Biology of the Crab
Crabs are found in the aquatic ecosystem. Aquatic ecosystem is two types: Fresh water ecosystem and Marine water ecosystem. We investigated the biology of the freshwater crab. Freshwater crabs are included in the aquaculture.
The freshwater crab belongs to the class: Crustacea, Subclass: Malacosreaca, Superorder: Eucarida, Section: Brachyura, Family: Potamidae (Potaminidae) (Kaestner, 1970). According to Kaestner (1970), the family Potamidae (Potamonidae); freshwater crabs include numerous species in subtropical and tropical areas. The family is now often split into the potamidae, Pseudothalpusidae and Trichodactylidae. The land crabs are belonged to four families such as Potamidae, Grapsidae, Gearcinidae and Pinnetheridae.
There are more than 4500 species of crabs. This is the largest and most specialized group. More than 600 species of freshwater crabs are known (Ng, 1998).
According to Ng (1998), there are several species of freshwater crabs of the families Potamidae and Paratelphusidae are consumed in many parts of South East Asia and Indo-China.
Bangladesh has got diverse freshwater habitat and ecosystem. Crabs are exportable fishery items and hidden resources of Bangladesh (Ahmed, 1991). Now a day, crabs play a significance role in the fishery wealth of many nations. (Chhappgar, 1991).
Freshwater crabs of Bangladesh are true crabs can distinguished from false crabs by not having 5th pair of pereiopods totally or partly concealed beneath the carapace, the antennae were always placed between the inner margin of orbit and fused pterygostomial region with endestome ( Ingle, 1983).
Shafi and Quddus (2001) reported that, there are four freshwater crab species in Bangladesh. There are Potamon (acanthotelphusa) Wood Masoni ( Ruthban), Potamon (acanthotelphusa) martinsi Wood mason, Paratelphusa ( Paratelphusa) spinigera Wood mansoni and Paratelphusa ( byrytelphusa) Lemellifrous. ( Alock, 1906)
According to Chowdhory and Hafizuddin ( 1991), there are ten 10 species of crabs in coastal water of Bangladesh. Siddique and Zafar (2002) reported 13 marine and 3 freshwater species of crabs from Chakaria Sundarban area of Bangladesh.
The crabs are themselves important detrivores, reducing the particle size of leaf litter and organize debris, presenting a source of nutrition to collector and filter- feeding freshwater fauna and enabling microbial activity. These crabs utilize energy from diver’s tropic levels and contribute to energy and resource recycling within the freshwater the freshwater ecosystem. (Gouws and Stewart,2001).
True freshwater crabs spend their entire lives in freshwater and it is shared defining characteristics to be able to complete their entire life cycle independently of sea water. All true freshwater crabs lay their eggs and rear their young in freshwater environments has involved a number of modifications of their reproductive system and behavior. These modifications include are production of relatively few eggs, each of a relatively large diameter, a complete reduction of the larval stages ( direct development ) whereby the eggs hatch directly into juvenile crabs ( “hatchlings”) and the protection of the hatchlings for several weeks after eggs hatching. Female freshwater crab produces fewer eggs about hundreds. The newly laid eggs of marine crabs are small (0.25-0.35mm in diameter) and double in size as they develop. In contrast, the newly laid eggs of freshwater crabs are much larger ( about 1mm is diameter) increase to between 3 and 5 mm in diameter as they develop, and remain attached to the pleopods of the female until hatching ( Neil, 1999).
Young freshwater crab leaves the eggs as small versions. The hatchlings are retained on the females’ pleopods in the female’s abdominal brood pouch for several weeks after, hatching and female freshwater crabs show a degree of maternal care. (Neil, 2006).
Most current studies on freshwater crabs focus on taxonomy. The biology of only a few species, e.g., Potamon fluviatile ( Herbst) by Pace et al., 1976 and Micheli et al., 1990; Spitolatrhelphusa hydrofroma ( Herbst) by Adiyodi 1968 and Pillai and Subramoniam, 1984; Barytelphusa cunicularis ( Westwood) by Diwan and Nagabhushnam, 1974; Sesarma ( Geasesarma) perracae Nobili by Soh, 1969; Meeopaulias depressus by Dissel and Schuh, 1992 and Dissel and Schuh, 1993; Sesarma jarvise Rathbun by Dissel and Horst, 1995 has been studied. Information on the biology of freshwater crabs and its ecology is evidently scanty.
