Crabs Gone Wild: A Look at the Mysterious Foraging Behavior of Rock Crabs

— Written by Gabbie Baillargeon


Two female brown rock crabs (Cancer antennarius)

Any research involving the study of living animals is simultaneously exciting and frustrating due to the fact that lab experimentation involves manipulation of individuals who have a brain of their own and don’t always want to cooperate nicely.  Although the general goal of my research project is to study the foraging behavior of brown rock crabs on California mussels, the specifics within addressing this research query have shifted in response to my preliminary observations of crab behavior.  Through this research project, I have come to find that doing science is a much more elaborate, drawn out, and unpredictable process of discovery than the simple flowchart of the scientific process in my high school biology book.

In order to start experimenting on the rock crabs, all of the crabs must undergo a set acclimation phase where they are allowed to get settled into their new, luxury tank habitat.  Following acclimation, the plan was to start all of the crabs in individual trials where both crab density, number of crabs per tank, and prey density, number of mussels per tank, would be systematically manipulated.  Simply, this experiment involves changing two variables: food availability and crab predator abundance, with the goal being to measure how the crabs respond to these changes by counting the number of mussels consumed and documenting behavior during feeding. All was going according to


A gravid female yellow rock crab

plan for the first few weeks as crabs came into the lab they were placed in tanks, fed, and played nice with one another. However, due to little previous research published or documented observations available for reference, I had the privilege of witnessing first hand a wide array of surprising crab behaviors before the experiment even began.  For those of you who, like myself, were blissfully unaware of rock crabs’ night time job as a ninja, I am here to tell you that they are exceptionally stealthy little beasts who are capable of destroying every plastic tank barrier I repair. Without the tank barrier in place properly, crabs could move freely between the tanks which makes it impossible to accurately measure which crabs consumed which mussels – a vital aspect of data collection.  Along the way, a few crabs were lost in the battle for scientific understanding as some crab fights would end in death for the loser.  There was even an instance of crab suicide, where the crab simply found a way to escape its tank and met its unfortunate end drowning in the open air.   Additionally, many of the female crabs collected started to become gravid, or possessing eggs, during the acclimation period. To avoid any bias in the study, all females who were not gravid were going to be used in the experiment.


Measuring and labelling a new batch of rock crabs to prepare for running foraging trials

A series of small pre-experiment foraging trials were conducted to get a sense of how the crabs interact with each other, how many mussels they consume, and how they utilize tank space.  The most surprising find of this small study was that groups of female crabs consumed many more mussels than groups of male crabs, indicating that there is a difference in their foraging behavior.  Given this new information, along with the predicament of females become pregnant, the experiment was reconsidered to answer a slightly different question within the same frame of investigating the interface of rock crab foraging and human impact on their populations.  My experimental design shifted to reflect this new path of investigation, as the study now tested only males. To accommodate a shortened timeline and restricted tank availability, the number of prey and predator densities was altered as well.   All in all, the crabs provide a good laugh, a frustrated yell, and sometimes a proud smile along the journey of designing and running foraging trials to better understand their feeding preferences and behaviors.



Leave a Reply