Harmony and Discord Under the Sea

            Mutualistic Relationships

Mutualistic relationships are often formed through the Graceful Decorator Crab's eating and camouflaging habits, providing protection, food sources, and even mobility to stationary creatures.  Sedentary animals such as bryozoans, hydroids, and sponges are frequent tools used by the crabs to conceal themselves but these animals benefit from this relationship as well (Weis 2012).    To learn about similar types of sponges that are used by the graceful decorator crab check out sea sponges and marine sponges These often immobile animals placed on the backs of the crabs are now allowed access to new water currents and food that was previously unattainable.  A prevalent example of this symbiotic relationship is the way in which the crabs decorate themselves with algae.  While the algae gets a relatively safe and secure place to live the crab is well hidden and also receives a food source in the algae (Weis 2012).   


       Feeding and Commensalism Relationship

   Other than algae, the Graceful Decorator Crab's diets consists of plankton, small and dead animals, and have even been known to occasionally eat other crabs (Larson).  Using their long pincers, these omnivores are able to extract food from crevices and the sea floor or can simply take the food growing on their back (Cattle Point).  These powerful pincers are also able to crack hard shells like those of other crustaceans.  During the course of feeding, Oregonia gracilis often times drops morsels of food, in turn feeding and supporting other creatures around them.  One such example would be anemones, which provide an important form of protection from many various predators.  A specific anemone that has similar qualities to the ones that the graceful decorator crab interacts with is the ritteri anemoneThese crabs have even been observed picking up and waving the anemones stinging tentacles at oncoming predators (Weis 2012).


            Threats often encountered by Oregonia gracilis are fish, sea otters, cephalopods such as the gaint pacific octopus, and other various predacious animals.  Being a bottom dwelling fish in the crabs main habitat, pacific halibut is one of the Graceful Decorator Crab's most common foes.  Both live in the cold, northern waters of the Pacific making encounters a common occurrence.  Another prevalent hazard encountered at the bottom of the ocean are cephalopods like octopi. (Walking Sideways 2009)Even other crabs have been known to attack and eat them making their choice in camouflage even more critical. 

 Parasitic Relationships

             While the crabs may spend their time worrying about halibut or cephalopods, there are other organisms which pose just as great, if not more of a threat to them; parasites.  Some of the most common parasites found on crabs are rhizocephalans, a type of parasitic barnacle (Weis 2012).  Sacculina is a rhizocephalan which invades the crab through vulnerable areas such as joints with soft cuticles and grows from within the crab.  Much like roots of a tree, Sacculina invades every tissue until it has completely taken over the crab and consumed the reproductive system (Weis 2012).  No longer able to effectively reproduce, the crab is only able to excrete the parasites larva instead of their own.  Some rhizocephalans are even able to alter the nature of the crabs so that they take over as caregivers to the barnacle's eggs, essentially taking care of them as if they were their own.  Other parasites have been known to make crabs sterile as well such as entoniscids, a parasitic isopod (Weis 2012).  Often times when these are present in a population, crab numbers are decimated. 

             Interaction With Humans

            Although not found on the dinner plate as with other crabs, interaction between humans and the Oregonia gracilis is possible, albeit sparingly.  Regularly found in the intertidal waters of the Pacific, humans with a keen eye may have the chance to catch a glimpse of one elaborately camouflaged looking for its next meal.  Some decorator crabs are even available for purchase to be kept in aquariums although it is most likely best to have in-depth knowledge of their behavior and habits before doing so.


<<<Reproduction                                                                                                                 Fun Facts>>>

Copyright 2008
University of Wisconsin-La Crosse
1725 State St. La Crosse, WI 54601

Contact us
Alecia Faber
Ben Tschoeke