There are another few studies on the biology of brachyuran crabs (Garcia-Guerrero and Hendrickx, 2004), whereas other groups of decapods crustacean have received much attention, and their embryology is better documented. Studies on the brachyuran crabs are recent and have included only a few species from different habitats, such as the hair crab Erimacrus isenbeckii ( Atelecyclidae) by Nagao et al. (1999). The estuarine or mud crabs Chasmagnathus granulate and Cytograpsus angulatus ( Varunidae) by Bas and Spivak (2000); fiddler crab or mangrove ghost crab Uca lacteal ( Ocididae) by Yamaguchi (2001); the mangrove crab Ucides cordatusi ( Ocypodidae) by Pinheiro and Hattori (2003); Goriopsis pulchra ( Grapsidae) and Aratus pisonii ( Seasarmidae) by Garcia-Guerrero and Hendrickzx (2004); the green shore crab Carcinus maenas ( Portunidae) by Chung and Webster (2004); the intertidal crabs Hemigrapsus edwardsii and H. crenulatus (Grapsidae) by Taylor and Seneviratna (2005) and Seneviratna and Taylor (2006); the blue crab Callinectes sapidus by Walker et al. (2006); and the blue king crab Paralithodes platypus by stevens (2006).
Crab is one of the popular and freshwater food items in many parts of the world in south East Asian countries. Its excellent and distinctive flavor makes it one of the most delicious and costly food item.
According to Rhaman and Hossain (1991), freshwater crabs are of commercially important species. It is esteemed as food in different countries. It is available in river, ponds, beels and inundated areas throughout Bangladesh.
Crab meat contains high protein level with all the essential amino acids recommended by FAO/WHO 1973. The meat is believed to cure asthma and chronic fever. ( Rhaman and Chidamveram, 1944). Now a days crabs are also used as aquaculture feed and poultry feed in Bangladesh. The waste materials consisting of gills, intestines and shell are good raw material for the preparation of fish meal and high quality chitin and chitosan.
The crab is important and beneficial arthropods for human consumption. In the present investigation the biology of fresh water crab Paratelphusa sp. Has been considered and also the food and feeding habit, fecundity and mating behaviour, incubation period, copulation, hatching have been presented.
MATERIALES AND METHODS
2.1. Materials used for the study
The materials which were used for the study are as follows-
i. Aquarium vii. Hand gloves
ii. Petri dish viii. Scalpel
iii. pH meter ix. Forceps
iv. Cast net (Khepla jal) x. Air pump
v. Khulson xi. Formalin
vi. Plastic container xii. Alcohol
xiii. Banana bark
2.2.1. Collection of specimen
The fresh water crabs were collected from the pond of Maidandighi in the district of Panchgarh from March 2010 to December 2010 for laboratory rearing (Fig no1) (Total of 30 freshwater crabs were collected from the natural habitat.
Separately, a total of 240 freshwater crabs were caught from the natural habitat to observe the ovigerous and non ovigerous females during January 2010 to December 2010. In both cases crabs were collected by the help of cast net (Khepla jal) and Kulson.
After collection the crabs kept in the plastic container with a little amount of water.The crabs kept in the plastic container without food in three days.Than the crabs kept in the pot of banana bark without water for transport them from natural habitat to the laboratory.
2.2.2. Crab identification
In the laboratory, the specimens were placed on tray and were washed with clean water to identify those scientifically using crab identifying keys. Crabs were identified as follows with the help of key of Alcock (1900)
Common experience were used in identify males and females. In case of male crab, it has a long, narrow and inverted T shaped abdomen whether females has an inverted U shaped abdomen.
Collected crabs were immediately transferred to the laboratory and were kept in aquarium. The aquarium was constructed with materials collected from the habitats of this species. Environmental conditions similar to those in the natural habitat were provided in the aquarium. The aquarium provided both dry and wet places for crabs. The crabs were feed with SIS fish, snail flesh (Pila globosa) and aquatic weed Hydrilla. Pond water was changed daily from the aquarium. The dead crabs were picked up to keep the environment afresh.
