Pacifastacus leniusculus (Dana, 1852), which is part of the subgenus Pacifastacus, is divided into three subspecies; leniusculus, klamathensis (Stimpson, 1857), and trowbridgii (Stimpson, 1857). Initially described as separate species, the similar, yet highly variable morphology shared by these subspecies has challenged taxonomists for decades (Larson et al. 2012). Miller (1960) was the first to describe these as subspecies of P. leniusculus. Genetic studies have since identified P. l. leniusculus and P. l. trowbridgii as being the most similar of the three subspecies, while P. l. klamathensis is the most distinct (Agerberg and Jansson 1995

Physical features key in differentiating P. l. leniusculus from P. l. trowbridgii include the presence of sharp spines on the post orbital ridge and a relatively narrow carapace. In contrast, P. l. trowbridgii typically have a robust carapace with rounded tubercles on their post orbital ridge. Small tubercles rather than spines are present in P. l. klamathensis’ post orbital ridge, and the white to blue-green pigmentation commonly found on the chelae of the other subspecies is often absent. Additionally, the rostrum of P. l. klamathensis is very wide relative to the length of its acumen (Riegel 1959; Miller 1960; Larson and Williams 2015). Due to the difficulty and complexity of distinguishing these subspecies, Larson et al. (2012) summarized Miller’s (1960) identifying criteria (Larson et al. 2012 - appendix S1). The characteristics summarized by Larson et al. (2012) are illustrated in the table below for comparisons.

The subspecies of Signal Crayfish are believed to have once been geographically isolated populations (Hobbs 1988). Mixing due to the prevalence of early introductions, a lack of historical records, and hybridization between subspecies has made describing their native range and taxonomic status problematic (Hobbs 1988; Lowery and Holdich 1988; Larson and Williams 2015). This has led the subspecies of Signal Crayfish to be commonly regarded as a single species (Hobbs 1988). Genetic tests have begun to shed light on this, but the extent of the native distributions of Signal Crayfish subspecies continues to be a contested subject (Larson and Williams 2015).

The subspecies P. l. leniusculus, is believed to be native to the lower Columbia River and its tributaries (including the Willamette River) in western Oregon and Washington state. It is also assumed to be native to the Umpqua River, which is believed to have had a historic drainage connection to the Willamette (Miller 1960; Larson and Williams 2015). Based on Miller’s (1960) accounts, it is probable that Pacifastacus leniusculus trowbridgii is also native to the lower Columbia River basin, and nearby coastal rivers, such as the Umpqua, which were likely once connected via stream capture (Miller 1960; Larson et al. 2012; Larson and Williams 2015). Pacifastacus leniusculus klamathensis is presumed to be native to the Klamath River in Northern California and Southern Oregon, but the extent of its native range beyond this basin is unknown due to decades of introductions that have resulted in the mixing of populations.

The Signal Crayfish occupies a range of habitats throughout its native and non-native distribution (Goldman and Rundquist 1977; Holdich and lowery 1988). Though P. leniusculus prefers low gradient streams typical of agricultural low-lands in western Oregon (Avault 1973), they inhabit both coastal and upland streams, lakes, and rivers (Lowery and Holdich 1988). Signal crayfish can be found in habitats ranging from clear, shallow coastal streams (Lowery and Holdich 1988), to major rivers with high turbidity (Ibbotson and Furse 1995), as well as eutrophic and oligotrophic lakes and reservoirs (Holdich and Lowery 1988). Pacifastacus leniusculus also occupies the saline and often turbid waters of major river deltas (Shimizu and Goldman 1983). Wheatly & McMahon (1983) revealed via a laboratory study that Signal Crayfish can occupy waterways with salinity as high as ~26 ppt (75% seawater), for several days. Additionally, Miller (1965) noted that Signal Crayfish have been observed copulating, molting, and laying eggs in brackish water.

The breeding cycle of the Signal Crayfish follows that of most temperate zone crayfish. Copulation occurs during the autumn months (September or October), and females carry the eggs throughout the winter (Holdich and Lowery 1988). Eggs then typically hatch in March and April as the water warms (Shimizu and Goldman 1983). The young from populations residing in cooler waters may hatch later in the year (June and July), since growth is temperature dependent. Once hatched, P. leniusculus grow rapidly and most individuals mature during their second summer. The time to maturity may also be delayed by cooler water temperatures, such as that of Lake Tahoe. Here, males may mature during their third summer, while females may not mature until fourth (Holdich and Lowery 1988). Abrahamsson and Goldman (1970) estimated that male and female P. leniusculus in the Sacramento River, CA., mature when they reach the size of 29-37 mm CL and 25-35 mm CL, respectively. Crayfish growth is also density dependent, which often results in small, newly established populations of P. leniusculus experiencing a short period of rapid growth (Hogger 1986).

Pacifastacus leniusculus is both a fast growing and long-lived species. It’s known as one of the fastest growing species of temperate zone crayfish (Holdich and Lowery 1988), and in general, the highest growth rates are associated with populations which have recently invaded an unexploited habitat (Hogger 1986). These rapid growth rates subside as populations establish and densities surge, presumably because of increased competition for food and space (Hogger 1986). Their potential for rapid growth has made them the focus of both aquaculture productions and commercial fisheries in several countries (Westman 1973; Furst 1977; McGriff 1983; Goddard and Hogger 1986; Lowery and Holdich 1988). Hogger (1984) found that individuals from a population of P. leniusculus in southern England had the potential to grow up to 62 mm CL in as few as three years when grown in ideal conditions.  Overall, the Signal Crayfish may survive up to 9 years or more when living in the wild (Goldman and Rundquist 1977).

The Mazama Newt (Taricha granulosa mazamae), which is endemic to Crater Lake, Oregon, is greatly threatened by the presence of the nonnative Signal Crayfish (Girdner 2018). Although newts remained in uninvaded regions of the lake, they were nearly absent from areas occupied by the crayfish. Mesocosm experiments revealed that P. leniusculus prey directly on newts, displace newts from cover, and have the potential to alter their overall behavior. Areas inhabited by the introduced crayfish also experienced dramatic decreases in benthic macroinvertebrate diversity (Girdner 2018). Crawford et al. (2006) found that the total number of invertebrates was significantly lower in sites where Signal Crayfish were present. Moorhouse et al. (2014) observed an inverse relationship between P. leniusculus and macroinvertebrate abundance and taxon richness.

The spread of American crayfish species in Europe during the 19th, 20th, and 21st century is closely associated with the spread of the crayfish plague (Holdich et al. 2009). Signal Crayfish serve as carriers of the crayfish plague, which is caused by a fungus-like organism (Aphanomyces astaci) (Cerenius et al. 1988). While P. leniusculus are highly resistant to the illness, Asiatic, Australian, and European crayfish are very susceptible to the plague’s ill effects (Unestam 1969). Although it was not introduced until the mid-1900’s, the Signal Crayfish is thought to be vector for spreading the plague (Lowery and Holdich 1988), and introductions of P. leniusculus to new regions in Europe are believed to contribute to the infection of new drainages (Cerenius et al. 1988).

Though uncommon, Signal Crayfish have been documented burrowing in river banks (Guan 2010). In the Great River Ouse, England, burrows were constructed at high densities (5.6 burrows per meter length), which increased the erosion of the river banks (Guan 2010).