BIJU DHARMAPALAN, M.S. JYOTHISH AND P.V. H ARINANDANAN

Unsung Heroes of Ecosystem—Mangrove

Feature Article Feature The survival of the mangrove crabs that are an integral part of the mangrove ecosystem is threatened today.

dead organic material into useful nutrients Crabs in the Mangrove and release these into the water. These Environment can be utilized by other organisms. So Crabs are covered with a thick exoskeleton even though the sediment in mangrove ANGROVE plants are salt-tolerant and armed with a single pair of chelate communities itself does not provide a plants that can withstand high (claws). They are universally seen in all good habitat for much marine life, the oceans, while many of them live in land and concentration of salts. They are M surrounding water provides a good habitat fresh water particularly in tropical regions. usually found in river deltas where river for a diversity of organisms. They differ in size from the pea , a few outputs are mixing with oceanic salty water. The networking of mangrove roots millimeters wide, and the Japanese spider Strong mixing of water during tidal forms a complex structure that provides very crab, with a leg span of up to 4 metres differences results in turbid water with low important nursery areas for marine life such (13 ft). visibility. A highly elaborate root system is one as reef fish, prawn and crabs. Crabs form The crabs constitute about 80% of the of the main adaptations of these plants, an important part of the and macrofaunal biomass and can reach providing support and enabling them to are a predominant . densities of 80-90/m2. They depend withstand physical stresses. Mangrove communities are only found in tropical or subtropical areas of the world. They require water temperature above 24ºC in the warmest month. Annual rainfall that exceeds 1250 mm is considered as optimum for mangrove communities. These requirements are similar to the needs of tropical coral reefs.

Very fine silt or mud particles are the main components of sediment around mangrove areas. The dense packing of these particles makes the sediment oxygen-poor. Bacteria are the only organisms that can survive and thrive in this oxygen-free environment. They can reduce sulphur instead of oxygen for respiration, producing a foul smelling gas

(H2S). The bacteria provide an important first step in the mangrove community food chain. They decompose and convert all Fishes near mangrove roots (Photo taken from Ayiramthengu mangrove, Kerala) 53 SCIENCE REPORTER, OCTOBER 2011 Feature Article

It is impossible to conserve the crabs without conservation of the entire ecosystem. In the long run the mangrove plants could get adapted to the changed environment, but for the little crabs that may not be possible. Once they are lost, they may be lost forever.

Grapsid crabs are amongst the most important crab species with regard to the number of different crab species as well the total number of crabs present. Most grapsid crabs belong to the subfamily Sesarminae and many also belong to the largest in Sesarmine, called Sesarma. Some of the more commonly mentioned species in literature are Neosarmatium meinerti, Neosamartium smithi, Sesarma messa, Sesarma moluccensis, Metapograpsus latifrons, and Metapograpsus thukarhar. There is considerable difference in the occurrence and diversity of mangrove grapsid crab species depending on the particular mangrove forest. Brachyuran crabs, which contribute a bioenergetically significant faunal component, play a significant role in maintaining a steady state of the mangrove ecosystem. Wide feeding habits are the main reason for the diversity of the mangrove crabs. The substratum, water level, salinity, Crabs clinging to mangrove roots temperature and floral distribution are (Photos taken from Ayiramthengu mangrove, Kerala) possible factors that influence zonation and distribution of crabs in the mangrove directly on mangrove areas for survival, by affects the productivity and reproductive environment. feeding on leaves and litter. Along with output of the vegetation, supporting the The ghost crab Ocypode macrocera numerous annelids and living hypothesis that mangrove crabs are a prefer sandy substratum and in the sediment the crabs have a keystone species. Macropthalmus species prefer only muddy significance in detritus formation, nutrient They are very much adapted to the substratum. Uca species is found in dry or recycling and dynamics of the ecosystem. sediment condition, tidal fluctuations, and grassy, elevated and muddy grounds. Crabs are good excavators—they make varying salinity found in the mangroves. Grapsid crabs are the dominant burrows that are used for refuge, for Mangrove crabs feed through a variety of species among the crab taxa. Grapsid feeding, as a source of water or for modes. Grapsidae and Ocypodidae are the crabs are considered significant seed establishing a territory necessary for two main families of crabs that are predators of mangroves and can be a threat mating. associated with mangrove ecosystem. Their to the successful regeneration or restoration The digging behaviour of crabs has a ecological significance depends on the of mangroves. Although grapsid crabs tend key role in mangrove aeration, draining and abundance of crab species. to consume a large amount of propagules, turning the dense waterlogged soil—a direct Except the Fiddler crabs, most crabs some studies have shown that they can be benefit to the plants that in turn give them tend to be nocturnal, with the majority selective, selection based on nutritive values shelter. Removing crabs from an area appearing at night on a rising tide and of the propagules and the density of the causes significant increase in sulfides and remaining in burrows by day to avoid specific mangrove tree species in the ammonium concentrations, which in turn predatory birds. canopy.

