Signal Crayfish

Members of this species are typically 6–9 cm (2.4–3.5 in) long, although sizes up to 16–20 cm (6–8 in) are possible. They typically weigh 60 g and 110 g at carapace lengths of 50 mm and 70 mm, respectively. Their dorsal surface is typically brown, but can range from bright red to blue. A distinctive white to pale blue-green patch is present near the claw hinge. The surface of the carapace and claws are smooth. P. leniusculus can often be confused with the noble crayfish (Astacus astacus), but lacks a row of spines on the shoulders of the carapace to distinguish them.

P. leniusculus is thought to be endemic to the Columbia River Basin in North America, which includes the Canadian province of British Columbia and U.S. states Washington, Oregon, and Idaho. However, substantial cryptic diversity exists, with morphology-based taxonomy and molecular evidence suggesting that three groups are highly distinctive from P. leniusculus in discrete geographic regions: the Chehalis River glacial refugium, central Oregon and the Okanagan Plateau. First introduced to the San Lorenzo River watershed in California in 1912, P. leniusculus has since spread through the state. The only native crayfish remaining in California (aside from Pacifastacus leniusculus klamathensis, a subspecies of P. leniusculus believed to be native to the Klamath River in Northern California) is the Shasta crayfish, of Shasta County, California (Pacifastacus fortis).

P. leniusculus are primarily nocturnal, conducting most of their activity at night. In a 12:12 light-dark cycle, an average of 72% of their activity occurred during dark hours. Research has shown that they have a circadian rhythm with distinct chronotypes, consistent and repeatable activity patterns, during early and middle night. Late-night behavior is far more variable and unpredictable. This is hypothesized to be driven by predator avoidance as a potential evolutionary strategy.

Pesticides in runoff continue to pose a problem for many aquatic ecosystems, including that of the P. leniusculus. Three of the most relevant pesticides affecting P. leniusculus are metazachlor (MTZ), terbuthylazine (TER), and thiacloprid (TCL). When exposed to MTZ, P. leniusculus exhibited a rapid increase in heart rate and induced movement. However, TER and TCL did not induce any significant differences in heart rate or movement. In experimental trials, a majority of exposed individuals did not exhibit any detectable response within the 10-minute period. No long-term physiological effects of the pesticides on P. leniusculus are known. As such, P. leniusculus are especially vulnerable to TER and TCL, as a lack of response may lead to prolonged exposure.