Katie

Interactions of the Mangrove Tree Crab (//Aratus pisonii//) within a Red Mangrove (//Rhizophora mangle//) Community.

flat = = = **Introduction** = A mangrove is a species of trees and shrubs that thrive in saline soil restricted to tropical zones worldwide; the most common are the white black, and red mangrove. As an intertidal system, a mangrove forest is characterized by physical factors that play a large role in testing the communities and the organisms within them. The habitat structure is complex and thus creates a 3D cover from predation for all invertebrates primarily crabs. The seasonal changes among predators emphasize the importance of foraging behavior and physiological range on the predation that goes on within the system. (Wilson et al 1989) The physical structure of mangrove communities creates for a particularly rich environment. The dense populations that make their homes among mangroves allow complex food webs to build from the various tree litter including leave, bark and wood decay. Most grapsid crabs consume this litter also known as detritus, but some species consume fresh leaves. Such a greater diversity of invertebrates are able to create their own niches and microhabitats among the great array of vegetation found among mangroves. (Whitney)

= **Organisms involved** =

The Red Mangrove, //Rhizophora mangle//, is one of two mangrove species that dominate Floridal’s tidal zones. Red mangroves thrive in frequently flooded “low marsh” zones, but can tolerate dry periods with seasonally low water available to them. Like all mangroves, R. mangle helps form and stabilize the shifting land they grow on, whose roots form a tangle that hold the tree to the ground. Like all mangrove species, the Red Mangrove produces popagules, which are seeds that become seedlings that are ready to root before they leave the parent tree. The prop roots of the red mangrove are essentially branches that have grown out, and down in the water planting themselves in the sand to form a new tree. These roots penetrate one to two inches in the soil and the aerial portion, that is, the part above soil level, contain spongy material on the interior that allows oxygen to diffuse through to the lower portion. Breathing pores of the lower trunk, known as lenticels, control oxygen flow between high and low tides. At low tide, the pores are open to let oxygen in but at high tide they close to prevent water from flowing in. Such adaptations enable mangroves of all species to survive in anoxic sediments. The Mangrove Tree Crab, //Aratus pisoni// i of the family Sesaridae, can be found from eastern Florida to northern Brazil, including the Pacific coasts from Nicaragua to Peru and throughout the Caribbean. The carapace is a mottled brown to olive green and legs also a brown to mottled color bear sharp tips at the end allowing the crab to climb mangrove trees. //A. pisonii// is predominantly found among red mangrove tree communities though populations have been found among black ( //Avicennia germinans// ), white ( //Avicennia schauriana// ), and tea mangroves ( //Pelliceria rhizophorae// ). They migrate vertically usually inhabiting tree canopies during high tide and exposed sediments during low tide. (SMS) Though absolute abundance measurements are few in number, past studies have noted that the crab sizes vary with habitat locations with the larger organisms among more mature forests and smaller individuals amid the younger, stunted trees. Though grapsid crabs are tolerant of the deoxygenated environment created from organic enrichment, the survival of crab populations is controlled by the development of landward mangroves. (Lee et al 1998).

= **Biological Interaction** =

Examination of the red mangrove arthropod communities demonstrate the way in which the mangrove tree crab is a key organisms in the trophic relationships that make up the above water food web (Beever et al 1979). The grapsid crab community is one of the most abundant and potentially the most important group of macrofauna inhabiting mangrove forests. Most grapsid crabs consume litter but few species such as the Mangrove Crab Tree consume fresh leaves of the tree. Up to 96 percent of herbivory in mangrove communities is due to living leaf consumption even in areas of low crab abundance. Most leaf consumption occurs in the fringing zones where the leaves are characteristically older. Such consumers of fresh leaves can climb as high as the top of the trees. (Alongi) As an organism that plays a critical role in biomass export in mangrove forest, //Aratus pisonii//. Though omnivorous, up to 42 percent of a mangrove tree crab’s diet can be made up of mangrove leaves. Crabs who feed on mangrove leaf tissue leave behind distinctive scraping marks, making observational studies somewhat easier. Such studies have found that //A. pisonii// can damage over 80 percent of red mangrove leaves including over seven percent of the leaf area. While examining biomass export through production of frass, it was estimated that an adult mangrove tree crab would introduce over eight centimeters cubed of grass per month into the aquatic system. (Lacerda) The provided shelter and nutrition by the red mangrove trees, give the mangrove tree crabs the ability to fuel a healthy system allowing mangroves to thrive among diverse communities.

= **Impact/Importance** =

The physiology and characteristics unique mangroves systems are suited to provide refuge from crab predators among microhabitats. This means, the crab must adapt to tolerate the seasonal variances experienced in these systems and consequently balance the constraints of physiologically tolerance and the avoidance of predation. (Wilson et al 1989). As a key member of the arthropod community, that dominates the red mangrove population, the mangrove tree crab plays the ecological role of primary herbivore, predator, and biomass and energy exporter via their offspring and frass (Beever et al). Through their feeding, grapsid crabs allow a large proportion of organic matter to be recycled within mangrove forests. Furthermore, the organic matter processed by these crabs for the base of the food web allowing energy to flow up the trophic level through small invertebrates. Surface topography is altered from crab bioturbation, affecting overall growth and production of mangroves. These changes can occur in particle size distribution, and/or degree of aeration, which can consequently affect the soil and water phytotoxin concentrations.

=Current Research=

Though there has been no significant observed relation between crab abundance and the distance from mainland or water flow intensity and/or direction, Diaz et al 1989 observted //A. pisonii// abundance related to fouling community richness and red algae ( //Catenella repens// ) of mangrove roots located in intertidal zones. This question requires further study, but complementary studies determining factors controlling the //A pisonii// abundance and distribution will give a more complete understanding of the population dynamics. (Diaz et al 1989)

=References=

“//Aratus pisonii// .” LH Sweat, Smithsonian Marine Station at Fort Pierce. Smithsonian Instituiton. 2009. Site visited 25 April, 2011 from, < [] > Beever, JW, Simberloff, D & LL King. 1979. Herbivory and predation by the mangrove tree crab //Aratus pisonii. Oecologia.// 43: 317-328. Humberto Diaz and Conde, Jesus Eloy. 1989. Population Dynamics and Life Hisotry of the Mangrove Crab Aratus Pisonii (Brachyura, Grapsidaw) in a Marine Environment. //Bulletin of Marine Science//, 45: 148-163 Wilson et al 1998 Alongi, Daniel M. //The energetics of mangrove forests// Drude de Lacerda, Luiz. //Mangrove ecosystems: function and management.// Whitney, Eleanor Noss, Means, D. Bruce, Rudloe, Anne. //Priceless Florida: natural ecosystems and native species.// //http://www.sms.si.edu/irlspec/aratus_pisoni.htm//

Page authored by Katie Carmody student of Dr. Michelle Lum at Loyola Marymount University, Los Angeles.