"Wait, wait, wait. What is all this?"


"All of you? You can't fit those horses in here. Is the King even expecting you? I wasn't told there was a siege scheduled today. Look, see, all I've got in my diary is, "Organise shoes into 'comfortable' and 'who made these, they make my feet bleed just by looking at them from across the room'", and I've done that."


"They're not my shoes, obviously."


"Inbred ki... Oh dear. You've got the wrong blackwater. You need Blackwater Bay."


"You could, but we don't have any porridge."



A new species of miniature, transparent fish has been discovered in the highly acidic Rio Negro in Brazil, the largest blackwater river in the world. Both the males and the females have bright patches of iridescent blue on their stomachs and the base of the tail.

"As soon as we had the little 'bluebelly' in the net, we knew it was something quite special," says Ralf Britz, an ichthyologist at London's Natural History Museum. Together with George M. T. Mattox from the Universidade de São Paulo and his team, Britz discovered the nocturnal fish during an expedition to the small village of Santa Isabel do Rio Negro in the Brazilian state of Amazonas. They named it Cyanogaster noctivaga, which means "bluebelly night wanderer".

The Neotropical zone that encompasses southern Mexico, Central and South America, and the West Indies is home to more freshwater fishes than anywhere else on Earth, and also has the richest collection of 'miniature fishes'. In 1988, Stanley Weitzman and Richard Vari from the Smithsonian National Museum of Natural History determined that all species under 26mm long should be classified as miniature. The smallest of all the miniature fish in the world is Paedocypris progenetica, the females stretching up to just 10.3 mm and the males 9.8 mm. The smallest mature specimen ever found was only 7.9 mm long, so smaller than the width of your fingernail.

P. progenetica was discovered by Maurice Kottelat from the Raffles Museum of Biodiversity Research in Singapore and colleagues in the blackwater peat swamp forests of Southeast Asia, and described in a 2006 edition of Proceedings of the Royal Society B. Until recently, the low-nutrient, highly acidic peat swamp forests were regarded as a species poor ecosystem, with low faunal diversity and few endemics. However, almost half of all known Asian miniature fish species have been discovered in peat swamps, their tiny bodies allowing them to thrive even when the water levels sink dangerously low. Even in dry periods, the peat acts as a wall to keep in clean and cold water, so tiny fish can survive droughts in shallow swamp pools; the burrows of other animals; or even in the soil, which permanently retains enough water to sustain only the most diminutive of animals.

At no more than 17.4 mm long, the recently discovered bluebelly night wanderer, C. noctivaga, appears to be thriving in the slow moving blackwater of the Rio Negro. Eleven specimens were scooped up at around 1.5 m depth, and described in a recent edition of Ichthyological Exploration of Freshwaters. "Of course we were surprised to find this really unusual fish in an area of the Amazon that can be considered one of the best known," says Britz, suggesting that the nocturnal species lives in much deeper water during the day, but comes up to the surface at night, possibly for food.

Because it was night time, and the water of the Rio Negro is the colour of a cup of black tea, spotting the transparent bluebelly was tricky, and the team had to look for flashes of blue in their nets to know if they'd caught any. And because the fish would turn a milky pink as soon as they died, which was moments after they were plucked from their murky home, getting a photo of their incredible colouring was no simple task. "The photo of the live fish was one of the trickiest photos I have ever taken, because Cyanogaster is extremely fragile and died within seconds after having been transferred to the photo tank," says Britz, adding that he didn't manage to get a single decent image on the first night.

"The second night I put the photo tank next to the waterline on the shore and the camera and flash next to it," he said. "Then George and I seined, and once we had some of the little beauties in the net, I transferred them with a big spoon to the phototank, so they never had to be out of water. That did the trick, and they survived for a few minutes, enough to get a few decent shots, one of which is the one in the paper. So the full colouration and beauty of the fish was only revealed after we looked at the photos. And then we were almost speechless."

Once the team had photos of the live specimens, they could zoom in to get a better look at the anatomy, and at that point they were pretty certain that they'd found not only a new species, but a new genus. "This initial gut feeling was then confirmed by our detailed studies later in the labs in Sao Paulo and London," says Britz.

Apart from the large eyes and unique colouring, C. noctivaga was distinguished from its closest relatives due to the number and arrangement of their teeth and fin rays. It belongs to the Characiformes order of ray-finned bony fishes, and this group often has specialised teeth with several cusps, or projections, on the crown. Called multicuspids, these complex teeth are also usually arranged in several rows in the jaw, and looking at their arrangement can be a key identifier for new species. "Cyanogaster is quite unusual in having only four multicuspid teeth in the inner row and just a single unicuspid (a conical tooth with one cusp) tooth in the upper jaw," says Britz. "Most other characiforms, if they have multicuspid teeth, have more than four in the inner and more than just one in the outer row."

Britz says that because the Rio Negro is such a vast, undisturbed river, there is no indication that the bluebelly nightwanderer is in any kind of trouble at this point. It's hoped that further research will reveal more about how this new genus of tiny fish fits within its larger family group.

And speaking of tiny fish, here's a great comparison of a Paedocypris and its humungous cousin, the critically endangered giant barb, Catlocarpio siamensis. Plus a man who is really mad that his fish is so big.

Papers cited:

Stanley H. Weitzman and Richard P. Vari (1988). Miniaturization in South American freshwater fishes; an overview and discussion Proceedings of the Biological Society of Washington, 101 (2), 444-465

Kottelat, M., Britz, R., Hui, T., & Witte, K. (2006). Paedocypris, a new genus of Southeast Asian cyprinid fish with a remarkable sexual dimorphism, comprises the world's smallest vertebrate Proceedings of the Royal Society B: Biological Sciences, 273 (1589), 895-899 DOI: 10.1098/rspb.2005.3419

Mayden, R., & Chen, W. (2010). The world’s smallest vertebrate species of the genus Paedocypris: A new family of freshwater fishes and the sister group to the world’s most diverse clade of freshwater fishes (Teleostei: Cypriniformes) Molecular Phylogenetics and Evolution, 57 (1), 152-175 DOI: 10.1016/j.ympev.2010.04.008

George M. T. Mattox, Ralf Britz, Mônica Toledo-Piza, & and Manoela M. F. Marinho (2013). Cyanogaster noctivaga, a remarkable new genus and species of miniature fish from the Rio Negro, Amazon basin (Ostariophysi: Characidae) Ichthyol. Explor. Freshwaters, 23 (4), 297-318


My book, Zombie birds, astronaut fish and other weird animals, has been released in the US through Adams Media this week, and is available from Amazon.