Baringa’s (1996) Crayfish
KeywordsDominance Hierarchy Agonistic Interaction Nervous System Function Command Neuron Dominant Animal
Social status and social behavior can alter the function of the nervous system within crayfish. This research details the experiments that show that dominant and subordinate crayfish can express different serotonin receptors in the nervous system as well as modulate the sensitivity of those receptors expressed.
Neuroethology is the study of the neural basis of behavior, and one of the assumptions of the field includes a concept that behavior arises from the underlying neural architecture of the organism’s central nervous system. Several studies have supported the concept that changes in the nervous system, either during development or via changes in neurotransmitter function, as a result of experiences, alters the types of behaviors demonstrated by the organism. For many years, this concept was considered a one-way process whereby the neurobiology influenced behavior. However, the classic studies by Don Edwards and his lab showed that behavior can alter not only the neural architecture but also the neural functioning. Specifically, the social system, including the suite of behaviors and the neurobiology of those behaviors, of the crayfish served as evidence that demonstrated that the social status of an individual (dominant or subordinate) can subsequently alter the modulation of behavior by the neurotransmitter serotonin. In addition, the release and modulation of serotonin can then alter nervous system structure, which influences the types of behavior being exhibited by crayfish with either a dominant or subordinate status. This work opened a new approach to neuroethology in that both the behavior and the nervous system of an animal are now considered plastic and flexibly adaptable to social and environmental influences.
Background on Social Behavior
Sociality carries with it several ambiguous ideas, and the degree to which an organism displays sociality is different across the animal kingdom. Social systems vary from highly organized and (potentially) genetically related organisms that exhibit cooperation (i.e., the social systems in eusocial insects, some species of birds, and primate communities) to the other end of the scale where organisms exhibit aggressive interactions during brief periods of the year, such as mating periods. Crayfish are a model system for studying social dynamics in general and, specifically, the neuroethology of social behavior. Crayfish exhibit agonistic behavior in the laboratory and in the field (Moore 2007). Within laboratory settings, social dominance and hierarchy formation are quite common (Goessmann et al. 2000; Gherardi and Daniels 2003). Thus, crayfish are organisms that display social behavior and can serve as a neural and behavioral model for the development of social hierarchies (Herberholz et al. 2003).
The first observations of social behavior and agonism in crayfish were based off of the obvious visual displays and the use of the large weaponry (claws) (Bovbjerg 1953). Since Bovbjerg’s initial observations, the agonistic behavior of crayfish under different conditions has been well established (see below). This large body of work has shown that the interaction of two crayfish leads to agonistic contests that progressively escalate until one of the opponents withdraws (Bovbjerg 1956). Through these interactions, a dominance relationship is formed. For crayfish, a dominance relationship is one in which subsequent social interactions between these pairs is highly predictable (Daws et al. 2002).
The formation of a dominant and subordinate relationship has consequences for time and energy budgets as well as the acquisition of resources. Indeed, the alterations in behaviors exhibited by dominant and subordinate animals formed the basis for the neurological studies of the Edwards lab (more on this later). Subordinate animals rarely seek to engage a dominant opponent and often retreat when approached by a dominant individual (Daws et al. 2002). Dominant animals can actively seek out subordinate crayfish to reinforce their status through short, low-intensity interactions. Although, it must be noted that most of these descriptions of behaviors are from laboratory studies often in confined aquaria. The social behavior demonstrated by crayfish in the field tends to be of shorter duration and lower intensity than those behaviors found in the lab (Bergman and Moore 2003). The continued reinforcement of status can not only have long-term effects on subsequent behavior but also on sculpting the nervous system of both the dominant and subordinate animals. To understand the role of neuromodulators, like serotonin, and how alterations in nervous system function can produce differing behaviors, the basic dynamics of crayfish agonistic interactions needs to be explained.
Crayfish agonistic interactions are highly ritualized meaning that a set of displays, sensory signals, and behaviors are consistently seen across different species of crayfish and different environmental conditions (Moore 2007). As with many decapods, when two crayfish are placed within a confined area, they will eventually engage in an agonistic interaction. The interaction begins with an approach behavior where the crayfish extend their chelae (claws) in a lateral fashion. Sometimes, the crayfish will modulate the height of their body, which is thought to be a threatening display. These two behaviors usually occur when the crayfish are separated by one to two body lengths of space. The next stage in a contest occurs when the crayfish open their chelae and actively push the opponent around the tank. What is interesting about this stage of the fight is that neither animal closes the chelae to use as a weapon. This has been termed “boxing” because the crayfish appear to be trying to move the opponent around the fighting arena rather than inflict serious harm. If neither crayfish retreats, the level of aggression usually increases such that the chelae are used to grab the opponent in an attempt to flip them over. The final stage escalates when the crayfish appear to try to injure their opponent by grasping with the chelae and ripping at different body parts. While infrequent, some interactions may result in injuries, lost limbs, or even death.