Crabs are omnivorous, eats remains of other crab, molluscs, insects, vegetation and detritus materials. Different plant and animal were supplied as food. Among the plant, Hydrilla was prominent and among the animal foods, small pieces of flesh of snail and SIS fish viz. Darkani, kholisa, mola, dhela etc. were supplied. Daily half of the aquarium water is changed before supplying the food.
2.2.6. Mating and copulation
Male and female crabs were observed daily to know whether they were performing mating or not. In laboratory condition all season females were observed reserving their abdomen are found the ovigerous female. But in nature the ovigerous females were found. Water condition especially, temperature, pH and other factors were observed to keep a constant condition in the aquaria.
During the period of rearing crabs were properly labeled for subsequent studies. The carapace length and carapace width were recorded to the nearest millimeter using a measuring scale and total body weight to the nearest gram by electronic balance. Excess water on the body of crabs was removed with the blotting paper to keep the accuracy of measurement.
2.2.8. Statistically analysis
To estimate the mean value and standard deviation as well as to determine the correlation among different variable (viz. Body weight (BW), Carapace width (CW), Carapace length (CL), and number of eggs etc. The analysis of tool pack of MS Excel was used.
3.1. Systematic position of the crab.
Phylum : Arthropoda
Class : Crustacea
Sub-class : Malacostraca
Super-order : Eucarida
Section : Brachyura
Order : Decapoda
Family : Potamidae
Genus : Paratelphusa
Species : Paratelphusa sp.
3.2. Morphological Characteristics
3.2.1. General characters
The carapace of the species was board, robust, deep, gentlyconvex soomth and almost polished. It had an H-shaped gastric groove and the free edge was sharp and slightly concave. The antrio-lateral margin curved, sharp and entire or crenulate faintly prominent with acute epibranchial spine atits far back.
Orbits small broad and outer angle prominent, with a wide antennal gap and devoid of large spines. Lateral epibranchial teeth and the front very close to each other. Chelipeds unequal in both sexes, with spines near the distal end of the merus;propodus of cheliped board , stout but not compressed; dactylus longer than palm; inerangle of carpus strong, stout and sharp pointed; legs stout with strong dactyls longer than the propodites. No setae or carinae present on dorsal surface but epibranchial and mesobranchial swollen. Eye moderate in size and the outer margin of eye with U to V shaped incision. Colour: variable; dorsal surface of carapace and cheliped reddish-brown, ventral surface of carapace light yellowish but periopod and ventral surface of cheliped light-brown. Dorsal surface of carpus
and outer margin of propodus and dactylus with brown spot and minutely tuberculate.
3.2.2. Female crab
The female crab was easily identified by the shaped of its abdomen. The female had an inverted ‘U’ shaped abdomen, four pairs of pleopods (or swimerates) on abdominal segment 2 through 5(Plate no .8). The first article or coax of a pleopod was attached to the body by a soft and flexible articulating member.The coxa was small and poorly calcified but the next articale the basis was, large and conspicuous. Two rami the exopod and endopod, are arise from the basis.
3.2.3. Male crab
The male crab was also easily identified by the shape of its abdomen. It had also an inverted “T’ shaped abdomen (Plate no.9). It had only two pairs of pleopods which were located anteriorly on the abdomen, on segment 1 and 2. In male the long curved tubular first pleopod was the gonopod .The second pleopod was much shorter than the first.
3.3. Food and Feeding
Crabs are opportunistic omnivorous, eating on a variety of food, with a preference for animal food, in general. The freshwater species Paratelphusa sp, is omnivorous, feeding on algae water plants, detritus and insect larvae. The animal food usually consists of bivalve molluses( Pila globosa) and SIS fish (Darkani, Mola, Kholisha, Dhela etc.).
3.4. Laboratory culture.
Feeding habit in aquaria
Table. 1. Represent the list of different food items supplied in the aquaria to feed the crabs. It shows Hydrilla sp, of plant as the most favorable food of crab where kolmilata and Halencha were taken as less favourable. On the other hand snail flesh of pila globossa and Darkani were taken as favourable food item among animal food.
3.5. Breeding in nature
The breeding season is the period when the female crabs carry eggs and perform sexual activities with the males. The vigorous Paratelphusa sp. Were found during April to November. The percentage of vigorous and non ovigerous female were found in experimental period are presented n the following .
