Watching a scary movie can quicken our breath and get our hearts racing. Even if we've seen it a thousand times, our brain remembers the scary parts and prepares us to stand and fight or to run away and hide. This preprogrammed response to stress and fear is something that animals from lions to fish to birds all share, but we all have a very similar nervous system. Could insects and crustaceans also have the same response even though their nervous systems are so different? Just like other animals, crayfish have social structures with one animal being dominant over the others, and they seem to remember which crustacean is in charge after ∼5 days. Armed with this knowledge, Iván Oliver-Domínguez, Aidee Lashmi García-Kroepfly, Mireya Osorio-Palacios, Karina Mendoza-Ángeles and Jesús Hernández-Falcón of the Universidad Nacional Autónoma de México wondered whether the red swamp crayfish (Procambarus clarkii) experienced the same fear-like responses as we do when they meet a more dominant individual.
To do so, the researchers first had to begin the immensely difficult task of recording the heart rate and breathing rate of these crustaceans. Once they had perfected these techniques, the team placed three crayfish into a circular tank that was divided into three sections: one animal in each. This way, the aquatic invertebrates could get to know who was who by sending chemical messages through the water to each other. The crayfish would stay in these chambers without getting to meet each other for 15 minutes and then the dividers would be removed, and they'd be able to interact for 45 more minutes before being returned to their individual aquaria. Doing this for 5 days in a row let these crustaceans learn which one was in charge and which ones weren't, and it was long enough for the crayfish to remember the other members of their group, both what they looked like and how they smelled. But would the submissive crayfish start showing signs of fear and anxiety when they were around the dominant one?
For the first few days, all the crayfish had the same heart rate (between 80 and 100 beats min−1), but on the fourth day, things began to change. The heart rate of the dominant crayfish dropped to under 80 beats min−1 by day five and the submissive crayfish had heart rates that soared to 120 beats min−1 when they could only smell each other. This suggests that the smell of the dominant crayfish was enough to start making the others nervous and that they could remember the scent. The more timid crayfish were obviously nervous about coming face to face with their dominant counterpart, causing their heart rate to rise; but would their breathing rate show a similar pattern?
Oliver-Domínguez and colleagues found something interesting: the two sides of the crayfish seemed to have different breathing rates. When the crayfish could only smell each other on the first day of the experiment, one set of gills was breathing quickly at over 120 breaths min−1 while the other set was under 80 breaths min−1. By the fifth day, the dominant crustaceans had lowered their breathing rate while the submissive crayfish kept breathing quickly. When they finally got to socialize with one another, the breathing rate in one set of gills in the submissive crayfish spiked to over 160 breaths min−1. So, the crayfish's breathing responds just like their heart rate, slowing in dominant ones while the less assertive animals had a spike in their breathing, getting ready to turn and flee. So, while we may not look much like a crayfish, when we're scared, we aren't that different after all.