Agonistic interactions are terminated and a dominant or subordinate relationship is formed when one of the crayfish retreats from the fight. Retreating can take one of two different forms. First, the losing crayfish simply backs away and fails to engage in further fights. The second method, known as a tailflip, is far more rapid. During a tailflip, the losing crayfish rapidly contracts the muscles on the lower part of the abdomen which causes the telson to curl under the crayfish quickly. This motion launches the crayfish backwards and off of the substrate in a rapid retreat fashion. [As a side note, there are actually multiple styles of tailflips each with their own different underlying neural mechanisms (Herberholz et al. 2001)]. The decision to retreat, either by tailflip or a slow retreat, determines the winner and loser of the fight and establishes the roles of dominant and subordinate crayfish. Thus, the neural mechanisms that influence the control of this retreat were the basis of the series of studies performed in the Edwards lab. Before those experiments are described, the different behavioral and environmental influences on social status will be outlined.
The Behavioral and Environmental Influences on Social Status
Dominance and levels of aggression in crayfish are influenced by several different factors that can be categorized as either extrinsic (environmental and sensory influences) or intrinsic (physical or physiological) to the crayfish (Moore 2007). Extrinsic factors include the presence of resources (shelters and food) and sensory signals (visual, chemical, and mechanical signals). Crayfish that have “ownership” over a resource like a territory, shelter, or food source are more likely to defeat and become dominant over intruding crayfish (Graham and Herberholz 2009). Unfortunately, the direct role of different sensory signals on dominant and subordinate status is still unknown, but chemical signals seem to be an important aspect of crayfish social interactions.
The role of intrinsic factors has received the bulk of the social status work in crayfish. The most obvious intrinsic factor that influences dominance is size of the animal or size of the chelae. Larger animals with larger weaponry are dominant over smaller animals (Pavey and Fielder 1996). The sex of the crayfish also influences dominance status as size-matched males tend to be dominant over females (Wofford et al. 2015). Reproductive status can also serve as a varying intrinsic quality that dictates dominance. Within many crayfish species, both the male and female cycle between a reproductive and nonreproductive state. Reproductive crayfish are dominant over nonreproductive crayfish (Figler et al. 1995). Previous social history and neurochemistry are critical factors in the dominant status of crayfish. As in many species, crayfish exhibit what is known as winner or loser effects (Daws et al. 2002). Crayfish with a previous social history of winning are more likely to remain dominant and win their next set of agonistic encounters. In an opposite fashion, crayfish that have recently lost encounters are more likely to continue to lose subsequent contests and become or remain subordinate. The winner and loser effects within crayfish are reversible signaling that any underlying neural changes that are associated with dominant and subordinate behavior must also be plastic. Since the retreat, by tailflip, determines the outcome of agonistic interactions, the Edwards group (Shih-Rung Yeh and Don Edwards with others) focused their neurological work on the flexibility and modulation of the tailflip circuitry in the crayfish. The ability to artificially manipulate dominant status through winner and loser effects gave them the experimental model which would directly connect social status and neurobiology.
The Underlying Neurobiology of Social Status
With the advent of modern techniques in neurobiology, several groups began working on the neural circuitry of dominant and subordinate behavior, with particular attention being paid to the neurons involved in a retreat response (tailflip) and the role of serotonin in dominant and subordinate animals (Edwards and Kravitz 1997). The tailflip circuitry of the crayfish has been a favored neurobiological prep since the pioneering work of Wiersma (1947). Since that initial work, the neurons involved in the tailflip response as well as the neuromodulation of that response have become fairly well known. Crayfish can exhibit three different types of tailflips, classified by the neural control necessary to perform the behavior (Krasne et al. 2014). The details of these three responses and their role in the ecology of the crayfish are beyond the scope of this entry, but the critical feature of these tailflips is the identified neurons that control each of their activations. Each of the tailflips are controlled by two giant command neurons: the medial giants and the lateral giants. The command neurons are found in bilateral pairs along the dorsal position of the ventral nerve cord of the crayfish. Within invertebrate systems, the diameter of the neuron determines conduction velocity. Thus, both giant neurons (up to 80 μm in diameter) have a high speed of conduction which evokes rapid tailflips. Both neurons activate structures called motor giants and fast flexor motor neurons which further explain the rapid response of the crayfish tailflip. Finally, these neurons are termed command neurons because they control the entire sequence of activation of the tailflip from neuron to behavioral output. Modulation of the sensitivity of these neurons can also modulate the speed and sensitivity of activation of the tailflip (Krasne 1969; Krasne and Edwards 2002). This modulation introduces the possibility that social behavior and dominance (as determined by which animal does not retreat) may be explained by the sensitivity of the neurons controlling the tailflip response. This hypothesis, coupled with the presence of winner and loser effects, is what the experimental work by the Edwards lab tested with their classic study (Yeh et al. 1996, 1997).
