Introduction and review

         A widespread practice included in humans' destructive activities is the introduction of "exotic" animals and plants into many areas of the world. Included in these transfers are various species of freshwater crayfishes. There are over 400 species of freshwater crayfishes included within the families Astacidae, Cambaridae and Parastacidae (Huner and Lindqvist, 1995). The most significant crayfish is Procambarus clarkii (Cambaridae) which had been introduced to the Egyptian Nile water during early 1980's (Ibrahim et al., 1995). This species is native and very common in many of freshwater bodies of south-central U.S.A., especially Louisiana (Huner, 1995).  It accounts for at least 80% of all wild and cultured crayfishes harvested around the world (Huner, 1989). Over 60,000 tons of P. clarkii are produced annually in the USA and China (Huner et al., 1993).

On the other hand, the White River Crawfish Procambarus zonangulus, which is a closely related species to P. clarkii, has been found in some localities along the River Nile.  It is known to be native to the south central United States (Hobbs et al ., 1989; Huner  1995).  Both P. clarkii and P. zonangulus share comrnon physiological traits, that have made their wide distribution and commercial aquaculture easier and feasible These include adaptability to burrow environments, aerial exposure, rapid growth, high fecundity and disease resistance (Huner and Lindqvist, 1995). But after a while, P. zonangulus disappeared completely from all water bodies of Egypt; and this is may be due to that the stagnant water environment is not suitable for its mode of living.

The majority of introductions of these crawfishes have had negative consequences (Sommer and Goldman, 1983). In Egypt, most of fishermen claimed that it harmed the fisheries productivity as it attacks the fish nests and consumes the eggs, while others claimed that it destroys the irrigation system, because of its burrowing behavior and tunnels made in ditches and channels Therefore, a thorough knowledge of their bionomics, biology, reproduction and distribution patterns in the new habitat was very important.

 So, many investigations and studies have been done to elucidate major features of the life cycle of both species, in addition to their mode of living, natural growth rates and their real update distribution along the River Nile system .

The ecological surveys indicated that the crawfish has been flourished and distributed all over most of the River Nile and its tributaries and also in some of the considered newly habitats in Egypt; reaching to Sinai desert through the water irrigation system. The range of the crawfish has clearly expanded northward until Damietta and Rashid and southward to Aswan, since the beginning of 1980's. Generally, they are more frequent in Qalyoubiya, Cairo and Giza governorates than in El-Menoufiya and El-Sharkyia. In Giza governorate, crayfish were found frequently in the main Nile and in Ibrahimia Canal . All water courses in this governorate near to Nahia, Warrak El-Arab, Abou-Rawash region, El-Zumur and El-Maryouteya canals were variably populated with the crayfish.

The qualitative life history and the natural growth curve of Procambarus clarkii have been studied at two localities in Egypt ( Ibrahim et al., 1997); one in Cairo governorate and the other in Qalyoubiya governorate. Despite observed field difference, no significant difference was observed between growth rates of the two populations. Procambarus clarkii exhibited a clearly difined 2-year life cycle with two separate breeding stocks; one in mid spring (April) and the other in late autumn (November). The crayfishes' active season began in late March when the temperature increased (22°C) and the water level in different channels and ditches is raised (after the winter closure). The animals emerged from burrows, moved to all parts of their habitats and began feeding on decomposing plant matter, snails and decayed fish. Egg-carrying females are found almost immediately after 2-weeks period of feeding, and are abundant throughout April month. Newly hatched crayfish are very abundant, on the back of their mothers during late April and early May. The young crayfish grow at a mean rate of 4.3 mm per week during summer, which decreased to only 2.9 mm per week during winter .

The observed life history of P. clarkii in Egypt, appears to be not similar to descriptions from the southern United States (Huner and Barr, 1981). The USA popu­lation has only one generation per year and a breeding peak occurring in early summer. Growth rate is little lower than that of the Egyptian population, and this may be due to relatively low temperature in USA in late autumn and winter when the water temperature markedly decreases (16 °C); the crayfish adults seem to retreat to burrows in banks of channels and ditches. Burrow analyses conducted in many localities have demonstrated that the animal has a strong capacity for survival under these conditions for a long time. Adult crayfishes are able to live in burrows throughout late fall, winter and early spring. Furthermore, burrow excava­tions revealed that the tunnels have no free water, or water that was dose to being anoxic. Hasiotis (1993) provided an excellent descripiton of the surface morphologies of burrows that promote water retention. Moreover, Hobbs (1981) and Horwitz and Richardson (1986) have classified crayfishes on the basis of their burrowing behaviour. P. clarkii and P. zonongulus excavate relatively simple diagonal burrows, 1-2 m deep. Huner and Barr (1991) and Huner (1995) categorized them as tertiary burrowers. They retreat to burrows only when surface waters disappear, temperature decreases or to lay and incubate eggs. McMahon and Hankinson (1993) mentioned that the very low oxygen level in barrows (0~3 mg L-1) is below that at which such species may successfully engage in continuous aquatic respiration. As a result, they must depend on atmospheric oxygen to meet their oxygen needs and do so very well indeed. McMahon and Hankinson (1993) showed that P. clarkii haemolymph oxygen affinity is two times that of a "water breathing" crayfish species. Moreover, when Huner & Lindqvist (1995) and Hobbs (1975) described adaptations that permitted crayfishes to live in barrows, they mentioned that their branchial chamber has become more vaulted to accommodate increased gill filaments-surfaces. This serves the crayfish well in use of atmospheric oxygen.

In the Lab, Ibrahim et al. (1995) and Sleem (2008) studied the feeding behavior of the exotic crayfish P. clarkii and they indicated that it is a polytrophic animal, has a large feeding capacity to any kind of food, even dead organisms. It consumes large number of snail species, hydrophytes and fish species commonly found in nature in Egypt. Regarding the snails, the crayfish tend to be choosy when it comes to them as a food. This choice depends on the hardness of the shell and the size of the snail. The Lab experiments showed that the snails, Biomphalaria alexandrina, Physa acuta, Lymnaea natalensis and Bulinus truncatus are easier preys to be attacked by P. clarkii . This is primarily because the shell is thin and easy to break, so getting to the soft parts is not a problem. They usually crush the shell between their large claws and eat the soft body of the snail. It is noticed that the crayfish could devour ten large-size snails from each of the four snail species during the first day of offering the diet to it.

The field surveys also showed high reduction and sometimes complete disappearance of these Schistosoma and Fasciola vector snails in irrigation channels, which have been invaded by Procambarus clarkii, while in water courses which do not harbor the crayfish, high densities of these vector snails were recorded .So, these studies are providing encouraging indication of the possible control of schistosomiasis and fascioliasis in Egypt by the freshwater crayfish Procombarus clarkii.

         Finally, it is very important to use this exotic animal in a positive way instead of leaving it in the River Nile causing many problems to the ecosystem. The major problem in Egypt for this new introduced species is the inability of Egyptians to use it for feeding as a  good, cheap taste diet.