Every invasive species is a native species somewhere else. Red swamp crayfish, Procambarus clarkii, aka Louisiana crayfish, also known as crawdads, and sometimes called mudbugs, are native to the south-central United States and northeastern Mexico. Everywhere else they’ve spread to – and that’s a lot of places, including Europe, Asia, and many states outside of their native range in the United States – they’re invasive.

Not all non-native species are invasive – non-native species are organisms that don’t naturally occur in a specific place, while those that are non-native as well as destructive in some way are considered invasive. Red swamp crayfish usually fall into the later category – when introduced to a new location, they typically dominate the local habitat, to the detriment of local crayfish populations. 

Red swamp crayfish were first officially detected in Washington State in 2000, in Pine Lake, a tiny lake (just eight tenths of a mile by four tenths of a mile at its widest point) 20 miles east of Seattle. Five years after they were first recorded in Pine Lake, the invasive red swamp crayfish population was much larger than the native crayfish population. (Native crayfish population was not reported in 2008.)

Note: 2008 values were calculated from 24-hour sampling periods, with the assumption that capture rate was equal throughout the 24-hour period - because crayfish are more active at night, the calculated values may be underestimates. 

Sources: First detection in Pine Lake from Mueller 2001; 2005 values from Mueller 2007; 2008 values calculated from Larson & Olden 2008.

(Figure by Emily Benson)

The problems that invasive red swamp crayfish can cause when they spread to a new location are well known, as are the most common mechanisms of introduction. Most red swamp crayfish dispersal is due to human activity – mudbugs are considered a culinary delicacy, and live crayfish have been stocked, farmed, and traded widely throughout the world. There are, however, other ways that crayfish can spread – a study published in Aquatic Ecology earlier this year suggests that ducks and other waterfowl may be a previously unappreciated vector for transporting crayfish between lakes and ponds.

The researchers who conducted the study were interested in whether or not juvenile crayfish could cling to the feathers of a duck as it flew between bodies of water, and, if they could, how long they could hold on for. They found that crayfish are capable of hitching a ride on waterfowl, particularly in shallower water depths. With trained homing pigeons standing in as proxies for ducks, the scientists found that crayfish could survive flights as long as 37 miles in mesh bags secured to the birds.

As the researchers write, “these findings indicate that waterbird-mediated passive dispersal should be taken into account to explain P. clarkii’s rapid spread and should be considered when managing its invasions.” Humans may be responsible for the majority of the spread of this invasive species, but we’re not the only culprits.

Red swamp crayfish are freshwater crustaceans, but they can survive out of water for up to sixteen and a half hours, depending on the temperature and humidity;  Anastácio and colleagues estimate that a crayfish could walk over 700 yards in that time. 

(Image by Entomolo via Wikimedia Commons)