Rakesh Kumar Gupta
Freshwater crabs are an ecologically successful group, occurring widely in tropical region of world. They are however, absent in northern and central Europe, northern and central USSR, Southern and Central Australia. In the tropical region of the world freshwater crabs of the family potamonidae are generally well established.
In India they are reported in almost all the states, more particularly in states having freshwater as source of aquatic fauna i.e. J and K, from Punjab to Nagaland, Bihar, Kerala and Tamil Nadu, etc. In Jammu region of J and K state Maydelliaptelphusa masoniana and Himalayapotam emphysetum of family Gecarcinucidae and Potamidae respectively has been recorded.
Habit and Habitat
Freshwater crabs are usually associated with lotic water (flowing water), however, their presence in lentic water (standing water e.g. lakes) also has been reported in north Bihar. In Jammu and Kashmir, only two species of crabs has been recorded viz. Maydelliatelphusa masoniana and Himalayapotaman epmphysetum, both the species are confine to river and streams. Further, being poikilotherms, the distribution of crabs has been confined to Jammu region where climate shows marked fluctutation whereas it remain cold in Kashmir and Ladakh region throughout year.
Freshwater crabs are mostly omnivorous scavengers, mainly feeding on plant matter, but some are opportunistic carnivores, feeding either on live prey such as fish and prawns or on dead animals that they encounter and cannibalism is not uncommon.
Reproduction
Maydelliatelphusa masonina is seasonal breeder while Himalayanpotamona emphysetum is continuous breeder. Parental care may be uniparental or biparental i.e. exhibited either by one or both the sexes respectively . In decapod crustaceans, parental care is usually recorded to be restricted to females only that carry eggs in the brood, the number of which varied from 100-900 depending on species.
Growth in crustaceans is characterised by the periodic replacement of the exoskeleton which takes place during the ecdysis process. Since crustaceans lose all of their hard parts during the moulting process, there is no method that can directly determine the age of these crabs.
Nutritious and Medicinal Value:
Crabs are good source of food to man as a good protein source. The nutritional quality of the crab proteins compare very favorable than that of muscle meat of mutton, chicken, duck and fish and are consumed in many parts of the world.
Detritius Food Chain
Trophic webs are considered some of the main forces that structure aquatic communities. Moreover, studies of the feeding ecology of animals contribute to the knowledge of the nutritional requirements of species, interactions with other organisms, and community organisation patterns over evolutionary time.
Scientific Value
The exoskeleton of crabs has been used for its antimicrobial activity as well as to prepare cover of medicines. Besides this, crabs has antileukemic, anticoagulant and cardio active properties. Crabs have plentiful amount of selenium which is an anti-oxidant, and cancel out the carcinogen effect of cadmium.
Bioindicator
Freshwater crabs act as good biological indicator. With the growing pollution level in stream or river, the increase in concentration of heavy metal can be detect in crabs where it get accumulated in the liver mainly. Besides this, any variation in such parameters has marked affect on population abundance of crabs resulting increase in their mortality rate, thus crabs act as an bio-indicator of pollution. They are also bioindicator of heavy metals. Crabs plays an important role in maintenance, modification and regulation of the benthic environment by influencing both the environment.
Ornamental Purpose
Freshwater crabs being small size and colourful are used in aquarium. They are easy to handle and culture in aquarium. Some of the important freshwater crabs used in aquarium are: Red clawed crabs, Fiddler crabs, Gold claw crabs, Soap dish crabs etc.
Employment
Freshwater crabs farming provide opportunity to farmer for growing paddy as well as freshwater crabs on same field, thereby helps to earn their living.
Threat to Freshwater Crabs
Different cause of freshwater extinction are as following:
1. Habitat Destruction
Freshwater crabs presently facing a great Threat due to habitat destruction and pollution. In the present studies, in Jammu region of J and K State, the dumping of waste in three streams viz., Jajjar stream, Tatdiyal and Gho-manhasan stream has lead to considerable reduction in the number of crabs. Seemingly dumping of Gulshan Langar waste caused considerable loss to inhabiting crabs in Ban Ganga stream. Besides dumping repeated trempling of stream basin force the crabs to shift upstream or downstream.
2. Overfishing
Fishermen while catching adult crabs caught smaller ones occasionally. Instead of releasing them back into river, they either used it as food or sell in the market. This led to considerable decrease in crab abundance. Besides this, fishing of crabs during their breeding season has much adverse affect on the population abundance..
3. Use of Poison or Water Bomb
Fishermen mostly use poison so that fishes get killed and start floating, however, during such procedure not only fishes bmalaut also other benthic fauna get lost their lives causing a huge damage to aquatic fauna.
4 Use of Pesticides in Fields
Pesticide is one of group which consists of clastogenic compounds leading to mutations in exposed organisms. Carcinogenicity checking is becoming important aspect as people are getting exposed to different chemicals directly and indirectly in day to day life. If concentration of pesticides reaches above certain limits in exposed organism, it leads to harms like change in metabolism, reproductive disorders, disruption of endocrine system. Death of the organisms occurs when pesticide is more toxic in nature.
5. Use of Detergents
Detergents discharge in the water bodies led to change in pH, total alkalinity, free carbon dioxide, DO, affect rate of photosynthesis.
Crabs exposed to detergents result decrease in rate of oxygen consumption due to presence of detergents in gill, thereby lead to increase in mortality.
6. Diversion of Stream for Construction Purpose:
Alternation of stream or river pattern result cause great damage to the habitat of freshwater crabs, thereby leading to their population loss. In Jammu region, Jajjar stream has gravel base, people brings truck to load stones from there, thereby causing diversion in the route of river. As a result of this, the flow of water get reduced considerably that ultimately lead to mass mortality of benthic fauna including freshwater crabs.
Freshwater crabs are hidden treasures that need to be explore. Now a day almost all the niches occupied by the fishes has been exploited upto their optimum level. There is great need to shift from fishes to crabs so that need of people for food can be met especially in states where freshwater is the only source of aquatic fauna. The study of ecology of freshwater crabs will set a venture for aquaculture point of view. Further, due to their medicinal value more research should be focused on freshwater crabs. As the knowledge is scanty regarding freshwater crabs so attempt should be made to maximise it.
Extensive studies on freshwater crabs has been carried out in our State by research scholars Department of Zoology, University of Jammu, under the kind supervision of Prof. Kuldeep K. Sharma and Prof. Seema Langer.
(The author is (Ph.D Research Scholar) Department of Zoology University of Jammu).
