Species at a Glance
The Allegheny crayfish (Faxonius Obscurus), also known as the obscure crayfish, is a medium-sized crayfish that typically inhabits rocky streams. This crayfish reaches maturity quickly after hatching, which enables it to spread quickly and compete for habitat and food resources with native crayfish species. In Pennsylvania, this species is native to western portion of the state; however, it has recently been identified in both the Delaware, Susquehanna, Erie, and Potomac Basins.
The Allegheny crayfish is found in a range of colors from light brown to olive green, with a dark brown wedge on the dorsal surface of the abdomen. Its rostrum (nose-like structure) is concave and lacks a ridge or keel running down the middle. The chelae (pincers) are large and straight with moderately long fingers and two rows of tubercles along the margin of the palm. Some individuals from the Mahoning River, Portage County, and the largest one from Ohio were found to have a maximum carapace length of 40 mm (1.6 in), while the largest from Allegheny County and Pennsylvania were measured at a total length of 93 mm (3.6 in).
The northern clearwater crayfish (Faxonius propinquus) may be confused with the Allegheny crayfish due to similar coloration. The key distinguishing feature between these two species is a small bump located on the rostrum of the northern clearwater crayfish that the Allegheny crayfish lack. Additionally, the claw tips of the clearwater crayfish are usually orangish-brown followed by a pale or yellowish band of color. The claw tips of the Allegheny crayfish are usually tipped with orangish-brown, followed by a dark green-blackish band, and then a pale-yellow band. These species have the potential to hybridize.
The Allegheny crayfish can also be mistaken for another invasive species, the rusty crayfish which usually has large red spots on the outside of the body where the head and thorax meet. These two species also have the potential to hybridize.
The Allegheny crayfish prefers slow-moving waterways and pools with rocky substrates, where it can hide beneath rocks and burrow into the sediment, creating inconspicuous tunnels along the water’s edge.
This species was most likely introduced outside of its native range through bait bucket releases. Once introduced, its high reproductive capacity allows it to quickly establish and spread to new areas. In Pennsylvania, live crayfish may be used as bait if they are collected from the same waters where the fishing is taking place. If moved elsewhere, the head must be immediately removed above the eyes before transport.
The Allegheny crayfish is native to Maryland, New York, Ohio, Pennsylvania, Virginia, and West Virginia. In Pennsylvania, this species is native to the Lake Erie, Genesee, and Allegheny and Ohio river drainages in the western portions of the state; however recent studies have revealed that it was introduced into the Susquehanna River drainage and portions of the Delaware River drainage.
Allegheny Crayfish threaten native crayfish species by directly competing for food, habitat, and resources, and through hybridization with native species.
United States Geological Survey (USGS) Nonindigenous Aquatic Species Database (2019)
Observation.org (2023) Hartzell, Sean (2020). Life History of an Invasive Population of Allegheny Crayfish (Faxonius obscurus) in Eastern Pennsylvania Stream. Northeastern Naturalist