Crustaceans are much more than they seem on the dinner plate or in the boiling pot. In fact, they serve the biological world in many roles. One of these is as an indicator species: those organisms that, based on their abundance, or lack thereof, help confer the condition of a given environment. So, it is extremely important to study a wide array of these organisms in detail. Because virile crayfish (Faxonius virilis) are an invasive species that threaten other crayfish populations, a team of researchers at both Arizona State University and the Federal University of Sāo Paulo saw fit to explore the organisms’ combat behavior.
In their article included in the March 2020 edition of Animal Behavior, entitled “Virile crayfish escalate aggression according to body size instead of weapon size,” Zackary A. Graham, et. al discovered that, unlike other crustaceans, virile crayfish do not use claw size information in determining their perceived dominance in a fight. To explore this idea further, we must make a small detour along the evolutionary tree of crustaceans. In a number of species, males use large claws to demonstrate dominance and attract mates. This creates space for individuals to dishonestly signal, or lie to their audience. In the natural world, size often relates strength and survivability, also displaying the individual’s energetic commitment to the trait. So, these “cheaters” will possess characteristics of dominance, but circumvent the cost of developing them. Think of street performance bodybuilders who inflate their muscles with synthol. Though they have “big” muscles, they have avoided the hard work of going to the gym day after day, and so after all is said and done, those “big” muscles have no real ability to produce force. In crustaceans, the claws of “cheater” individuals often serve as their “big,” but dysfunctional muscles.
Virile crayfish do not play to this particular tune. Graham and colleagues found a weak and extremely varied relationship between claw size and strength in both males and females. This means that virile crayfish assume little about an opponent’s strength by way of their claw size and must therefore engage them in combat to actually find out. Within the 49 experimental encounters the researchers produced, they found that the encounters were rarely based upon the relative claw size or sex of the combatants. To the contrary, a body size model based on the length of the individual’s carapace was nine times more likely to predict a combat scenario (see figure 1).
The researchers obtained these results by capturing 124 virile crayfish at Saguaro Lake, Arizona, U.S.A, and then acclimating them to the lab environment: individual plastic tanks filled with dechlorinated water. Claw size was then measured on both right and left hand sides. Additionally, carapace length was measured to serve as an index for body size. The researchers then measured maximal claw strength using a custom-built force transducer that the virile crayfish pinched repeatedly. After all measurements were made, the games could begin. Pairs of males and females were placed in a large aquarium with a removable screen divider, which prevented any premature physical contact. Once the screen divider was removed, the violence could commence. The winner was determined by whether an individual chased their opponent or fled from them.
Even if claw size did determine claw strength in virile crayfish, analysis of the aggression dynamics is crucial to establish the actual signalling significance of claw size. Without assessing combat between individuals, the researchers could have concluded that virile crayfish signal dishonestly due to the weak relationship between their claw size and strength. But based on the aggression data the researchers gathered, they discovered that while both claw size and body size aliment an individual’s dominance, only body size serves as a direct signal for such. When it comes to the generality of crustacean dishonest signalling, these data mix things up. This is due to the fact that though a trait might operate as a signal in one species, it might not in another. But this is only natural, as more data on a broad subject increases the likelihood of conflicting evidence appearing.
Although these creatures are common in your creeks, streams, and lakes, the over 650 species of crayfish serve a wide range of important ecological roles. This names the importance of the continued study of aggression dynamics within these communities; to seek a greater understanding of broad ecological conditions through the understanding of minute ecological relationships.
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