The details of their experiments are elegantly simple (Yeh et al. 1996, 1997). They first placed two socially naïve crayfish in an aquarium and waited until a dominant and subordinate relationship had firmly been established. After this relationship was established, both crayfish were dissected and the lateral giant neurons were tested for their sensitivity to serotonin. Using electrophysiological techniques, the researchers applied different concentrations of serotonin to the bath surrounding the dissected neurons from subordinate, dominant, and control (i.e., socially naïve) animals. During application, they measured the excitatory postsynaptic potential of the lateral giants to test their excitability to different concentrations of serotonin. The results showed that serotonin had the ability to modulate the excitability of the lateral giant neurons, meaning that serotonin sets the gain of the neurons. The exact effect of serotonin on modulating this response depended upon the social status of the crayfish. In dominant animals, serotonin increased the sensitivity of the lateral giants which increased the probability of firing and the opposite effect was seen with the subordinate animals. Thus, a first set of conclusions that can be drawn from this work is that the modulation of social neural networks by serotonin is dependent upon the social history of the crayfish.
However, this initial result was counterintuitive. If dominant animals are more sensitive to serotonin and their lateral giants are more likely to fire, why does this lead to dominance? The researchers used a set of different compounds on the dissected neurons in an attempt to understand the exact physiological mechanism of change that is occurring at the neural level. They found that different classes of serotonin receptors were being expressed in the dominant and subordinate animals. One class of serotonin receptors increases the excitability of the lateral giant neurons and a second class of receptors reduces the excitability of the lateral giants. Dominant crayfish expressed more of the first class of receptors (excitability) and the subordinate animals expressed more the second class of receptors (less excitable). Work in another lab (Ed Kravitz from Harvard) has shown that the lateral giant neuron is also involved in activating other neurons that release serotonin in other areas of the crayfish nervous system (Kravitz 2000). These other areas are involved in modulating levels of aggression in crayfish. Thus, an increase in excitability of the lateral giant, which is mediated by the expression of certain receptors, also increase the level of aggression in crayfish by activating a global release of serotonin. Thus, a second conclusion from this work is that changes in social status can change which serotonin receptors are expressed within crayfish, which in turn affects sensitivity to circulating serotonin.
These changes in sensitivity to serotonin are not permanent and are reversible. In a continuation of their experimental design, the Edwards group placed two subordinate crayfish within a tank and waited for one of the previously subordinate animals to become dominant. Again, the lateral giant neurons were dissected from the animal and placed in a petri dish to measure their responsiveness to serotonin. The responses of the lateral giant were reversed and instead of the activation being suppressed by serotonin, the responses were enhanced. The neurons now responded as if they came from a dominant crayfish. This change in firing sensitivity took around 2 weeks to occur. In a final test of their findings, the researchers paired two dominant crayfish in a tank to test whether dominant crayfish altered their neurophysiology in a similar fashion to the subordinate animals. Surprisingly, the dominant crayfish exhibited much higher levels of aggression and their fights ended in death at a far higher rate than other fight conditions. After one of the crayfish eventually exhibited subordinate behaviors, Yeh and Edwards tested their lateral giants for serotonin sensitivity. Unlike the change in physiology found in the paired subordinate fights, the “losing” dominant crayfish still exhibited the enhanced sensitivity to serotonin. Yeh and Edwards demonstrated that dominant animals may exhibit subordinate behavior, but their nervous system still exhibited the dominant physiology for long periods after losing their dominant status. Although social status can change a nervous system’s sensitivity to serotonin, both the initial change and any subsequent change in physiology is dependent upon the social status of the crayfish. Subordinate crayfish exhibit loser effects due to a decreased sensitivity to serotonin, but this change in neural sensitivity is reversible on a short time scale. Conversely, dominant crayfish exhibit enhanced sensitivity to serotonin and this change in enhanced sensitivity is a long-lasting change, exhibiting the possibility that winner effects are more potent than loser effects in crayfish.
The results from this study demonstrated three essential points for the plasticity of the nervous system. First, behavior, in general, and social status, specifically, alters the functional properties of the nervous system of the crayfish. From a neuroethological point of view, behavioral experiences alter nervous system function. Second, this change in nervous system function can vary depending upon the prior experience. Serotonin functionality in crayfish enhanced excitation of escape neurons in dominant animals and suppression excitation in subordinate animals. Third, the temporal properties of this change are also dependent upon the behavioral experience. Changes in serotonin sensitivity were longer lasting in dominant animals than subordinate animals. Taken together, these results have clearly demonstrated a more plastic or flexible nature of the nervous system that is highly dependent on the type and intensity of behavioral experiences.
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