Orconectes rusticus (Rusty Crayfish), an invasive species in the native waters of Wisconsin

by Austin Cumber
BIOL 490, Fall 2014

Key taxa: Arthropoda, Malacostraca, Decapoda, Orconectes

Over the past 50 years, the numbers of Wisconsin native aquatic species has declined due to a biological threat of a particular crustacean. This crustacean, known as the rusty crayfish (Orconectes rusticus), is a highly aggressive invasive species found in aquatic habitats throughout north central regions of the United States. The unique rust-colored band running down the center on the back side of its abdomen allows this arthropod to be distinguished from native crayfish species (Olden et al., 2006). Orconectes rusticus was first introduced in the early 1960’s by the live bait trade which led to the illegal dumping of these crayfish into native waters (Karen et al., 2005). Since then, these crustaceans have been out-competing other native species for common resources by taking over the habitats of native species and destroying them. This destruction of habitats has become a major problem in almost all of Wisconsin’s waters.

Authors have recorded enough data about the spatial patterns and temporal distributions of the rusty crayfish over the past 40 years to conclude that these crustaceans are currently widespread over the state of Wisconsin. During the first twenty years of a study by Oldon and colleagues, the percentage of waters infested by the rusty crayfish was seven. However, during the last 20 years, that number increased to 36% making them the most dominant crayfish species found in Wisconsin. As of 2014, they have been observed in almost 400 water sources throughout the state and comprise 17% of all crayfish species found within the area. The northern native crayfish (Orconectes virilis) compensated for 62% of total water sources during the first 20 years of the study, but during the next two decades, their numbers decreased to 34% because of the Orconectes rusticus (Olden et al., 2006). These gathered data suggested a major change in species dominance because aggressive behavior between these two species was more dominant in the rusty than in the northern. (Morsel et al., 2010)

Rusty crayfish are capable of taking over entire ecosystems by destroying the complex trophic levels and reducing native species numbers. One of the first observations was done in Vilas County, Wisconsin. Fifteen modified minnow traps containing fish pellets were placed in ten different locations equidistant from one another around Trout Lake and were checked annually. This study was done in 1979 when Orconectes rusticus first appeared in a small portion of the lake to determine how fast these crustaceans can disperse to other areas and how their presence affects other organisms. Within a few years, there was already a decrease in the abundance of native species present in the lake. Nineteen years later, the rusty crayfish had successfully invaded all parts of the lake, which negatively impacted the environment. Piscivorous fish numbers remained the same because they preyed on other fishes and crayfish regardless of species difference whereas all other native species experienced a decrease in abundance levels. (Byron & Wilson 2001).

The biggest impact on the entire ecosystem was the pulling and grazing of macrophyte communities by the rusty crayfish. By foraging on these plant communities and uprooting young seedlings to make burrows, these crustaceans decreased macrophytes and the macro invertebrate populations that fed on them by over 80% in some areas. This effect on the two communities also negatively impacted the snail densities from over 10,000 snails per square meter to less than five (Wilson et al., 2005). The lower amounts of aquatic vegetation caused omnivorous fish, such as bluegills and pumpkin seeds, to fall in numbers because they had to switch to a more carnivorous diet. The aggressive behaviors of rusty crayfish forced many native crayfish out of their cobble substrates to macrophyte and sandy habitats where predation rates were much higher. The native species were unable to adapt in the sandy environment because the substrate made it difficult to build sufficient nests. This decreased the amount of coverage made these arthropods more vulnerable to predation which led to a higher consumption rate than the invasive species (Byron & Wilson 2001).

A similar study was done involving predator-prey interactions among common fish of Wisconsin. Yellow perch, walleye and three species of bass were observed eating three types of crayfish species (rusty, northern, and northern clearwater). Roth and Kitchell (2005) tested predator success on invasive species versus native species in North and South Turtle Lake in Vilas County, Wisconsin. They were curious whether selective predation had any influence on the consumption rate of crustaceans. Six locations of different depths were used as the feeding sites for their experiment. The walleye and rock bass showed no signs of selective behavior towards a certain arthropod, whereas yellow perch, largemouth bass, and smallmouth bass displayed a strong selective behavior towards the two northern species (Nilsson et al., 2012). The authors concluded that rusty crayfish tend to have larger chelae than the two native species causing them to become more aggressive. This aggressive behavior and morphological feature makes the rusty crayfish less vulnerable to predators because they are able to appear bigger and more dangerous than the northern and clearwater species. The rusty crayfish also maintained a larger body size which led to having a higher survivorship curve.

A larger size and higher survivorship curve was positively correlated with a greater food intake and explains why rusty crayfish populations become abundant faster than native species. Since taxa in the genus Orconectes are mainly nest predators, they must have adapted some type of behavior that allows for excellent egg consumption. In a study involving northern lakes of Wisconsin, Nilsson et al. (2012) looked at the different feeding behaviors on fish eggs of substrate nesting fishes between the rusty and virile crayfish. They wanted to determine if an increase in the predation of fish eggs could lead to a reduction in fish abundance. They focused on which of these species would consume eggs at a faster rate. The two species were observed feeding on bluegill eggs at different elevations. Each species of crayfish ranged in size from newly hatched juveniles to large adults and were starved over a 24 hour period before given the eggs.

