I. Background and Significance

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 loss can put a limit on the number of individuals reliant on a given area (Clark, Backwell 2017). Male fiddler crabs live-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 effect’s habitat loss induces is the increase in competition that comes with the decrease in available resources and territorial space. When species lose range of their territory 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).  

II. Objectives and Hypothesis

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: Interspecific competition of Fiddler crabs will exhibit more intense aggression than intraspecific competition.

Prediction: If we introduce two species of individual fiddler crabs to each other, then they will exhibit a higher frequency aggressive encounters compared to their intraspecific aggression.

III. Methods

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 U. pugilator and U. 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 are able to 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.  Up to four crabs can live in a 10-gallon tank in a laboratory setting and live in temperatures of about 75 to 85 degrees Fahrenheit.

Housing and Feeding 

The crabs will be housed in two 10-gallon aquarium tanks with no more than four in each tank. The two species will be separated at first arrival. They will have access to saltwater and a land area with enough sand for burrowing and protection. They will be placed where they can get plenty of sunlight and warmth. A heat lamp will be available if supplemental heating is necessary to keep the tanks between 75°-85°F. All crabs will be given ample food so they are not hungry. They will be fed freeze-dried plankton and shrimp every other day.

Experimental Design

To observe if the crabs will be more aggressive towards a different species when introduced we will start by housing 7 of each species separately in two 10-gallon tanks, so that 7 crabs of the same species are housed together in one of the tanks.  In those tanks the individuals themselves will be separated by dividers, to cancel out the variability of familiarizing themselves with each other and also so that we don’t miss competitive behaviors. The two tanks will be conditioned to the same temperature, equal amounts of feeding/water availability, and terrain environment.  All the seperated sections in both tanks will have a damp sand area and a saltwater area.  The two tanks will be labeled based on the initial species that occupied that tank.  If the difference between the two species is visibly unclear, then appropriate labeling techniques and incorporating numbering of the individual crabs will take place prior to combining the two species. After setting up the tanks, U. pugilator will be placed in Tank 1 and the U. panacea will be placed in Tank 2. Initial observation will take place for 30 minutes to record the behaviors that take place. We will then create an ethogram based on the behaviors that we know they can exhibit.  They will be fed and held under the same conditions as previously described for a duration of a week.  We will first take out the 7 crabs in Tank 1, U. pugilator, and place them in the third 10-gallon tank with just conditioned saltwater so that they can 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 is up they will be placed back in their divided sections in the tank and maintained for a day before they are placed in the third tank again for observation. We will repeat 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 are completed, we will take 3 crabs from Tank 1 and 3 crabs from Tank 2 and place them together in the third tank and record the frequency of the behaviors listed in the ethogram for 30 minutes and repeat 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.

Sample Data Sheets

Interspecific: U. pugilator vs U. panacea

TrialExtendManus AlignDactyl SlideInterlace
1    
2    
3
4
5
6

Intraspecific: U.pugilator

TrialExtendManus AlignDactyl SlideInterlace
1    
2    
3

Intraspecific: U.panacea

TrialExtendManus AlignDactyl SlideInterlace
1    
2    
3

 The frequency of each behavior will be tallied for each interspecific and intraspecific competition over the course of one week.

Ethogram:

Combative Behaviors

Extend- claw is swept toward opponent, no contact

Manus align- Opponents face each other with manus of one claw held adjacent to the other, no shoving

Dactyl slide- pollex and dactyl of the claw of each opponent are intercrossed and slid back and forth at their distal ends

Interlace- intercrossed pollex and dactyl are clamped tight on opponents’ claw; vigorous shoving and pinching occurs

Statistical Analysis

To test for differences in aggression between intraspecific and interspecific competition we used one-way ANOVAs with post-hoc Tukey tests. A Tukey test will tell us the significant difference between intraspecific competition within U. pugilator, intraspecific competition within U. panacea, and interspecific competition within U. pugilator and U. panacea.

IV. References

Becerril-Morales F, Macías-Ordóñez R. 2009. Territorial contests within and between two species of flies (Diptera: Richardiidae) in the wild. Behaviour 146, 245–262.

Clark, H L, and P R Y Backwell. “Territorial Battles between Fiddler Crab Species.” Royal Society Open Science, The Royal Society Publishing, 18 Jan. 2017, www.ncbi.nlm.nih.gov/pmc/articles/PMC5319326/.

Crane, J. 1975. Fiddler crabs of the world . Princeton University Press, Princeton, NJ, USA.

Christy, J.H. 1983. Female choice in the resource‐defense mating system of the sand fiddler crab, Uca pugilator.Behavioral Ecology and Sociobiology , 12: 169–180

 Fish MR, Côté IM, Gill JA, Jones AP, Renshoff S, Watkinson AR. 2005. Predicting the impact of sea-level rise on Caribbean sea turtle nesting habitat. Conserv. Biol. 19, 482–491.

Galbraith H, Jones R, Park R, Clough J, Herrod-Julius S, Harrington B, Page G. 2002. Global climate change and sea level rise: potential losses of intertidal habitat for shorebirds. Waterbirds Int. J. Waterbird Biol. 25, 173–183.

V. Resources

  • Uca pugilator fiddler crabs
  • Uca panacea fiddler crabs
  • 3 10-gallon tanks
  • 2 heat lamps(optional)
  • Water
  • Aquarium salt/sea salt, water conditioner
  • Sand
  • Freeze-dried plankton and shrimp/sinking pellets
  • Stopwatch
  • Lab bench space to hold tanks
  • 10-12 sheets of plexiglass (must fit in the tank)