1. Crayfish Plague: A Fungus That Spares American Crayfish but Sickens Local Crayfish
The cause of crayfish plague is the fungus Aphanomyces astaci. It is not a bacterium, but belongs to a group called oomycetes, which are related to water molds [1]. Crayfish native to North America carry this fungus but do not die from it. Research explains that this is because the host and the pathogen have reached a balanced relationship over a long time [1]. Crayfish from other regions are different. If they become infected, it is thought to lead to death [1]. Imagine grass seeds stuck to clothes. The seeds do not harm the clothes, but they might sprout in a new garden and change it. However, the actual fungus spreads through a different mechanism than seeds. When the carrier is healthy, it is harder to notice that the disease has been brought in. How real is this situation in Japan?
2. Crayfish Plague Causing Mass Deaths of Japanese Crayfish in Japan
The Ministry of the Environment lists the transport of crayfish plague as one of the impacts caused by American crayfish. There is concern about the effect on the native Japanese crayfish [2]. This concern is becoming reality in Hokkaido. In the mid-2010s, mass deaths of Japanese crayfish occurred in Sapporo. The fungus detected was the same type carried by American crayfish. This is reported as the first report in native crayfish outside Europe [1]. Europe has a precedent. It is said to have started in northern Italy around the mid-19th century and spread across the continent, causing many native crayfish to disappear [3]. The exact starting year varies depending on the source. These carriers are not limited to crayfish. Next, we look at frogs.
3. African Clawed Frogs and the Fungus That Causes Chytridiomycosis in Frogs
It is speculated that the fungus causing chytridiomycosis in frogs originally came from African clawed frogs. This frog was transported worldwide in large numbers for experiments and pets, and it is thought that the fungus spread with them [4]. Please note that this is described as a "speculation" or "thought to be." In Japan, a research group from Azabu University confirmed this disease domestically for the first time in December 2006, from frogs imported as pets [5]. Afterward, the Ministry of the Environment conducted field surveys in various locations. According to 2007 committee materials, out of 4,360 samples collected, 693 were tested. DNA of the chytrid fungus was confirmed in 2 samples from Okinawa [4]. These were provisional results at the time and do not reflect the current situation. Globally, a 2019 international study reported that this disease is estimated to be linked to the decline of at least 501 amphibian species over the past 50 years, with 90 species believed to have gone extinct. On the other hand, 12% of the declining species show signs of recovery [6]. Invasive species are not the only cause, but this example shows the significant impact of transported pathogens. Note that chytridiomycosis is not caused solely by invasive species, and extinction numbers cannot be read simply as the responsibility of invasive species.
4. Hawaiian Birds and Disease Spread by Mosquitoes Brought to Hawaii
Hawaii originally had no blood-sucking mosquitoes. The first mosquitoes are said to have been brought by ship in 1826. This means the arrival of mosquitoes dates back about 200 years [8]. The year "1826" is the common understanding, and the detailed history is unclear. Currently, the invasive southern house mosquito carries the pathogen for avian malaria, Plasmodium relictum [7]. This is a different disease from human malaria. Here, there is a difference from the crayfish situation. Forest birds in Hawaii evolved in an environment without this disease, so they have almost no immune response to avian malaria [9]. Under specific conditions, for the apapane (a native bird), it is said that 90% die after being bitten just once by an infected mosquito [9]. Furthermore, rising temperatures are allowing mosquitoes to move up to cooler high-altitude forests, spreading to areas where birds had been taking refuge [10]. Hawaiian honeycreepers are birds whose numbers have dropped significantly. So, is there a way to reduce the mosquitoes themselves?
5. Hawaii's Plan to Reduce Mosquitoes by Releasing Male Mosquitoes (IIT)
In Hawaii, a method called the Incompatible Insect Technique (IIT) is being advanced. The mechanism involves releasing male mosquitoes with a different type of Wolbachia bacteria than the wild ones. When these males mate with wild females, the eggs do not hatch [9]. Male mosquitoes do not suck blood and do not transmit diseases [9]. Also, this is not genetic modification; it uses the difference in bacterial types. In previous sections, the small passengers brought by invasive species were the harmful side. In this method, a different small passenger is playing the role of an ally. According to reports, releases that began in late 2023 on Maui and Kauai involve flights twice a week on each island. The mosquitoes are dropped in biodegradable capsules from drones or helicopters [11]. The amount released varies by time period. This attempt is ongoing, and we are at a stage where we must wait for future results to see if it works. It is not yet known if it will be successful. So, where can we verify this story?
6. Activity: Find Out Which Water Creatures Near You Are Native or Introduced
Look up what creatures live in the ponds or rivers near your home using encyclopedias or the internet, and divide them into native species and introduced species. The Ministry of the Environment's page on "Invasive Crayfish" is available in Japanese [2]. Always go to water bodies with an adult, do not enter the water, and observe from a distance. As you investigate, you will see that the impact of invasive species is not just about eating or taking space, but also about carrying disease. The follow-up reports on the mosquito efforts in Hawaii are also something you might want to check on someday.