About the fish, gJapanese rose bitterlingh
 The Rose Bitterling is a small fish, which is about 3cm long.We call this species, which lives only in Japan, the Japanese Rose Bitterling.  There is another similar species, which lives in China  called the Chinese Rose Bitterling.  The Japanese Bitterling can be easily distinguished from the Chinese one.  The Chinese Bitterling has a white line on its ventral fin, but Japanese one does not. 
    The Japanese Bitterling used to live throughout the western part of Japan, but now can only be found in a few places in the Osaka, Kyushu and Shikoku areas.
There are a number of reasons why the population of Rose Bitterling has been decreasing.  The Rose Bitterling lives in farm ponds (reservoirs), where it feeds mainly on plankton. 
The male Rose Bitterling becomes reddish during
the reproductive period in order to attract the
female.  This reddish colour is just like a beautiful
rose flower, that is why it came to be called
the Rose Bitterling. Eggs are deposited into
a fresh water mussel called Dobugai. 
  As a result of such unique behavior the eggs and the young fish are protected from predators.The female has a tube or ovipositor, through which eggs are   deposited into the mussel.  This ovipositor grows to the same length as the fish itself. The female puts it into the siphon where the mussel takes out water.Eggs grow in the mussel and swim out of it as young fish.
  Therefore this mussel is essential in order for the Rose Bitterling to reproduce. Baby Dobugai cannot feed by themselves.  They grow sticking to other fishes and absorbing nourishment from them.Most of baby Dobugai stick to a fish called Yoshinobori or goby. Dobugai cannot complete their reproductive cycle by themselves, either.There used to be a lot of farm ponds all over Japan where this particular eco-system existed but their numbers have been in serious decline in recent years.  It means these fishes have been losing their habitats.Some habitats supporting the Rose Bitterling have had bigger fish such as Black Bass or Blue Gill introduced from foreign countries.  These non-native species have destroyed local fish populations.  
  There is other reason why The Japanese Rose Bitterling is decreasing. The Japanese Rose Bitterling and the Chinese Rose Bitterling were originally the same fish.  They have evolved into two sub-species over about one million years in different places, but it is still possible to cross their offspring.  Such young fish are a hybrid of Japanese and Chinese Rose Bitterling. 
  However we cannot strictly name this hybrid fish a Japanese Rose Bitterling. Neither Black Bass nor Chinese Rose Bitterling are indigenous to Japan.  Man has introduced them into Japan and they have as a result spread throughout the country.  It is as a result of these reasons that the Japanese Rose Bitterling has been decreasing in numbers.  It may even be extinct in a few years.We really hope to protect the Japanese Rose Bitterling and the farm ponds that still remain around us.