3.6. Mating behaviour.
Mating occurred either in the water or on land, and in one case they were sheltered under a stone in the water. They mat in nocturnal (usually before midnight). The males were aggressive, initially grasping a female by the chelipeds, turning her over with the aid of his pereiopods. The male then mounted her forcefully, simultaneously inserting his abdomen under that of his partner to begin mating.
3.7. Copulation in aquaria.
In the aquaria, first copulation occurred in May 2010. The male and female crabs were taken part in copulation by laying head-to-head and sternum-to-sternum contact, with their abdomens relaxed so that the abdomen of the female overlaps that of the male. This brought the female opening into contact with the gonopods which swung out away from the sternum when the male abdomen was relaxed. The terminal articales of gonopod 1 connected with, and were inserted into, the paired vulvae of the female sited on sternite 5.
The female showed her interest of sex by rocking and waving her claws in and out. She approached the male, turned around backwards, and attempted to wedge herself under him. She was continuously waving her claws and the male responded by tapping and rubbing her claws with his. Soon the female quieted down, tucking her claws in to a submissive posture, allowing the male to clasp and carry her with his walking legs. She was right side up and racing forward showing a precopulatory embrace.
The ability of an animal to produce offspring is known as fecundity. In Paratelphusa sp, the number of eggs carried by females varied from30-142, and almost all crabs carried more than 100 eggs. The mature females of this species reproduce once a year.
3.9. Incubation period.
In laboratory condition observed the incubation period 17 days. It may be due to the maintenance of cons constant temperature and some other environmental factors in the aquaria.
3.10. Hatching : The crabs Paratelphusa sp, in the aquaria hatched directly to the juvenile without passing any larval stage.
Freshwater crabs are of commercially important species (Rahman and Hossain,1991). The freshwater crabs, Paratelphusa sp, was studied to know the morphological characteristics, food feeding habit, abundance in nature, maturity, fecundity. Different food items (viz. Hydrilla, snail flesh, SIS fish, darkani, kholisa,mola, Dhela,etc) were supplied in the aquaria to feed the crabs. It is noted that the Hydrilla sp, kept the water quality good and provided oxygen to the aquarium water. At the same time the plant given the crabs a good place to make hide themselves from the attack of other one. Similar work on freshwater crab, Sudanonauts floweri was done by Franciso et al. (2008)
Copulation of Paratelphusa sp. under laboratory condition was observed. Crabs were taken part in copulation by laying head-to-head and stearnum-to- stearnum contact. Nail Cumberlidge(1999) reported the same process of copulation for the potamonautidae crabs. Work on copulating behavior of Sesarma sp. was also performed by Zimmerman and Felder (1991) and of perisesarma bidenes was done by Sarker et al.(2009).
In the present work, when supplied only vegetation food there is not found in ovigerous female .when supplied vegetation and other animal protein found in ovigerous female. But in nature observation ovigerous female are found. In nature the present observation reveals that the percentage of ovigerous females is somewhat dependant on the temperature, relative humidity, sun shine and rainfall. The highest percentage of ovigerous female (65%) was observed in July 2010 when the mean temperature was 29.98 C, relative humidity was 85.83%, sun shine 5.39 h day, and rainfall 6.003 mm respectively, and no was found (0%) in January –march and December 2010.when the mean temperature was 18.44C, 21.35C ,25.70C, 20.02C; relative humidity was 83%, 68%, 63%, 86%; sun shine 5.45 , 8.89, 7.33, 4.88, and rain fall 0mm , 0.2mm, 0.9mm, 0 mm, respectively .
The abundance of gravid Potamon sp. was surveyed in Padma River by Rahaman et al. (2005). An increasing event of maternal care which was served by the mother crab keeping the eggs and hatching into their abdomen was noticed during the observation of Paratelphusa sp, Simillar finding on potamon sp,was observed by Rahman,et al (2005)
In the present work, the study on fecundity of Paratelphusa sp, shows that the number of eggs carried by females varied from 100-142 and the mature female reproduce once a year. This result supports the work of Hung-Chang Liu et al. (2005) on freshwater crab, Candidiopotamon rathbunae. Cumberlidge and Reed (2003) worked on the West African freshwater crab potamoautes dybowskii and reported that the female freshwater crabs produce few eggs about 100.
