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Palaemonetes sp, commonly known as grass shrimps, are clear, translucent shrimps with a few brown stripes across its body from side to side. They are approximately 2.5 cm long, with a head width of around 5 mm and narrowing to a tail width of around 2 mm. These crustaceans have a segmented exoskeleton for protection, and on the head segment, protrude a pair of long antenna and a pair of black spherical eyes. Organs are visible inside the head segment as a brown spot. They have five pairs of appendages on the head segment, with the foremost pair having obvious visible claws. On the tail segments, they have many pleopods and a tail fin shaped similar to a fan. Grass shrimps use their appendages, pleopods and tail for locomotion, while their antennas and eyes are for sensing the environment.
Palaemonetes sp. are abundant and widespread in estuaries, salt marshes and bays along the Atlantic and Gulf of Mexico coasts of North America and along the European coasts from the Baltic Sea and the North Sea through the Mediterranean (Bauer 2004). They are more abundant in areas of relatively high turbidity near river mouths or shallow tidal areas, since the suspended sediments in the water provide some protection against predators (Anderson 1985). Brackish Palaemonetes sp. are most commonly found in salinities ranging from 5 to 39 ppt, while the salinity range of freshwater Palaemonetes sp. is from 0 to 10 ppt (Coen and Wenner 2005). They can tolerate a range of water temperatures, from 5áµ’ C to 35áµ’ C, but found most commonly in the temperature range of 18áµ’ C to 25áµ’ C (Anderson 1985).
Two abiotic factors important to Palaemonetes sp. include salinity and temperature, both of which affect the whole area of their natural habitat. Their habitat is prone to changes of salinity due to the influx of ocean flow, river flow, rainwater and evaporation. Salinity affects grass shrimp as individuals residing in relatively higher salinity water grow larger, are able to mature and reproduce earlier than ones in waters of lower salinity (Masterson 2008). Larvae cannot tolerate salinity extremes that the adults able are to withstand, and when salinity levels are optimal, they pass through fewer larval stages and have a lower mortality rate (Anderson 1985). Grass shrimp are normal found near the water's edge at low tide, and are prone to exposure to daily fluctuations in temperature. In winter and in summer months, when temperatures are at their extremes, grass shrimp may move from shallower waters to deeper waters, where the temperature fluctuates less and is more stable (Coen and Wenner 2005). A higher temperature would cause the shrimps to have a higher metabolic rate, and therefore, growth is generally faster in the summer than the winter, but longevity also decreases (Alon and Stancyk 1982). At lower temperatures, almost no reproduction happens, as grass shrimps use their energy for metabolism (Vernberg and Piyatiratitivorakul 1998).
Palaemonetes sp. are susceptible as an intermediate host to a number of different parasites, including Microphallus turgidus, which are usually encysted in a grass shrimp's abdominal muscles (Khan et al. 2003). M. turgid could increase the predation of grass shrimp, as infected shrimp displays lethargy, have a lower swimming stamina, and spends less time motionless and hiding from predators (Kunz and Pung 2004).
In its natural habitat, Palaemonetes sp. are detritivores, and they mechanically break down organic substances so that decomposers can then chemically breakdown these substances to releases nutrients. Grass shrimp are also important because they transfer energy from one trophic level to another, as they break down organic substances, consume producers, and in turn, eaten by other consumers. The excrement of grass shrimp are rich in nutrients and contains large amounts of phosphate and ammonia, allowing microflora and other decomposers like bacteria and fungi to proliferate (Welsh 1975).
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