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Noble Crayfish

Astacus astacus

The Noble, or European Crayfish, is the largest of native crayfish species. The conspicuously large and mottled front claws serve not only to catch and hold on to food, but also to build hiding places and defend itself.

Description
The body of the Noble Crayfish varies from reddish brown to dark brown, but blue specimens may also be seen. An important differentiating characteristic to other crayfish is the dark red undersides of the claws and claw joints. (The signal crayfish, or Pacifastacus leniusculus, has a bright signal spot on the upper side of each claw which serves as another point of differentiation.). Noble Crayfish males grow to a length of 20cm and weigh up to 350g; females are smaller. A hard exoskeleton protects the front part of the crayfish. Because this shell does not grow along with the crayfish body, it is periodically shed, or moulted, during the growth process and a new shell is formed. This process is repeated around 15 times, or until the crayfish is fully grown. The most dangerous moments in the life of a crayfish occur during moulting: on the one hand, the process is extremely strenuous, and if the crayfish cannot shed its shell in less than 15 minutes, it may die of exhaustion as a result. On the other hand, the new exoskeleton is soft and malleable at first, giving rise to the nickname "butter crab" – and in this instance the crustacean is easy prey for its many enemies. 
Astacus astacus has two pairs of feelers, or antennae, whereby one is around one-third as long as the entire body, while the other is smaller and can only be seen from up close. Both are used in order to smell and for a sense of balance.

Distribution
The Noble Crayfish is native to all of Europe except for the Iberian Peninsula and Ireland. In the Donau-Auen National Park, Astacus astacus may be observed in well-oxygenated bodies of water with firm ground at the bottom.

Endangerment and Conservation Status
The Noble Crayfish is highly sensitive to chemical contamination such as that caused by pesticide run-off from fields or industrial waste. Consistently high levels of nitrates negatively impact the general health of this crustacean and result in lower survival rates of juveniles.

Behaviour
Noble Crayfish are natural health inspectors in the water. By removing dead and/or dying fish from the waters, they help limit the effects of fungal diseases. In general, the Noble Crayfish is not a picky eater and will eat most anything – whether detritus (dead plant matter) or dead molluscs, and it may even resort to cannibalism and eat members of its own species. Given the availability of banks with sufficient hiding places (tree roots, deadwood, stones etc.) the Noble Crayfish may survive long despite the presence or many natural enemies. Ideal are water temperatures of 18° to 21° C in the summer. In order to better dig the burrows it spends time in, the Noble Crayfish avoids muddy waters. In the reproductive process, the female carries up to 400 eggs attached to her legs for around 26 weeks. In late spring, the almost fully-developed crayfish larvae hatch and disperse. Opaque at first, they are enveloped by a yolk sac during this initial phase. Only around 15% of the female's eggs survive to become juvenile crayfish.

Special Characteristics
The greatest threat to the Noble Crayfish is posed by the freshwater fungus Aphanomyces astaci, a fungal disease which was first observed in Austria in 1880 and which is spread by way of spores attached to muddy boots, fish trimmings or wet equipment. The Noble Crayfish had been very common until it was nearly eradicated by this plague, which had its origins in the Americas. In an effort to re-establish the ecological balance by introducing a similar species, the American signal crayfish, the situation was made even worse. It was discovered that the fungus can survive in the signal crayfish without hurting it while it continues to kill off the Noble Crayfish. Only if the invasive signal crayfish is eliminated will it be possible for the Noble Crayfish population to recover.