According to Rahman et al. (1991), the fecundity of the berried crabs varied with the length of carapace and body weight. In present study it also observed that, female crabs with a carapace length (CL) 37 mm, carapace width (CW)51 mm and total body weight (BW) 34.88 g have the highest fecundity(142). And females with a carapace length (CL) 25 mm, carapace width (CW) 35 mm and total body weight (BW) 10.6 g have the lowest fecundity (100).
The incubation period depends on the species and temperature. At low temperature, the incubation period is prolonged, while it is shorter at higher temperature (Yamaguchi, 2001). In the present study, it is found that the females of Paratelphusa sp, have become ovigerous earlier than the nature.The incubation period laboratory condition 17 days. It may be due to the maintenance of constant temperature and some other environmental factors in the aquaria.
In many crustacean embryos, hatching occurs after a long period of incubation. Light is a major factor inducing the release of larvae. Release of larve occurs under well-defined conditions of light, tide, and several other exogenous and endogenous factors. But, in the present study, the crabs (Paratelphusa sp), in the aquaria hatched directly to the juvenile without passing through any larval stage where enough sun light and free passage of fresh air was lacking. Effects of environmental factors on especially crabs were reported by many authors. Saigusa and Hidaka (1978) worked and reported on semi-lunar rhythms in Seas. Larval release at night by the filder crabs was observed by De Coursey (1981). She suggested that nocturnal larval release is advantageous for both adult and larvae to avoid predators.
A number of direct and in direct methods have previously been used to determiner the maturit of crabs, (Hartnoll, 1995). However, in Paratelphusa sp, mature male were smaller than the smallest mating male. In females, some crabs larger than the smallest breeding female were still without developed ovarious. The mature size of both sexes in Paratelphusa sp, is not a definite value, but ranges 21.1-25.5 mm of CW.
There is no apparent courtship in many crabs’ species. The male simply grasps the female and forcibly adjust her to the appropriate position (Warner 1977). Paratelphusa sp uses this forced copulation strategy as well .Similar behaviour has been reported in two other species of freshwater crabs. Potamon fluviatile (see micheli et al. 1990) and Geothelphusa dehaani(see minei 1976)
Mating of freshwater crabs has been recorded in only a few species, (see Adiyodi, 1998, sp) as Paratelphusa sp. In present work observation the laboratory condition the freshwater crabs Paratelphusa sp were mating.
Prior to ovulation, the females of many decapods become more secretively in their movement and some cases they stay in a burrow or crevice for much of the incubation period (Hazlett, 1983). The behavior of ovigerous female of Paratelphusa sp. is same from that of S. hydrodroma whose ovegerous female stay in original habitat and can be easily found during a survey (Pillli and Subramoniam, 1984; Micheli et al., 1990).
Reproduction output per brood in crustaceans almost always increases with body size (carapace width) or weight (Hartnoll, 1985). However in Paratelphusa sp. the female with a CW of 51 mm have the highest fecundity female with a wider CW often have fewer eggs than 30mm CW female. In this study area female larger than 30mm CW comprise about 50% of the mature female population.
The breeding of crabs is influenced by diverse environmental factors, such as the availability of food and water, temperature, salinity, lunar-phase, tidal cycles and competition between congeneric species (Adiyodi, 1988). In this present work laboratory condition the supply food only vegetation. So there is no found ovegerous female because lack of availability of food and water, temperature, salinity. But another aquarium supplied vegetation and animal protein this aquarium found ovigerous female in there breeding season.
This crab depends on seasonal rainfall for breeding and the brood is released only during the rainy season. However in Paratelphusa sp, the brood is released at the end of the rainy season because there is never a shortage of water in its natural habitat at the rains during the wet season are so heavy they often induce destructive flash floods which destroy the habitat, it is reasonable that Paratelphusa sp. released their brood after the rainy season to minimize injury or damage by the hostile floods. Brood released at the beginning of the dry season also provides the longest period of nonfoods condition for the juvenile to grow. The juvenile crab has little chance to encounter a flood for half a year. They are then sufficiently mature to overcome the damage of flood before the next wet season of the following years.
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