Rusty Crayfish (Orconectes rusticus)

Rusty crayfish
Rusty crayfish (Faxonius rusticus) in Guelph, Ontario, Canada
Rusty spots on sides of carapace
Conservation status
Least Concern  (IUCN 3.1)[1]
Secure  (NatureServe)[2]
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Family:	Cambaridae
Genus:	Faxonius
Species:	F. rusticus
Binomial name
Faxonius rusticus (Girard, 1852)
Synonyms[3][4]
Cambarus rusticus Girard, 1852 Orconectes rusticus (Girard, 1852)

The rusty crayfish (Faxonius rusticus) is a large, aggressive species of freshwater crayfish which is native to the United States, in the Ohio River Basin in parts of Ohio, Kentucky, and Indiana.^[3][5][6] Its range is rapidly expanding across much of eastern North America, displacing native crayfishes in the process.^[7][8] The rusty crayfish was first captured in Illinois in 1973, and has been collected at over 20 locations in the northern portion of the state.^[9] In 2005, F. rusticus was found for the first time west of the Continental Divide, in the John Day River, Oregon, which runs into the Columbia River.^[10]

Adult rusty crayfish can reach 10 centimeters (4 inches) in length, although they reach maturity at about 4.4 cm (1.7 in),^[11] and can range in color from greenish grey, to reddish brown,^[12][13][14] They can be recognized by two "rusty", reddish colored spots variably present^[15] on the sides of their posterior carapace^[11] and their large front claws with black bands around the tips.^[16] Male astacoid crayfish have small hook-like features, or copulatory stylets,^[17] that are used to hold onto a female while mating,^[7] prominent in rusty crayfish on the ischia of the third pair of pereopods for those males in their form I (reproductive) molt.^[7][15]

• 
  Male in Illinois

Many species of vertebrates that live in communities together utilize a dominance hierarchy to establish order, and studies have shown that some species of invertebrates do as well.^[18][19] The dominance hierarchy is an important aspect of a crayfish's biology and behavior. Crayfish tend to form dominance hierarchies with the other members of their population in a particular environment. The largest male will generally demonstrate the most dominance over the others by being the most aggressive, and picking fights with the other, smaller crayfish. The crayfish that wins the most fights is placed at the top of the hierarchy with the other members generally ranking in descending order based on size and sex.^[20] Studies have suggested that the largest determining factor in the formation of dominance hierarchies is size rather than sex. That means that female rusty crayfish can rank higher in the dominance hierarchy than male rusty crayfish if they are larger than them.^[20]

The larger size and aggressive nature of rusty crayfish that have been introduced to a body of water makes it harder for them to be preyed upon by native species of fish, which are not accustomed to crayfish fighting them back.^[21] Instead of running away like the native crayfish species do when they come in contact with a predator, the rusty crayfish will take an attack stance with its claws raised above its head, which will generally scare away most predatory fish. Additionally, adult rusty crayfish can be too large for some fish to consume.^[22] Because the rusty crayfish are able to avoid predators fairly well, their population in these new aquatic ecosystems was able to grow extremely quickly and within twenty years the rusty crayfish population had exploded and become an invasive species in the Northern United States and parts of Canada.^[23] Because these rusty crayfish populations have basically taken over the natives species' habitats and forced them out of their homes, many populations of native crayfish have experienced drastic decline over the past fifty years and the rusty crayfish has become the dominant species in much of the Midwestern United States.^[13]

While chemicals that kill crayfish exist, none are specific to rusty crayfish.^[24] Harvesting for consumption is expected to solely impact the adult population.^[24] Once a population of rusty crayfish is introduced to a body of water, it may be very difficult to completely eradicate them. Therefore, the best control strategy is to try to prevent any further spread of the rusty crayfish. The best methods to prevent spread of invasive species are to learn to identify them, and to avoid using invasive species as bait or to otherwise transport them to bodies of water where they are not already present.^[16]