The Heartrate Reaction to Acute Stress in Horned Passalus Beetles (Odontotaenius Disjunctus) Is Negatively Affected by a Naturally-Occurring Nematode Parasite
Total Page：16
File Type：pdf, Size：1020Kb
insects Article The Heartrate Reaction to Acute Stress in Horned Passalus Beetles (Odontotaenius disjunctus) is Negatively Affected by a Naturally-Occurring Nematode Parasite Andrew K. Davis *, Brandon Coogler and Isaac Johnson Odum School of Ecology, University of Georgia, Athens, GA 30602, USA; [email protected] (B.C.); [email protected] (I.J.) * Correspondence: [email protected] Academic Editors: Kenneth Wilson, Fleur Ponton and Sheena Cotter Received: 18 July 2017; Accepted: 16 October 2017; Published: 18 October 2017 Abstract: There are many events in the lives of insects where rapid, effective stress reactions are needed, including fighting conspecifics to defend territories, evading predators, and responding to wounds. A key element of the stress reaction is elevation of heartrate (HR), for enhancing distribution of blood (hemolymph) to body compartments. We conducted two experiments designed to improve understanding of the insect stress reaction and how it is influenced by parasitism in a common beetle species (Odontotaenius disjunctus). By non-destructively observing heartbeat frequency before, during and after applying a stressor (physical restraint) for 10 min, we sought to determine: (1) the exact timing of the cardiac stress reaction; (2) the magnitude of heartrate elevation during stress; and (3) if the physiological response is affected by a naturally-occurring nematode parasite, Chondronema passali. Restraint caused a dramatic increase in heartrate, though not immediately; maximum HR was reached after approximately 8 min. Average heartrate went from 65.5 beats/min to a maximum of 81.5 (24.5% increase) in adults raised in the lab (n = 19). Using wild-caught adults (n = 77), average heartrates went from 54.9 beats/min to 74.2 (35.5% increase). When restraint was removed, HR declined after ~5 min, and reached baseline 50 min later. The nematode parasite did not affect baseline heartrates in either experiment, but in one, it retarded the heartrate elevation during stress, and in the other, it reduced the overall magnitude of the elevation. While we acknowledge that our results are based on comparisons of beetles with naturally-occurring parasite infections, these results indicate this parasite causes a modest reduction in host cardiac output during acute stress conditions. Keywords: insect stress; heartrate; passalus beetle; Odontotaenius disjunctus; nematodes; Chondronema passali 1. Introduction Insects, like all animals, are often faced with stressful events in their lives, such as evading predators, fighting with conspecifics and coping with injuries. All of these events require a functional stress response, which in vertebrates as well as invertebrates, is designed to elicit specific changes in physiological functions that temporarily promote survival during the stress event [1–3]. Such changes include heightened immunity [4–6], enhanced predator evasion [7] and importantly, increased cardiac output, which serves to increase blood flow (hemolymph in insects) and nutrients to muscles. Cardiac output can be gauged by monitoring heartbeat frequency, or the pulsations of the dorsal vessel (the insect ‘heart’), which distributes hemolymph throughout the insect. It has long been known that when insects are faced with a physically demanding activity, such as flight, the heart beats faster. In fact, one of the earliest examples of this comes from a study conducted in the 19th century; Newport [8] Insects 2017, 8, 110; doi:10.3390/insects8040110 www.mdpi.com/journal/insects Insects 2017, 8, 110 2 of 13 scraped the upper abdominal scales off of a hawk moth (Sphinx ligustri) to view the pulsations of the dorsal vessel. He noted when the specimen was at rest, the heart rate was 42–49 beats/min, and after the moth had “flown around his sitting-room” for several minutes it was 127–139 beats/min. Thus, insect heartrates become elevated during situations when greater blood distribution is needed. Similar to the response to physical activity, a positive relationship between acute stress states and heartrate elevations in insects is evident in the entomological literature, although mostly indirectly. A considerable body of work has been completed showing how stress leads to increases in circulating levels of octopamine, the insect equivalent to adrenaline [9–12]. Meanwhile, other work has shown that heightened levels of octopamine has a stimulatory effect on insect heart contractions [13–15]. From these two separate bodies of work, one can logically surmise that acute stress (which increases octopamine) leads to an increase in heartrate in insects, though there are many questions about this relationship that could be