Cambaridae and Applied Interest of Invasive Introductions
by Amber Honadel
BIOL/WATER 361, Fall 2013
The freshwater crayfish family of Cambaridae can be very invasive if introduced into an ecosystem. Invasive crayfish tend to out compete and kill native species of fish, macroinvertebrates, snails and plants. Crayfish are not unlike any other invasive species spreading though out freshwater systems. Almost every introduction is a negative one, except in Africa where it is a form of biological control. Removal methods usually do not work because of the high densities and regeneration of plants is very difficult when crayfish are still present.
The rusty crayfish (Orconectes rusticus) was introduced by fishermen, who emptied their bait buckets and lake users who wanted problem plants gone. It was also commercially released for harvesting (Wilson et al., 2004). Now that the rusty crayfish has been established, it has spread from lake to lake by moving up streams and rivers (Wilson et al., 2004). Crayfish are generalist, which means they will consume a wide range of food items. This can include leaf litter, algae, plants, snails, invertebrates and fish eggs.
While small crayfish are commonly eaten by fish, larger crayfish are generally not often consumed (Wilson et al., 2004). Being generalist and large in size allows the rusty crayfish to move into a lake ecosystem and cause imbalances and decreases in native species which can occur within a few years after an invasion. The good news is that crayfish are slow dispersers which means a lake may take 16+ years to become fully invaded (Wilson et al., 2004).
Lake Ottawa, Michigan discovered its first invasive rusty crayfish in 1987, and at that time, it made up 20% of the crayfish in the lake (Rosenthal et al., 2006). By 1997, it made up 75% of the population, and finally since 2001, 100% of the crayfish in Lake Ottawa are now rusty crayfish (Rosenthal et al., 2006). With the increasing population of rusty crayfish, the lakes in Michigan saw decreases in species richness and abundance of plants, snails and macroinvertebrates. This is due to crayfish grazing and uprooting plants which are home to the marcoinvertebrates. Because crayfish consume snails and invertebrates, this causes outcompeting of bluegill and pumpkin seed fingerlings for food. Rusty crayfish are also capable of extirpating native crayfish species because of the competition (Wilson et al., 2004).
The decrease in marcoinvertebrates and plants can upset the trophic levels. Some scientists believe that invasive crayfish can unbalance food webs by removing alternate prey species (Wilson et al., 2004). Predator fish will switch to diets of primarily rusty crayfish when they invade a lake, and this causes other prey species to thrive and have population growth, which causes the imbalances of food webs. The rusty crayfish is spreading throughout eastern North America, and soon it will start moving into Canada causing imbalances and decreases in species diversity. Its invasion is well into establishment which makes it almost impossible to stop (Wilson et al., 2004).
In every case when a crayfish is introduced into an ecosystem, it destroys the native habitat. However, in rare cases, these introductions can be used as a positive bio control agent. It is estimated that in Africa, 1-2 million people die from schistosomiasis, which is caused by the parasites Schistomsoma haematobium and S. mansoni (Loker et al., 1991). This parasite requires a snail in the genera Biomphalaria or Bulinus in its life cycle (Mkoji et al., 1999). The infective stage, called cercariae, are released from the snail and enter the skin when a human enters the infected water. Currently, there is no vaccine for schistosomisasis though one is in the process. Scientists wanted to find a way to decrease the number of snails, which would decrease the number of schistosomiasis cases. Africa has no native crayfish, but in 1960 they were introduced for aquaculture purposes (Mkoji et al., 1999). They found that where the Louisiana red swamp crayfish (Procambarus clarkii) was found, Biomphalaria and Bulinus populations were not (Mkoji et al., 1999). The crayfish not only killed all the snails in the lakes, but also consumed native plant species that the snails live on. It may consume native plants species, but introducing crayfish is cheaper than molluscicidal chemicals to kill snails which would need to be applied every year and can cause environmental damage (Loker et al., 1991). Because crayfish are able to burrow during dry seasons, they can adapt to Africa’s wet/dry season. Scientist also found that when the Louisiana red swamp crayfish was introduced it consumed the problem snails even when other food was present (Loker et al., 1991).
Scientists tested whether the crayfish was decreasing the number of schistosomiasis cases. Children from six schools, three control and three experimental were used in the experiment. Near the three experimental schools, crayfish were introduced into a nearby water source, the three control did not have crayfish introduced (Mkoji et al., 1999). After crayfish were established, scientists treated the children that were infected with the parasite. Through a two year study new infections and reinfections were compared between the schools. At the schools where crayfish were introduced, infections decreased significantly. Where the crayfish were not introduced, reinfections were as common as before treatment (Mkoji et al., 1999). In this case, the scientists found that under certain environmental conditions, crayfish introduction can result in the reduction of snails, which means a reduction of Schistomsoma haematobium and S. mansoni.
