# Mangrove crab

Mangrove 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. Mangrove crabs have a variety of phylogenies because mangrove crab is an umbrella term that encompasses many species of crabs. 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. Mangrove crabs primarily live in the Indo-West Pacific region in mudflats along tropical coasts. The largest habitats for mangrove crabs are in Southeast Asia, South America, and Northern Australia. 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). The main difference between the two crab groups is their foraging habits. Litter ingested by sesarmid crabs forms fragmented organic material that helps stimulate microbial respiration, in contrast fiddler crabs remove reactive organic carbon. Mangrove crabs are a part of the Animalia kingdom and are put into the Arthropoda phylum, Malacostraca class, and Decapoda order. Mangrove crabs can be classified into six different families: Camptandriidae, Dotillidae, Macrophthalmidae, Ocypodidae, Sesarmidae, and Oziidae.

### Types of mangrove crabs

* Sesarmid crabs
* Fiddler crabs
* Aratus pisonii, Americas
* Haberma, genus of small mangrove crabs, Indo-Pacific, including:
** Haberma tingkok, Hong Kong
* Metopograpsus messor, Indo-Pacific
* Metopograpsus thukuhar, Indo-Pacific
* Neosarmatium meinerti, Indo-Pacific
* 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, monkeys, hawks, and raccoons. The larvae of mangrove crabs is a major source of food for juvenile fish in waterways near the crabs. Adult mangrove crabs are food for the crab plover among other protected species. To protect themselves the crabs can climb trees. The only other crustaceans that climb trees are hermit crabs.

### Habitat and ecosystem engineering

Mangrove crabs often construct and inhabit burrows in mangrove sediment. 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. Fiddler crabs generally have very simple 10–40 cm “J-shaped” burrows, while sesarmid crabs that burrow often create complex, branching burrows that can reach over 100 cm in depth. Both types of crab significantly increase the surface area of the sediment and water/air interface to similar extents when scaled for relative abundance. These burrows also result in significant burial and downward travel of mangrove leaves. 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.
Mangrove crabs modify particle size, nutrient availability, particle distribution, redox reactions, and organic matter. Aeration allows for additional microbial decomposition, oxidation of iron, and reduction of sulfur by anaerobic microbes. This leads to extremely high pyrite concentrations in mangrove soils, 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.
The feces of mangrove crabs may help form a coprophagous food chain which contributes to mangrove secondary production.

### Biofilms

Biofilm endosymbiosis occurs on the gills of some mangrove crabs, namely Aratus pisonii and Minuca rapax. Each species of these mangrove crabs likely have distinct bacterial compositions. These microbial biofilms are locations of nitrogen transformation, particularly nitrogen fixation. Bacteria like Cyanobacteria, Alphaproteobacteria, Actinobacteria, and Bacteroidota have been found on mangrove crab carapaces. The biofilms served as a net nitrogen sink and a source of ammonium and dissolved nitrogen to the environment. The importance of the biofilm may be dependent on if the crabs live primarily in burrows or outside burrows. Crabs that live outside burrows may consume their nitrogen from microphytobenthos, while crabs that live inside their burrows may rely more on their associated microbes.

## Human impacts
### Climate change

Ideal mangrove crab habitats rely heavily on coastal depth and surface temperature. Climate change due to anthropogenic activities is likely to create fluctuations in these two factors, driving the mangrove crab habitats to higher latitudes. As a result, it is predicted that mangrove habitats will continually shrink for the majority of crab species. This shrinking of habitat space isolates crab communities and shrinks genetic diversity, making many species more vulnerable to extinction.

### 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. This not only negatively affects the health of the crabs, but could affect the health of humans who consume them.

### Land use change

Around 6,000 km$^2$ of mangrove was deforested between 1996 and 2016, usually redeveloped for fish and shrimp aquaculture, rice cultivation, palm oil plantations, and sometimes urbanization. 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.