SCIENCE REPORTER, OCTOBER 2011 54 Feature Article

Tree crab (Photo taken from Ayiramthengu mangrove, Kerala) Tree crabs are adapted to Mud crabs (Photo taken from Ayiramthengu mangrove, Kerala) climb on mangrove trees, sharp tips at the end of the legs are one such adaptation. Mangrove tree crabs are Mud Crab up to 3.5 kilograms in size and have a shell Scylla serrata is also known as mud crab or width of up to 24 centimeters. commonly seen in red mangrove crab. It is an economically mangroves. important crab species usually seen in Fiddler Crab estuaries and mangroves of Australia, Africa This is a semi-terrestrial marine crab sediment into little balls. The occurrence of and Asia. Usually its shell colour varies from belonging to the family Ocypodidae and these sediment balls near the entrance of a deep, mottled green to very dark brown. the genus Uca. The name Fiddler crab a burrow is a good sign of its occupation. It can be served as a seafood delicacy. In derives from the fiddle-shaped claw By sifting through the sands, they aerate the South Asian countries it is considered to be present on the males. They are usually substrate and prevent anaerobic among the tastiest of crab species. In the found in sea beaches, brackish inter-tidal conditions. Thus, the fiddler crabs play a Northern states of Australia and especially mud flats, lagoons and swamps. Fiddler vital role in the preservation of wetland Queensland, these crabs are common and crabs shed their shells as they grow. If the environments. even a bit more expensive than other sea large fiddle claw is lost, males will food. regenerate it on the opposite side after the Mangrove Tree Crab These crabs are highly cannibalistic in next molt. Aratus pisonii is one of several species of nature, the hard shelled ones will consume The crab’s smaller claw picks up a bulk crabs belonging to the family. the moulting crabs, which have of sediments and brings it to the mouth, Its carapace is mottled brown to olive- comparatively soft shells. The females can where its contents are sieved through. After green in colour and is widest at the front, give birth to 1 million offspring that can grow salivation of edible things, they convert the tapering posteriorly. Eyes are situated at the front corners of the carapace. Its legs are brown to mottled and the claws are hairy. These crabs are adapted to climb on mangrove trees, sharp tips at the end of the legs are one such adaptation. Mangrove tree crabs are commonly seen in red mangroves.

Grapsid Crabs They are small in size and are commonly seen in rocky shores and mangroves. Grapsid crabs are nocturnal. They are colourful crabs, and are active during night. Fiddler crab Grapsid crabs are also known as shore (Photo taken from Ayiramthengu crabs. They possess a purplish hairy leg mangrove, Kerala) peeking out from the crevice. These crabs are very well adapted for scrambling over rocks and other slippery surfaces. They can 555555 SCIENCE REPORTER, OCTOBER 2011 Though the sediment in mangrove communities itself does not provide a good habitat for much marine life, the surrounding water provides a good habitat for a diversity of organisms.

seedlings in mangroves. They also actively Grapsid crabs take part in the recycling of nutrients by (Photo taken from “ploughing”. Ayiramthengu mangrove, Even though the grapsid and Kerala) ocypodid crabs are considered to be of no economic value, they are involved in the breakdown of particulate organic material by exposing them to microbes. Their burrowing habit helps in the oxidation stay out of water for some time. Hooks on Conservation Efforts of sulphides. Degradation of mangrove leaf the tips of their long flattened legs are Crabs are good sources of proteinaceous litter by searmid crabs plays a key role as a adaptations to survive in rocky shores. In food for marine organisms as well as man. major link between primary and secondary some species, males have larger pincers Crab protein is advisable as compared to production. Leaf consuming mangrove than females. Grapsid crabs are muscle meat of mutton, chicken, duck crabs have a key role in the initial scavengers and can consume almost and fish because of its high nutritional processing of litter. Because of the anything. quality. Besides, mangrove crabs are also herbivorous nature of searmid crabs they good food for birds, snakes and predatory can be considered as a vital factor in the The dense packing of mud fishes. degradation and thus in biogeochemical Brachyuran crabs are detritus feeders particles makes the sediment cycles. of micro particles of plant and origin. oxygen poor. Bacteria are the Even though conservation of The detritus therefore is a biogenic material mangrove plants has been an area of only organisms that can survive in various stages of microbial decomposition prime focus, the real heroes who maintain and represents a potential energy source and thrive in this oxygen-free a healthy ecosystem seem have to been for the consumer species. environment. neglected. The major problem faced by The faecal matter of crabs contains these unique organisms is the disruption The bacteria provide an carbon, nitrogen, phosphorus and trace of mangrove forests by human metals that can serve as food for other important first step in the interference. consumers. Besides, their burrowing habit Nowadays developmental activities mangrove community food provides aeration and free circulation of are progressing at a fast pace near water, which promotes the growth of chain. mangrove habitats. Even though large stretches of mangrove forests get the attention of conservationists, some places Construction of like Ayiramthengu mangrove in the Kollam road through district of Kerala are still neglected. It Ayiramthengu seems the local people and the mangrove in government are still unaware of the Kerala significance of the mangrove community. It is impossible to conserve the crabs without conservation of the entire ecosystem. In the long run the mangrove plants could get adapted to the changed environment, but for the little crabs that may not be possible. Once they are lost, they may be lost forever.

Shri Biju Dharmapalan (E-mail: [email protected]), Shri M.S. Jyothish and Shri P.V. Harinandanan are with the Department of Plant Biotechnology, Mar Athanasios College for Advanced Studies, Tiruvalla, Kerala-689101 SCIENCE REPORTER, OCTOBER 2011 56