|A Colorful Spiny Crayfish|
Crayfish are small freshwater crustaceans resembling lobsters. They are known for their remarkable ability to regenerate lost limbs and for their open circulatory system. Crayfish are also known for their ability to live underwater, and yet survive on land for short amounts of time by storing water in their gill chambers. Crayfish are found around the globe, and are often considered a fine treat when it comes to human consumption.
Crayfish have a hard exoskeleton that supports and protects their body and is made up of chitin. Chitin is a carbohydrate fabrication of calcium carbonate and protein.(Porch and Batdorf 176) As a crayfish grows, it often molts (loses its old shell and grows a new one and then eats the old shell.  Molting occurs six to ten times during the first year of rapid growth, but less often during the second year. For a few days following each molt, crayfish have soft exoskeletons and are more vulnerable to predators. 
A crayfish’s body is divided into two body segments: the cephalothorax (which is covered by the carapace) and the abdomen. These segments are further divided into segmentations called somites. There are nineteen along the entire body.
Each section possesses one pair of appendages, although in various groups appendages may be reduced or missing.  The crayfish has eight pairs of walking legs (pereiopods), and one pair of large pincers or claws called chelipeds. These chelipeds are used for cutting, capturing food, attack, and defense. The legs are used to fan fresh water to the gills, bring food to the mouth, and transport the creature to other locations. Crayfish are sessile and will use their legs to skitter across a surface or flip their tail in a swishing motion to propel themselves backwards. The legs can also regenerate, which means they can be regrown if they are broken off.
They have two pair of sensory antennae and a pair of eyes on movable stalks. Their eyes, called onmatids are individual, complex, and faceted; there are about 7,000 to 30,000 of them on each stalk. Even with these compound eyes, crayfish primarily orientate themselves by touch. The antennae and atennules in front of the mouth are their sensory organs, and aid in swimming and feeding in larvae forms. At the base of each antennule is a statocyst, an organ of balance, lined with tiny sensory hairs and containing a few grains of sand. As the body tilts, the grains shift against the hairs, which then send impulses to the brain. (Porch and Batdorf 479)
The sensory information gathered through the crayfish’s antennae and sensory eyes is received through a variety of organs, in part of what is called a ventral nervous system. This system consists of a “brain” (a pair of ganglia), located over the esophagus and is connected by two main nerves to a ganglion below the esophagus, and a ventral nerve chord. This position for the ventral nerve chord provides supreme protection and a short passage for the information traveling from the nerve system to the appendages, the structures most reliant on constant interaction with the system.
The crayfish’s gills are used to extract oxygen that is dissolved in water and lead it into the crayfish’s open circulatory system. This type of circulatory system doesn’t limit the blood to blood vessels; instead the blood collects in the pericardial sinus, the cavity surrounding the dorsal heart. (Porch and Batdorf 477) From there the dorsal heart pumps the blood through short vessels, which empty into internal cavities and bathe the organs. After the blood bathes the organs and drains from these cavities, it flows into the large sternal sinus. The blood then passes through the gills, becomes oxygenated, and returns to the pericardial. The green glands, located near the base of the antennae, filter out waste as the blood circulates. Then the waste is excreted through a pore just in front of the Crayfish's mouth. Crayfish can store water in their gill chambers so that they can walk on land; they hold their water on land like we hold our breath in water. (Porch and Batdorf 478)
A Crayfish's coloring as a juvenile is light tan, but adults are deep red. Their color also depends on their diet; some can be orange, green, or even blue. Their colors can also differ in various locations to accommodate their ability to camouflage with a specific environment.
Crayfish are usually about 7.5 cm (3 inches) long. The smallest is the 2.5-centimetere-long Cambarellus diminutus of the southeastern United States. The largest is Astacopsis gouldi of Tasmania, its length may reach 40 cm and its weight about 3.5 kg (8 pounds). 