examined further, and in a more direct manner. Specifically, investigating how heartrates of (intact) insects change while insects are subjected to a stress event would be useful. This would help to address a number of questions relating to the timing of the stress reaction. For example, how quickly does heartrate increase when stress begins, and how long does the stress effect last following the stress event? Also, how is this reaction affected by the health of the insect? Can insects harboring parasites react as well to stress? We addressed the questions above using a natural host–parasite insect system that is ideal for investigations of insect stress and parasite effects. The horned passalus beetle, Odontotaenius disjunctus (Coleoptera: Passalidae, Illiger; Figure1), is a medium-sized saproxylic insect that inhabits rotting hardwood logs in forests throughout the eastern seaboard of the United States [16–18]. It creates cavities within the logs, where it raises its larvae in the summer months. Since it consumes the wood as food, this species can be readily maintained in a laboratory setting by providing rotting wood. Importantly, it is host to a number of external and internal parasites [19], with one notable one being a nematode, Chondronema passali (Leidy), that lives in its abdominal cavity (Figure1)[ 20]. This parasite is not well-studied, thus it is not known how it is transmitted (though probably from parent to offspring), and its taxonomy is not even clear. This nematode is known to be highly prevalent in field collections of passalus beetles, with most collections having between 60 and 100% parasitism rates [16]. Moreover, it can be extremely abundant within its host, with beetles often harboring many hundreds, and sometimes thousands of worms. Despite this, the nematode seems to convey little outward negative effect to the host. Recent projects in our lab have begun to examine how this parasite affects its host [21–23], with particular attention given to behaviors and events in the lives of beetles where its impact might be felt most strongly, that is, during times of stress, when energy demand is high. Here we conducted two experiments using field-collected passalus beetles, in which we monitored heartrates of intact specimens before, during and following a stressor in the laboratory (restraint). Moreover, we employed a non-destructive observational approach to directly count pulsations of the dorsal vessel of the living insect under a low-power microscope (see methods), even as the stress was applied, allowing us to know the exact timing of the heartrate elevations. This low-tech approach differs from that used by most previous investigators studying insect cardiac activity, where experiments often involve various electronic devices and probes attached to, or inserted into, the insect [24–26]. One prior study we are aware of has employed a similar, observational approach to monitor heartrate of a hemipteran, Rhodnius prolixus [27]. The goals of the current project were to: (1) document the magnitude and time course of the heartrate response during acute stress; and (2) determine if the C. passali nematodes influence either the resting heartrate, or the beetle heartrate during acute stress. The results from this project should be useful for improving understanding of the insect physiological stress response. Insects 2017, 8, 110 3 of 13 Insects 2017, 8, 110 3 of 13 Figure 1. Intact horned passalus beetle, Odontotaenius disjunctus , left. Right Right top image shows a beetle with the lower portion of the elytra elytra removed removed to allow allow visualization visualization of of the the dorsal dorsal vessel vessel (white (white arrows). arrows). A video of the dorsal vessel pumping is available in the Supplemental files.files. Lower right image shows nematode parasite, Chondronema passali,, thatthat inhabitsinhabits thethe abdominalabdominal cavitycavity ofof O.O. disjunctusdisjunctus.. 2. Methods 2.1. Beetle Collections There were two two collections collections of of passalus passalus beetles beetles used used for for this this project. project. For Forboth both collections collections the beetlesthe beetles were were hand-collected hand-collected from from hardwood hardwood forests forests near near the theUniversity University of Georgia of Georgia campus, campus, in Athens,in Athens, GA, GA, USA. USA Beetles. Beetles were were extracted extracted from from rotti rottingng hardwood hardwood logs logs on on the the forest forest floor floor (using hatchets) and placed in plastic containers for transporttransport back to thethe lab.lab. There, they were housed individually in 900 mL plastic containerscontainers filledfilled with moist, rotting hardwood.hardwood. Beetles for the firstfirst experiment werewere aa setset ofof 19 19 specimens specimens that that were were collected collected as as larvae