injected active frogs with urea and allowed them to recover for 10 days, to determine whether high levels of urea in the body – which are typical in frogs nearing hibernation to maintain water and ion balance during their slumber – increases urease activity in

the gut bacteria. The group determined RAPID ADAPTATION that high urea levels in the blood did not influence the number of bacteria present Common knowledge says that evolution

HIBERNATION in the hindgut but did increase the activity takes a long time, right? It might take of the urease enzyme by 2.7 times. thousands of years for us to detect the Hibernators, or animals that ‘sleep’ effect of a selective pressure. But for through the winter to conserve energy, The researchers then collected guts from steelhead trout, evolution has happened have always fascinated scientists. To date, active frogs during the summer for much faster. Game fisherman stocked the researchers have studied the adaptations comparison with guts from the hibernating freshwaters of Lake Michigan with that allow these animals to be inactive for animals. Even though the digestive tracts of steelhead trout from the saltwater coast of so long and their ability to come back the hibernators weighed 61% less and were California in the late nineteenth century. from their slumber at the beginning of 25% shorter than those of the active frogs, Since then, the steelhead have settled into spring. However, no hibernator has been there was no difference between the size of their new Great Lakes home and adapted more captivating than the wood frog, the hindgut portion in the two groups. In to freshwater life in just over 100 years. Rana sylvatica. That is because this little fact, Wiebler and his colleagues reported critter is able to freeze during the winter that the hindguts of the hibernators had Janna Willoughby and her colleagues from and then resurrect as if nothing has 33% fewer bacteria than those of the active Purdue University, Michigan State happened. We know a great deal about frogs, but the amount of the urease enzyme University, and Oregon State University, how this animal protects itself from the ice and activity was 2–3 times higher in the USA, were interested in how a saltwater crystals that form in its body as it freezes hibernators. In addition, the authors fish could make the move to freshwater. but very little about the role that the reported that the types of bacteria present in How had the genetic code of these fish symbiotic bacteria in its gut play during the hibernators were different from those in changed and evolved to allow them to Journal of Experimental Biology

3 OUTSIDE JEB Journal of Experimental Biology (2018) 221, jeb170084. doi:10.1242/jeb.170084 grow, reproduce and live in a completely which allowed it to cope better with diversity in the small population of new environment? To uncover the freshwater living. transplanted fish. Only a tiny portion of answer, Willoughby and her colleagues the California population was moved sequenced the entire genome for over The third gene region identified by the to Lake Michigan in the late 1800s, 250 steelhead trout, some from the team plays a key role in wound healing, creating a population bottleneck that modern California population and others indicating that freshwater steelhead could reduce the population’s genetic from the Lake Michigan population a likely heal more effectively than their diversity. Less diversity could potentially century on from their original trans- saltwater counterparts. This change restrict their opportunity to evolve in their continental road trip. might seem strange at first glance, but the new home. Yet, this new study excitingly ability to heal rapidly should prevent the highlights how animals can evolve and The team then scrutinized the two loss of internal salts following an injury adapt rapidly to changing environments genomes, looking for sequences that or attack. Additionally, the introduced and, more promisingly, shows that certain varied between the two populations and fish are vulnerable to attacks by parasitic animals, such as the steelhead trout, can found three regions that differed lampreys – residents of the Great Lakes succeed even if they started with fewer dramatically. Two of the three genomic that latch onto fish with razor-sharp teeth cards in their genetic deck. regions play a role in osmoregulation, – which they would never encounter in which is the ability of fish to maintain their native California coast. The ability 10.1242/jeb.170134 their internal salt balance; the freshwater to heal quickly from a lamprey attack fish have to work hard to keep salt ions would be another bonus for freshwater Willoughby, J. R., Harder, A. M., Tennessen, J. A., Scribner, K. T. and Christie, M. R. (2018). in their bodies, while the saltwater fish steelhead. Rapid genetic adaptation to a novel environment have to work hard to pump excess salt despite a genome-wide reduction in genetic ions out of their bodies. This means that in Fascinatingly, the rapid evolution that diversity. Mol. Ecol. doi:10.1111/mec.14726. just 100 years, the transplanted saltwater Willoughby and colleagues have Erin McCallum (0000-0001-5426-9652) trout population had experienced revealed in the steelhead genome Umeå University dramatic changes in its genetic code, occurred despite the limited genetic [email protected] Journal of Experimental Biology