thumb|Red mangrove crab<br />Neosarmatium meinerti

thumb|Mangrove crab

alt=Mangrove crab|thumb|Mangrove crabMangrove crabs are crabs that live in and around mangroves. They belong to many different species and families and have been shown to be ecologically significant by burying and consuming leaf litter. By shredding and burrowing the leaf litter, mangrove crabs are able to prevent tidal export, which in turn helps retain the nutrients present in their habitats.This makes the nutrients around mangrove trees more available to other organisms, and thus they have an important role in maintaining ecological relationships and processes within their environments. Two of the most common families are sesarmid and fiddler crabs. They are omnivorous and are predated on by a variety of mammals and fish. They are distributed widely throughout the globe on coasts where mangroves are located. Mangrove crabs have wide variety of ecological and biogeochemical impacts due to the biofilms that live in symbiosis with them as well as their burrowing habits. Like many other crustaceans, they are also a human food source and have been impacted by humans as well as climate change.

Species and distribution

Current estimates place the number of mangrove crab species at 481 in 6 different families, with new species being discovered frequently. Their role as propagule predators, or consumers of reproductive units of plants, is also critical in the maintenance of the environments in which they reside in through the broad impacts they have onto the vegetation structure and regeneration. As their name suggests, they are primarily found among mangrove tree forests and form symbiotic relationships with the trees, restricting their habitat to where the trees can grow.

Phylogeny

A variety of different species are what makeup the umbrella term of mangrove crabs. The two main crabs that typically dominate mangrove ecosystems are the sesarmid (Grapsidae) and fiddler crabs (Ocypodidae). Litter ingested by sesarmid crabs forms fragmented organic material that helps stimulate microbial respiration, in contrast fiddler crabs remove reactive organic carbon.

  • Neosarmatium smithi, Indo-Pacific
  • Parasesarma leptosoma, western Indian Ocean
  • Perisesarma, genus with 23 species, primarily Indo-Pacific, with two West African species, including:
  • Perisesarma bidens, Indo-Pacific
  • Perisesarma guttatum, western Indian Ocean
  • Scylla serrata, Indo-Pacific
  • Scylla tranquebarica, Indo-Pacific
  • Sesarma, genus with close to 20 species, many of which live in mangroves, Americas, Indo-Pacific
  • Ucides cordatus, western Atlantic Ocean

Ecology and biogeochemistry

Diet and predators

When young, mangrove crabs get most of their nutrients from polychaete worms and a multitude of microorganisms found living in the sediments and leaves of their environment. As they grow older mangrove crabs are generally detritivores with their diet consisting of already dead organic material. Mangrove crabs consume a large amount of plant material but are primarily omnivorous. In the mangrove swamp this includes dead leaves and corpses of other crustaceans, even that of their own species. In some cases, mangrove crabs may also eat fresh mangrove leaves. Mangrove crabs are predated on by wading birds, fish, sharks, Adult mangrove crabs are food for the crab plover among other protected species. To protect themselves the crabs can climb trees. However, the mangrove crab has also been known to jump off of trees in order to escape avian predation as well. Ultimately, this puts the species more at risk for fish predation and thus, the mangrove crab proves to be vulnerable whether they are up in the trees or residing near the water. Their ability to move around within the mangrove tree habitat does show a sentence of adaptability, which is not present in many other crustacean species, besides notably hermit crabs.

Habitat and ecosystem engineering

alt=Mangroves|thumb|A mangrove

Mangrove crabs often construct and inhabit burrows in mangrove sediment, preventing factors such as loss of nutrients within their ecosystem while simultaneously promoting decomposition. These burrows aid them in enduring the extremes that can be found in mangroves at high and low tide, allowing them to maintain more constant and ideal temperatures and oxygen levels. These constants can additionally aid other small benthic fauna, like polychaetes and juvenile crabs. Mangrove crabs may plug their burrows at intervals determined by their circadian rhythms, or they may leave them open. The variety in structures and maintenance of these burrows may lead to a variety of different impacts on mangrove sediments, such as increasing or decreasing erodibility. while sesarmid crabs that burrow often create complex, branching burrows that can reach over 100&nbsp;cm in depth. The burrowing dynamics of mangrove crabs dramatically impacts ecosystems, these dynamics were impacted by both abiotic factors like soil composition, and biotic factors like root depth and tree density. Aeration allows for additional microbial decomposition, and removal of sulfides that negatively impact plant growth. Surface soils are similarly impacted when mixed by mangrove crab legs.

Depending on its nitrogen content, burial of detritus in crab burrows can stimulate microbial growth and activity and lead to variation in mangrove soils' carbon dioxide efflux, ammonium content, and nitrate content.

Biofilms

Biofilm endosymbiosis occurs on the gills of some mangrove crabs, namely Aratus pisonii and Minuca rapax. This nitrogen fixation has even been found to be enriched within the crab's intestines and sediments, in addition to sulfur associated compounds. These compounds are thought to help the species degrade organic compounds through their roles as terminal electron acceptors in the processes which help the organism produce energy. which helps the species convert ammonia to amino acids while additionally helping avoid negative side effects of their environment such as toxicity from sulfur compounds and influxes of carbon monoxide. The animal-microbe associations themselves serve as a net nitrogen sink, with nitrogen fixation exceeding nitrogen losses, and thus a source of ammonium and dissolved nitrogen to the environment. Another fact which further helps support the increased prevalence of N-fixation in mangrove crab habitats is the abundance of the stable N isotope in fiddler crab associated biofilms, due to the depletion of <sup>15</sup>N isotope. All crab species however, can impact the spatial abundance of nitrogen within their environments, through the process of selective grazing and moving around within their habitats depending on mating season, predatory threats, and low tides.

Crabbing

Like many other crustaceans, mangrove crabs have historically been caught, prepared and eaten by people all over the world. Crab meat can be prepared simply by boiling the crab either dead or alive until the shell turns from black to red. This practice may be threatened by human activities, however, as microplastics have been found to be abundantly common in the gills of mangrove crabs due to human pollution. Diversity of mangrove crabs does not seem to be negatively affected in abandoned aquaculture plots, though logging has significant negative effects on mangrove crab diversity.

See also

  • Fiddler crab
  • Sesarmid crabs
  • Grapsidae
  • Mangrove ecoregions
  • Neosarmatium australiense

References

  • East African mangrove crabs
  • Spotted Mangrove Crab (Caribbean and Florida)
  • Mangrove Crab Aquaculture
  • Mangrove crabs at Sungei Buloh Nature Park
  • Mangrove crab Ucides cordatus (Brazil)
  • Complete larval and early juvenile development of the mangrove crab Perisesarma fasciatum (Singapore)
  • Laboratory cultured zoeae and megalopa of the mangrove crab Metaplax distincta
  • Regulation of pulmonary blood flow and of blood pressure in a mangrove crab (Goniopsis cruentata)
  • Haberma nanum, a new genus and new species of mangrove crab from Singapore