The Rusty Crayfish

(Faxonius rusticus)

INTRODUCTION

The Rusty Crayfish, (Orconectes rusticus) occurs in lakes, rivers, streams and ponds. It prefers areas with rocks, logs, orother debris as cover but substrate types may also include clay, silt, sand and gravel. It can inhabit slower, deeper pools as well as shallow fast water areas of streams.

The Rusty Crayfish is thought to be native to the Ohio River Basin and the state of Ohio, Kentucky, Michigan and

Indiana. It has however been widely introduced in North America and now in Europe. In Canada found throughout Ontario, in Eastern Manitoba and in western Quebec.

It is an aggressive species and a prolific breeder which, as a result has spread significantly and has displaced resident native species (Virile Crayfish, Northern Clearwater Crayfish and Appalachian Brook Crayfish), Big water Crayfish in numerous lakes and waterways in eastern Noth America. Larger size, earlier breeding season, higher fecundity, faster egg, larval and juvenile development, appear to be some of the factors contributing to its dominance. In addition, the species may be able to better compete for food, withstand pressure from fish predation, and hybridize with native species.

The expanding dense populations of this species therefore not only pose a threat to the biodiversity of the native aquatic invertebrate fauna, but also potentially endanger aquatic vegetation as well as sport fish (in terms of competition for aquatic prey, predation on fish eggs as well as impacts on fish nursery habitats). Many of these effects have been documented in the United States where the species has expanded into 27 staqtes and in Illinois, and Wisconsin it aggressively displaced native crayfishes and had a significant impact on aquatic ecosystems.

All photos P. Hamr unless otherwise stated

Rusty crayfish by M. J. Larson

Illinois-Indiana Sea Grant

Virginia DWR.

Canadian & Great Lakes basin rerion distribution. iNaturalist.com 2026

Description

A medium to large crayfish. It has a brown cephalothorax and abdomen, green claws with dark black bands near tip, green legs, prominent (dorso-lateral) rusty spots on each side of the carapace and small rusty spots also present on abdominal segments. Blue morphs of this species have also been recorded in the wild . It can be distinguished from the similar Northern Clearwater Crayfish by absence of rostral carina and presence of rusty markings on exoskeleton. The other similar native species. the Virile Crayfish. is generally more blue in colour (without rust markings) with broader shorter chelae bearing distinct yellow tubercles. whereas the Rusty Crayfish has larger more elongated fingers of claws without tubercles.

Life History

In southern Ontario. the species has been shown to breed very early in spring (March-April) when water temperatures rise above 4'C (39'F). Eggs are laid 11-12 days after copulation and are carried for 6-8 weeks. The fecundity ranges from 35-351 and eggs are about 2.4 mm (0.094 in.) in diameter. The average fecundity and egg volume of this species is greater than that of the native Virile Crayfish and Northern Clearwater Crayfish. Males alternate from a breeding form (F1) to a non-breeding (F2) moulting from F1 to F2 in May and then back into Form I ifrom late June to early July.

The Rusty Crayfish grows rapidly with maturity being reached early in both sexes, in some cases at the end of the first summer of life and young Rusty Crayfish then may participate in mating within less than a year of hatching. Maturity is attained between 14-23 mm (0.63-0.9 in.) carapace lenght (CPL) and the maximum life span is 3-4 years at a CPL of 40-45 mm/1.6-1.8 in. Maximum size measured to date is 54 mm carapace.

In southern Ontario densities may reach very high numbers - average densities of 6-64 crayfish per m2 have been counted in rivers draining into Rice Lake (maximum counts as high as 113/m2 or 371/fe). The sex ratio is about 1: 1 but adult males are generally more active throughout and therefore make up the majority of hand samples and trap catches.

The Table below summsrizes Lifehistory events in a creek in Southern Ontario (from Hamr &Verhoef 2022).

A 4 month, a 2 year & a 4 year old.

A rare bilateral color mutationfrom Chagrin R. Ohio. (S. Williams)

Blue Rusty from McRae Provincial Parkon Lake Simcoe, Ontraio. (A. Fyon)

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