Project Proposal

Open Science Notebook


Habitat loss in the environment often causes interactions between species to increase, forcing them to move closer to each other following damage to their habitat/home.  As the available surface area of livable space decreases, the chance to run into and interact with members of both your own as well as different species increases (Fish, et al. 2005).  Reducing the habitat area can limit individuals to a given area (Galbraith, 2002). Habitat loss affects a wide range of species and has the potential to cause a variety of damaging effects to individuals. Reduction of habitat can put a limit on the number of individuals that can be supported by a given area (Clark, Backwell 2017). Male fiddler crabs are often found in high-density populations where each individual inhabits its own territory containing their burrow used for protection, feeding and courtship displays (Clark, Backwell 2017). If a male loses their burrow due to habitat loss, they must either dig a new burrow in undisputed territory outside of the population or fight for the burrow of another (Clark, Backwell 2017).

One of the common effects of habitat loss is the increase in competition that comes with the decrease in available resources and territorial space. When species lose territory range, it pushes species closer together, limiting their domain and forcing them to partake in more competitive interactions. Intraspecific competition is when individuals of the same species compete over the same resources. In contrast, interspecific competition is when members of different species compete over resources. These can both ultimately lead to a decrease in fitness and even survival, whether it’s on an individual level or an entire species level (Becerril-Morales, 2009).  The increase in competitive interactions in an area that has experienced habitat loss will often drive the dominance of one species over another. 

Aggressive behaviors will arise with the increase in competition when an individual is threatened and must fight over territory or resources.  Habitat loss forces these very combative actions and behaviors more frequently between species when many individuals lose their homes and are on the hunt for new territory and food in order to just survive (Crane, 1975).  To have a clearer understanding of the effects on biodiversity loss, we must further investigate the competitive processes following habitat loss. Fiddler crabs are an ideal model organism to study interspecific and intraspecific competition due to their semi-territorial aggressive nature and the fact that multiple species occupy similar areas (Crane, 1975).  

Our main objective is to observe the difference between intraspecific and interspecific competition. Specifically, we want to observe aggression behaviors between two species of fiddler crabs, U. pugilator and U. panacea. We want to determine how aggression changes when the crabs are forced to share territory with a different species after establishing an initial territory with the same species.  In specific, we will investigate if the aggression increases with the introduction of a different species after they have already established their territory and coexisted with the same species for a period of time.

Hypothesis: Fiddler crabs will be more aggressive when interacting with individuals of different species (interspecific competition) compared to their interactions with individuals of the same species (intraspecific competition).

Prediction: Two individuals of different species when placed together, will exhibit a higher frequency of aggressive encounters compared to two individuals of the same species.

Image 1. Uca panacea

Image 2. Uca pugilator

Study Species

Fiddler crabs, Ocypodidae, are a semi-terrestrial species that occupy mangroves, salt marshes, and sandy or muddy beaches of West Africa, the Western Atlantic, the Eastern Pacific and the Indo-Pacific oceans.  The species Uca pugilator and Uca panacea will be observed in this study. They are commonly recognized by their one, oversized chela or claw that they use in mating courtship displays for females and in combat towards other competing males, (Crane, 1975).  They can court female mates by waving and gesturing their claw away from their body to attract females into their burrows, only if the female decides the claw is large enough, (Christy, 1983).  Fiddler crabs search for food and nutrients by digging in the mud and sand in marshes for algae, bacteria and detritus by using their small claws to scoop the food into their mouths.  Since they are territorial crustaceans, they have shown to compete interspecifically and intraspecifically for resources. 

Experimental Design

We housed 7 of each species separately in two 10-gallon tanks. In those tanks, the individuals themselves were separated by dividers, to cancel out the variability of familiarizing themselves with each other and so we didn’t miss competitive behaviors. The two tanks were conditioned to the same temperature, equal amounts of feeding/water availability, and terrain environment. After setting up the tanks, U. pugilator was placed in Tank 1 and the U. panacea was placed in Tank 2.  We then created an ethogram based on the behaviors that we know saw them exhibit.  They were fed and held under the same conditions as previously described for a duration of a week.  We first took out the 7 crabs in Tank 1, U. pugilator, and placed them in a third 10-gallon tank with just conditioned saltwater so that they could interact with each other for 30 minutes while we tally the amount of times a certain behavior listed in the ethogram is exhibited. When the 30 minutes was over, they were placed back in their divided sections in the tank and maintained for a day before they were placed in the third tank again for observation. We repeated this for a total of 3 times every other day along with the 7 crabs in Tank 2, U. pugilator. After the intraspecific observation replicates were completed, we took 3 crabs from Tank 1 and 3 crabs from Tank 2 and placed them together in the third tank and recorded the frequency of the behaviors listed in the ethogram for 30 minutes and repeated this every other day for a total of 3 times. For data analysis purposes, treatment 1 is defined as intraspecific competition within species 1 (U. pugilator), treatment 2 is defined as intraspecific competition within species 2 (U. panacea), and treatment 3 is defined as interspecific competition between species 1 and species 2.

Image 3. Dividers placed in both tanks – one crab per section. 


To test for differences in aggression between intraspecific and interspecific competition we used one-way ANOVAs with post-hoc Tukey tests. There was no significant difference between interspecific and intraspecific competition for the manus align and extend behaviors. However, the tests show differences between Treatments 1 and 3 for the interlace behaviors (Tukey HSD = 10.17, p = 0.031).  Dactyl slide behaviors showed statistical significance between Treatments 1 and 3 (Tukey HSD = 7.5, p = 0.025) and between Treatments 2 and 3, (Tukey HSD = 9.16, p = 0.008).

FIG 1-4. Treatment 1 is defined as intraspecific competition within species 1 (U. pugilator), treatment 2 is defined as intraspecific competition within species 2 (U. panacea), and treatment 3 is defined as interspecific competition between species 1 and species 2. Letters above bars indicate significant differences between treatments in competitive aggression – bars not sharing letter flagging are significantly different using post-hoc Tukey tests.


Our data supports our hypothesis that interspecific interactions are more aggressive in fiddler crab populations.  We predicted that when individuals of two different species were placed together the frequency of aggressive behaviors will be higher than when individuals of the same species are placed together. The results demonstrate that interspecific and intraspecific competition aggression levels depend on the specific behavioral action.  The dactyl slide and interlace behaviors are more aggressive and intense behaviors than the extend and manus align actions.  Yet these more aggressive actions showed significant enough difference between intraspecific and interspecific frequencies, further supporting our hypothesis. Our experiment provided evidence that when it comes to dactyl slides and interlace behaviors, interspecific aggressive interactions are higher than intraspecific aggressive interactions.  However, the extend and manus align interaction frequencies were not statistically different between intraspecific and interspecific interactions.  Our results were consistent with other studies where establishing territory between two species of fiddler crabs was observed, one species will dominate and have higher fitness over the other species, this is usually based on physical size differences (Clark, Backwell 2017). Dominance can also lead to elimination of another species in a habitat.  In our experiment, U. pugilator generally was observed to be the more dominating species considering it was larger in size in comparison to U. panacea.  We would predict that in nature U. pugilator would be the dominating species over U. panacea and would generally have a higher fitness, since we observed their physical advantages over the smaller species.  This small study entails predictions over a larger scale of how interspecific and intraspecific competition changes the dynamics of a habitat. 


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