Pacifastacus leniusculus, commonly known as the signal crayfish, is a species of freshwater crayfish in the family Astacidae, native to western North America. They are thought to have originated from the Columbia River Basin.
Physically, they are characterized by a smooth carapace and a distinctive white to pale blue-green patch at the base of the claw joint. Reminiscent of the white flags that signalmen used for directing trains, this light patch is responsible for the species' common name. Due to its heightened aggression and opportunistic polytrophic diet, They typically weigh 60 g and 110 g at carapace lengths of 50 mm and 70 mm, respectively. 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). Efforts are being made to create a barrier to signal crayfish invasion in this region. Within North America, it has also been introduced to Nevada, and the populations in Utah may be the result of introductions.
P. leniusculus was first introduced to Europe in 1960 and have spread across the continent since, both intentionally and not. As of 2009, they can now be found in 27 European countries and regions, making it the most widespread invasive freshwater crayfish species in Europe. Consequently, P. leniusculus has continued to spread and outcompete native crayfish.
The signal crayfish is now the most widespread alien crayfish in Europe, residing in 27 countries, from Finland to Great Britain and from Spain to Greece. It was first introduced to Great Britain in 1976, and is now widespread across the British mainland as far north as the Moray Firth. It has also been observed on the Isle of Man, but not in Ireland,
Multiple studies have been published to identify effective mitigation strategies for the detrimental impact of the settlement — and subsequent overpopulation — of invasive signal crayfish in Europe. This includes studies regarding effective upstream barriers against signal crayfish that do not adversely affect fish migration,
Invasiveness and European range
The success of P. leniusculus is often attributed to the Enemy Release Hypothesis, where they escape approximately 75% of their native parasites and predators when entering a new range. They also display higher aggression than other established invasive spiny cheek crayfish (Faxionus limosus'). This may factor into their success and fitness against native species, allowing for P. leniusculus to have rapid range expansion.
In Europe, P. leniusculus has been included in the list of Invasive Alien Species of Union concern (the Union list) since 2016. This implies that this species cannot be imported, bred, transported, commercialized, or intentionally released into the environment in the whole of the European Union. The signal crayfish is often considered a nuisance species amongst anglers in Europe.
In the Iberian Peninsula, invasive P. leniusculus have been recorded to significantly impact native fish communities. On average, native fish were observed to dominate in only 4% of encounters with adult P. leniusculus compared to 25% with juveniles. In particular, the Pyrenean stone loach (Barbatula quignardi) dominated 50% of juvenile encounters, but exhibits an evasion response greater than 30% in adult encounters.
A case study of Rabaçal and Tuela River basins of northeast Portugal revealed that the presence of P. leniusculus had nearly negligible effects on overall native fish abundance, biomass, or species richness. However, more nuanced species interactions were observed. In P. leniusculus invaded sites, there was a significant decrease in Iberian chub (Squalius carolitertii) populations and brown trout (Salmo trutta) physiological conditions.
Habitat
P. leniusculus typically lives in a variety of streams, rivers, and wetlands. The species is environmentally tolerant but does not occur in waters with a pH lower than 6.0.
P. leniusculus display heighten aggressive behaviors compared to other freshwater crayfish. In interactions with the invasive spiny cheek crayfish (Faxionus limosus), P. leniusculus established dominance consistently. P. leniusculus had a greater tendency to engage and maintain high-intensity fight. F. limosus conversely retreated at a lower fight intensity.
Exposure to different semiochemicals, communicatory or informative chemical signals, drastically influences P. leniusculus behavior. Exposure to haemolymph, an alarm cue that elicits strong anti-predator responses, triggered heightened activity. Compared to responses to food odors, P. leniusculus exhibited significantly more movement in distance traveled at a faster swimming velocity. Though able to locate food odor sources more accurately, P. leniusculus often remained within the vicinity of haemolymph sources for longer once detected. This behavior suggests that P. leniusculus may remain within proximity to the alarm cue to gain more information on the potential threat while staying out of direct contact and danger. Adults can adapt to a broad range of salinity, substrate types and the presence of aquatic vegetation. This has been found experimentally, where injecting WSSV into different tissues in the signal crayfish resulted in positive WSSV gene probe results, changes in the proportions of different haemocytes, and symptoms similar to those of WSSV-infected shrimp. However, no white spots or red body color were observed, which are typical symptoms. In 1960, Miller was the first to describe them as subspecies and later genetic studies have since confirmed this relationship, with P. l. klamathensis being more diverged than P. l. leniusculus and P. l. trowbridgii.
It was previously described that these subspecies of P. leniusculus were geographically isolated. However, early introductions and a lack of historical records complicate the delineation of any formal native ranges or potential overlaps. P. l. leniusculus and P. l. trowbridgii are thought to be indigenous to the lower Columbian River Basin and its tribunaries, potentially diverging due to stream capture. P. l. klamathensis is thought to be indigenous to the Klamath River.