16 Mangrove Ecosystems Sesarma are small (usually less than cm in breadth) and inconspicuously colored They are amphibious, retreating into burrows at high tide and foraging on the exposed mud at low tide Respiration in air is achieved partly by recirculating water from the gill chambers over the carapace where it can be reoxygenated: evaporative cooling during this process also serves to reduce the dangers of high air temperature Water loss can be offset by the acquisition of soil water through tufts of root-like hairs Sesarmines are euryhaline, although differing degrees of salt tolerance probably contribute to the zonation of crab species along estuaries or with shore level In some cases, sesarmine crabs climb trees to feed on fresh leaves or buds In East Africa, Sesarma leptosoma undertakes synchronized mass migrations twice daily from refuges among mangrove roots to forage on the tips of the branches of the trees The virtually indistinguishable Caribbean species Aratus pisonii spends most of its time in trees, only rarely descending onto the mud Most sesarmines, however, subsist on fallen leaves or propagules Mangrove leaves are often rich in tannins and other aversive materials, and several crab species have been shown to select leaves from the more palatable species of tree Many leaves are collected as soon as they fall and cached in crab burrows As decomposition proceeds, tannin levels decrease and nitrogen content increases through the accumulation of microbial biomass: storage therefore increases leaf palatability Much of the leaf material eaten is not assimilated but redeposited onto the mud as feces, available for microbial decomposition It has been estimated that processing of leaf material by crabs increases the rate of breakdown of leaf litter 75-fold compared with the rate of decomposition under microbial action alone Therefore, sesarmine crabs collectively play a very important role in facilitating energy flow through the mangrove ecosystem By eating propagules, they also affect species distribution and community structure of mangrove trees (see Tree Distribution) However, there are geographical differences: in Southeast Asia and Australia, sesarmines are crucial in litter breakdown and selective removal of propagules, whereas in Florida and the Caribbean they are of lesser significance Ocypodid Crabs Some crabs of the family Ocypodidae, such as the Central American hairy land crab Ucides, consume mangrove detritus The majority are deposit feeders Among these, the most conspicuous are the gaudily colored fiddler crabs (Uca spp.), widespread throughout the mangroves of the Old and New World (Figure 8) The common name derives from the one greatly enlarged claw of male fiddlers, which is used in courtship and in deterring rival males The smaller claw of males and both claws of females are devoted to feeding Mud is scraped into the buccal cavity in which, by a complicated process of flotation and manipulation by the mouthparts, fine organic particles are separated from the mineral components The former is ingested and the latter deposited as a ball of sand, or ‘‘pseudofecal pellet.’’ The process of separation may be quite selective In some species, what is extracted consists almost entirely of microbial cells rather than, for example, frag- Figure Fiddler crab (Uca) in a Mozambique mangrove (photograph courtesy of D Barnes) mented leaf material Others have subtly different extraction techniques and may specialize in the smaller meiofaunal animals There may be as many as 60 fiddler crabs per square meter, resulting in 500 g of soil being processed daily The toll on meiofauna is probably considerable, and the effects on soil texture and composition are profound Other Mangrove Crustacea Other crabs found in mangroves are important predators The most conspicuous is the mud crab Scylla serrata of the family of swimming crabs (Portunidae) Scylla reaches a carapace width of up to 20 cm, making it the largest invertebrate predator found in mangroves Equally formidable predators are the mantis shrimps (Stomatopoda), which live in burrows in the mud and lacerate prey by rapidly shooting out their viciously spiked raptorial appendages Other rarely seen burrowing crustaceans include pistol or snapping shrimps (Alpheus spp.) and the mud lobster Thalassina (see Crustacea as Ecosystem Engineers) More general mangrove scavengers include hermit crabs, particularly Clibanarius, which forage on the mud surface at high tide Shrimps may also be abundant in mangroves and mangrove creeks Penaeid shrimps, which in at least some parts of the world depend heavily on mangroves for feeding and breeding, are an important commercial crop The shrimp Merguia apparently lives only in mangroves and has the distinction of being the only semiterrestrial shrimp: it actually climbs trees Only two species are known One occurs in the Indo-West Pacific region, from Kenya to Indonesia, and the other occurs in Panama, Brazil, and Nigeria Indo-West Pacific and Atlantic regions differ in the composition of their mangrove floras, and the separation of the two species of mangrove-associated shrimps may have occurred in parallel with the divergence of the mangroves themselves Crustacea as Ecosystem Engineers All species have an impact on their environment, at the very least exchanging materials in the form of food, waste