Crayfish reproduce sexually and are capable of regeneration. Crayfish are either male or female, the males being the larger of the two. Usually mating occurs in autumn or spring, however the females can store the sperm from a fall mating until the spring when the eggs are fertilized and laid. The females can protect their eggs by carrying them around under their tails. During this period, when the eggs are stuck to the swimmerets, the female is said to be “in berry”. The eggs are laid from late May to early June and hatch by July. 
When the eggs hatch, all of the hatchling’s appendages are developed: claws, eyes, antennae, etc. Freshwater crayfish are ‘mothered’ by the female for a period of time following birth.  The hatchlings look like miniature adults, and can grow to be about 2-3 cm long by the fall.
The lifecycle of the crayfish doesn’t last very long. The males will usually die after mating, at about two years old. The females die after the eggs are fertilized and laid, also approximately two years. Some will live a while longer, but none are known to have survived beyond their fourth spring. 
Crayfish are capable of deliberately discarding a limb for protection without excessive blood loss. This is possible because of a self-sealing double membrane in each limb. After a crayfish looses a limb, it will gradually grow a new appendage to replace the one lost. (these cavities, it flows into the large sternal sinus. The blood then passes through the gills, becomes oxygenated, and returns to the pericardial. The green glands, located near the base of the antennae, filter out waste as the blood circulates. Then the waste is excreted through a pore just in front of the Crayfish's mouth. Crayfish can store water in their gill chambers so that they can walk on land; they hold their water on land like we hold our breath in water. (Porch and Batdorf 479)
Crayfish are commonly found in streams and lakes, where they hide themselves by burrowing under large logs or rocks. 
Most crayfish cannot tolerate living in polluted water, but some are exceptions, like the Procambarus clarkii, which can tolerate the harsh conditions. Some live in saltwater, but those from the family Astacidea are found only in freshwater.
Crayfish burrow into the ground for protection from predators, and some have been found as deep as 3 meters underground.
Crayfish are important in terrestrial and freshwater food chains because they are omnivores, eating small fish, earthworms, snails, tadpoles, and plants. They are not just predators, but in many cases also the prey. They are hunted by large fish (like bass), salamanders, birds, snakes, mink, raccoons, etc. 
Older crayfish will feed at night, or on dark cloudy days, feasting greedily until dawn. The younger crayfish however, will feed during the day, more so on bright sunny days. 
Crayfish in the family Astacidae are found west of the Rocky Mountains in the northwest United States and into British Columbia, Canada, and over in Europe. Crayfish from the family Cambaridae are found on the east side of the mountains in mid and eastern parts of America.
The Crayfish Plague
Aphanomyces astaci, is a water mold that arrived in Italy with gravel and dirt filled waters from a North American ship in 1960, and spread quickly through Europe. It is a mold that infects crayfish, and in England by the 1950s it had wiped out large populations of the Astacus. Scientists and locals tried to find a replacement crayfish, and in 1960 they settled on the Signal Crayfish. The Signal Crayfish, although more resistant than Astacus, is also a carrier of the plague. Some made efforts to revive the original European Crayfish population, but all efforts have been unsuccessful because of large implantations of Signal Crayfish diminishing the native's food sources.
Implantations of the Signal Crayfish caused the spread of the disease to Ireland and the UK. Transport of Signal Crayfish, Red Swamp Crayfish and infected native European freshwater crayfish between waters is the main cause for contamination. Other causes of contamination, such as transmission through items that have been in contact with contaminated water, are unlikely because the transfer would have to occur within three days, the survival period of the spores.
A key sign of infection is large amounts of crayfish visible during the daylight, because they are nocturnal. Another sign of difficulty with coordination is if you turn a crayfish on its back and it is not able to flip itself back over. Most often, however, the disease is not noted until large numbers crayfish are found dead. For a conclusive diagnosis, laboratory examinations are always performed.
In Sweden the number of Signal Crayfish has also started to decline significantly over the last years, and researchers now suspect that the Signal Crayfish may be less resistant to the plague than they were previously thought. Research, however, is not yet finished. The fungal spores of the crayfish plague disappear from an infected water system a few weeks after the last infected crayfish is gone. Yet, it is possible for a lake to become infected again if it is still connected with another infected water source. 
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