Some may believe that because native snails and plants are decreasing, the ecosystem is falling apart, but large fish such as largemouth bass and tilapia species have done well from feeding on introduced crayfish (Loker et al., 1991). The crayfish populations have grown enough that they are regularly harvested for food, which boosts the economy. Crayfish have now become one of the four commercially harvested species in Africa, with 80% of the catches being exported to other countries (Loker et al., 1991). In this introduction, crayfish were able to decrease the problem of parasite transmissions and increase the economy (Loker et al., 1991). The crayfish did decrease the amount of native plants, but in Africa the positives of saving lives and creating an economy, out weigh the negatives of losing water plants.
In most cases invasive crayfish are not desired because of the destruction they cause on an ecosystem. There are many methods that can be used to try and control the invasive populations including mechanical, physical, biological control and biocides (Gherardi et al., 2011). Mechanical methods of control are done using traps to catch the crayfish. This methods does not work well because hundreds of traps would need to be set for many continuous weeks, this takes more time and money. Trapping tends to not capture females because they are less active than males, and in streams crayfish can hide in rocks (Gherardi et al., 2011). Physical methods such as draining rivers will not work because crayfish can burrow in times of drought. Biocides could be placed inside the burrows, but this would take time to find every burrow in the lake (Gherardi et al., 2011). Biological control would require introducing predators of crayfish such as birds, diseases or bacteria. However, with any type of biological control there can be consequences such as diseases or bacteria that infect non-target species. Introduction of a disease or bacteria could cause more problems than the crayfish did (Gherardi et al., 2011). All these methods are available to rid an ecosystem of crayfish, but many of them are expensive, and most will not reduce the population enough for the environment to recover.
Rosenthal et al. (2006) looked into the possibly for restoration after a crayfish invasion. They found that after the crayfish density is reduced, low species diversity seed banks prevent lakes from regenerating. After attempting to regenerate many types of plants and algae, they found that when crayfish are still present, many will not grow due to the 15+ years of herbivory from crayfish. They concluded that if any ecosystem restoration would be successful, plants would need to be manually planted after the crayfish populations were reduced to almost nothing (Rosenthal et al., 2006).
When invasive crayfish populations get out of control, ecosystems can collapse. Crayfish out compete native crayfish, bluegills and pumpkinseeds for invertebrates, consume many macroinvertebrates and uproot seedlings and plants. This all causes unbalance in an ecosystem. Many methods to remove the invasive crayfish do not work because usually the densities are too great. Bio control or biocides would work, but at the risk of killing non-target species. High densities prevent regeneration of plants in the seed banks which means lake ecosystems may never return to normal. In Africa, the introduction of crayfish has decreased the number of deaths from schistosomiasis because it consumed the entire population of parasite transmitting snails in select lakes and the plants they lived on. Whether the introduction of crayfish was accidental or on purpose, in both cases it becomes invasive, consuming and destroying anything in its path spreading through water systems. With new lakes being invaded every day, it seems that regaining control of fragile freshwater ecosystems is out of reach.
- Gherardi, F., L. Aquiloni, J. Diéguez-Uribeondo & E. Tricarico. 2011. Managing invasive crayfish: Is there a hope? Aquatic Sciences 73: 185-200.
- Loker, E.S., B.V. Hofkin, G.M. Mkoji, J.H. Kihara, F.K. Mungai, B.N. Mungai & D.K. Koech. 1991. Procambarus clarkii in Kenya: Does it have a role to play in the control of schistosomiasis? Aquaculture and Schistosomiasis: Proceedings of a Network Meeting Held in Manila, Philippines: 272-282.
- Mkoji, G.M., B.V. Hofkin, A.M. Kuris, A. Stewart-Oaten, B.N. Mungai, J.H. Kihara, F.Mungai, J. Yundu, J. Mbui, J.R. Rashid, C.H. Kariuki, J.H. Ouma, D.K. Koech & E.S. Loker. 1999. Impact of the crayfish Procambarus clarki on Schistomsoma haematobium transmission in Kenya. The American Society of Tropical Medicine and Hygiene 61(5): 751-759.
- Rosenthal, S.K., S.S. Stevens & D.M. Lodge. 2006. Whole-lake effects of invasive crayfish (Orconectes spp.) and the potential for restoration. Canadian Journal of Fisheries and Aquatic Sciences 63: 1276-1285.
- Wilson, K.A., J.J. Magnuson, D.M. Lodge, A.M. Hill, T.K. Kratz, W.L. Perry & T.V. Willis. 2004. 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.