Once given the eggs, the consumption rates were recorded for both species and compared to determine if feeding rates were correlated with carapace length. The duration of feeding times were longer for Orconectes rusticus than for Orconectes virilis regardless of different elevation levels. In the rusty crayfish, the carapace length and foraging rate displayed a positive relationship because their feeding rates increased as the carapace grew longer. Since the rusty crayfish had a higher feeding rate than the virile crayfish, they were able to capitalize on unguarded fish nests during brief time periods more efficiently (Nilsson et al., 2012). This provided them with better predator avoidance and increased their chances of survival. Orconectes rusticus also had higher metabolic rates than Orconectes virilis which gave them the ability to consume more eggs at a faster rate. This higher metabolic rate allows the rusty crayfish to reach adulthood sooner and complete their reproductive life cycles faster than native species (Morsel et al., 2010).

As this invasive crustacean becomes larger, it forages more which further destroys the aquatic habitats of native species. The rusty crayfish exhibits a more aggressive behavior that is capable of driving native crayfish to different food sources when competing within the same area. This aggressive adaptive behavior makes them a better egg predator and more successful at invading native environments. Even though northern crayfish consumed eggs, their rates were too low to affect fish abundances when compared to the high feeding metabolism of rusty crayfish. This high feeding rate affected substrate nesting fishes because a large portion of their eggs were consumed by the rusty crayfish. The reduced number of fish caused a lower survival rate for predatory fish which led to a negative impact on the ecosystem (Morsel et al., 2010).

Since carapace length is correlated with body size and consumption rates increase as the crayfish gets larger, another study wanted to compare these findings to another variable which was temperature. Mundal and Benton (1990) were curious whether temperature played an important factor in crustacean development. They conducted an experiment that focused on the thermal ecology of Orconectes rusticus and the effects of four water temperatures (16, 20, 25 and 29° C) on the survival and growth rates of juveniles. In this study, rusty crayfish were collected from rivers before the experiment and placed in aquaria with four different treatments to test for temperature selection between juveniles and adults. The greatest survivorship curve observed during this investigation was from 20-25° C with the highest mortality rate at 16° C and 29° C. Maximum carapace length was observed at 26.3° C and weight at 27.5° C which suggested that growth rates were temperature dependent. At higher temperatures (26-28° C) rusty crayfish achieved their maximum size the fastest but had shorter lifespans. At cooler temperatures (20-22° C) they exhibited the highest survivorship but did not reach maximum size. Post molting mortality was the highest at the lowest and highest temperature tested.

Thermal tolerance levels were higher in juveniles than in adults with juveniles being more likely to be found in warmer water temperatures. This is because warmer conditions will increase juvenile growth rates and give them an enhanced fecundity with competitive abilities earlier in life. Adults were found in cooler waters to favor maximum survival and maintained a higher survivorship curve. The rusty crayfish’s aggressive behavior and high growth rate have led to the displacement of many native crayfish from their habitats (Mundal & Benton 1990). The broader temperature tolerance range of this species allows them to live along the fringe of shorelines which no other crayfish species has been able to do. This gives the crayfish more space and access to resources in order to reproduce and expand its invasive range into more water territories (Phillips et al., 2009).

These types of Wisconsin ecosystems have the highest endangerment and extinction rates because of the huge threat invasive species such as the rusty crayfish poses to their environment. The rusty crayfish is currently forcing other native crayfish species westward and into Canada. This is because these other species do not have the aggressive behavior to compete over resources with them (Phillips et al., 2009). It is only a matter of time before this exotic species destroys other ecosystems causing certain species to go extinct and destroy the freshwater fishing industry in Wisconsin. By educating fishermen about rusty crayfish and the negative ecological impacts they have, we can reduce future introductions to other areas and be warned of invasion progression by this harmful species.

References Cited

• Byron, C., & Wilson, K. 2001. Rusty crayfish (Orconectes rusticus) movement within and between habitats in Trout Lake, Vilas County, Wisconsin. Journal of the North American Benthological Society 20: 606-614.
• Morsel, W.J., Baldridge, K.A., & Sargent, W.L. 2010. Invasive crayfish Orconectes rusticus (Decapoda, Cambaridae) is a more effective predator of substrate nesting fish eggs than native crayfish (O. virilis). Crustaceana 86:387-402.
• Mundal N & Benton M. 1990. Aspects of the thermal ecology of the rusty crayfish Orconectes rusticus. Oecologia 82:210-216.
• Nilsson, E., Solomon, T., Wilson, A., Willis, V., Larget, B., & Vander Zanden J. 2012. Effects of an invasive crayfish on trophic relationships in north-temperate lake food webs. Freshwater Biology 57: 10-23.
• Olden, J., McCarthy, J., Maxted, J., Fetzer, W., & Vander Zanden J. 2006. The rapid spread of rusty crayfish (Orconectes rusticus) with observations on native crayfish declines in Wisconsin (U.S.A.) over the past 130 years. Biological Invasions 8: 1621-1628.
• Phillips, I., Vinebrooke R., & Turner M. 2009. Ecosystem consequences of potential range expansions of Orconectes virilis and Orconectes rusticus crayfish in Canada. Environmental Reviews 17: 235-248.
• Roth, B., & Kitchell, J. 2005. The role of size-selective predation in the displacement of Orconectes crayfishes following rusty crayfish invasion. Crustaceana 78: 297-310.
• Wilson, K., Magnuson, J., Lodge, D., Hill, A., Kratz, T., & Willis, T. 2005. A long-term rusty crayfish (Orconectes rusticus) invasion: Dispersal patterns and community change in a north temperate lake. Canadian Journal of Fisheries and Aquatic Sciences 61: 2255-2266.