The peculiar tale of Xenopus laevis in Hamamatsu City, Shizuoka Prefecture
There are a few pathways for invasive populations of African clawed frogs, Xenopus laevis. The first, and oldest, has been the accidental (or deliberate) release of animals associated with laboratories using animals for pregnancy testing or as lab models. More recently, the number of invasions due to released pets has been on the rise, and we have seen examples of this in the USA, Belgium and China. Most of the invasions in Japan also appear to be related to pets, but not the population in Hamamatsu City.
In addition to being the laboratory model amphibian, the tadpoles of Xenopus are also used in a method known as FETAX (Frog Embryo Teratogenesis Assay – Xenopus) where they are used to measure toxicity of the water. I presume that it was this reason that led the City of Hamamatsu to release Xenopus tadpoles into one of their polluted Lake Sanaru in 1982 (see Arao & Kitano 2006). This means that the population found by Arao & Kitano in 2005 in an aquaculture area raising eels was over 20 years old at that time, but is now over 40 years old.
I was reliably informed that the animals were still there 5 years ago, and so I teamed up with Dr. Hisanori Okamiya from the Museum of Natural and Environmental History in Shizuoka. After spending a frutless morning searching in Makinohara (which apparently also had a population 5 years ago), Hisanori and I started looking around the eel farms.
A farmer at the first farm that we asked at pointed to a dead floating frog floating in a newly sterilised eel tank and asked us if this was the type of frog we were looking for. I fished it out with a very long net, and could immediately see that it was indeed a (very) dead Xenopus laevis.
The farmer confirmed that he gets these frogs in his eel tanks all the time, but that the population appeared to be getting smaller over the last 5-10 years. We placed out traps in a large outdoor pond, the presumed source of frogs, and came back the next day very disappointed to find that there were no animals at all inside.
I suggested that we keep on asking at eel farms and see whether anyone else was finding animals, or perhaps could point to their source. As we headed down the row of farms along the road next to the bullet train track, sightings appeared to be getting less and less. We decided to take a break and have some lunch. It had been pouring with rain all night and all morning, and Hinasori and I had been getting very wet despite waterproofs. It seemed fitting that we have eel for lunch, and so I splashed out.
After lunch, we headed back the way we came. We stopped in the neighbouring aquaculture farm (not an eel farm, but a farm for koi carp). We couldn't find anyone around. We searched around all of the buildings, but couldn't find anyone. As we were walking through a big puddle at the side of an eel pond, Hisanori saw a frog swimming in the water. Happily, I filmed what happened next, so you'd best watch the video below:
We managed to find the owners of the farm who allowed us to fish around in one of their empty fish ponds, where we found lots of tadpoles and lots more metamorphs, but no adults.
So it seems that 42 years after releasing tadpoles into Lake Sanaru, African clawed frogs are still alive and well in Hamamatsu City.
Further Reading:
Arao, K. and Kitano, T., 2006. Xenopus laevis from Hamamatsu City, Shizuoka Prefecture, Japan. 爬虫両棲類学会報, 2006(1), pp.17-19.
