Out of place: a study of the mangrove crab Cardisoma carnifex on Mo’orea, French Polynesia

Background. Invasive plant species pose a significant threat to island biodiversity and ecosystems. Invasive mangrove trees in particular have been shown to be devastating to the shorelines of islands across the Pacific. Previous studies have assumed that the mangrove crab, Cardisoma carnifex, plays a significant role in controlling Rhizophora stylosa mangrove populations on Mo’orea, French Polynesia. Found across East Africa, Indonesia, and the Pacific, C. carnifex’s behavior can change drastically depending on the environment it’s found in.

Methods. From October-November 2016, a field study was conducted on Mo’orea to assess C. carnifex habitat and food preferences. A series of 30 meter transects were done to determine the population density of crabs in mangrove and non-mangrove sites. In addition, a set of food preference experiments were run to determine if C. carnifex preferred to eat R. stylosa leaves, propagules, Hibiscus tiliaceus leaves, flowers, and Paspalum vaginatum marsh grass.

Results. Cardisoma carnifex was found to be more prevalent in mangrove sites although the results were not statistically significant. There were no food preferences found regardless of habitat and the flow of food through both habitats was the same.

Discussion. Results do not give a clear indication of C. carnifex’s ability to impact R. stylosa populations. However, results do demonstrate C. carnifex’s opportunistic nature, suggesting that they are extremely adaptable and do not rely on one habitat or food source for survival. Cardisoma carnifex’s role in its habitat is still poorly understood and needs to be studied in more detail in the future.