A flurry of social media attention recently warned that the arapaima, the largest fish in the Amazon, is facing possible extinction. A study revealed that these river giants are already extinct in some areas and under great pressure in others from overfishing.

The conservation status report came with good and bad news. Happily, in areas where fishing is regulated, populations are doing very well, suggesting that similar measures elsewhere could restore the arapaima before it is too late. Unfortunately, barely more than a quarter of the communities surveyed have such rules in place.

So, just how big is the Amazon basin's largest fish? Really big. Reaching 3m (10ft) in length and weighing as much as 180kg (28st 8lb) or more, these are truly impressive creatures. Until recently, all the arapaima were believed to belong to a single, widespread species, Arapaima gigas. That "species" was the only CITES-listed freshwater fish in South America and because it was harvested for markets along hundreds of miles of river there were serious concerns for its future.

The single-species theory had stood unquestioned for 145 years. It goes back to a publication in 1868 by Gunther, who summarily sank three previously named species into A gigas. The three had been described by the great French anatomist Cuvier with co-author Spix in 1829 and Valenciennes in 1847. Cuvier, incidentally, had named A gigas too, in 1822. It was Dr Donald Stewart of the College of Environmental Science and Forestry, Syracuse, New York, who took a closer look at Gunther's claim. He compiled extensive data on a representative population from the Mamiraua reserve, Brazil, to establish the first serious baseline for comparisons, then studied museum specimens and what little has been written about species-level taxonomy in Arapaima. He concluded that Cuvier and company had been right all along, resurrecting the three species suppressed by Gunther and recognising a new and fifth species in the genus.

Stewart named the new species Arapaima leptosoma from the Greek words leptos, slender, and soma, body, due to the comparatively long and narrow form of its body. The first new species in the genus since 1847, it must be inferred to be threatened along with the others and for the same reason, overfishing. Teasing apart the arapaima's five species is a first and necessary step to assure their long-term survival. Now that the five fishes can be distinguished, it will be possible to gather data on each of their geographic distributions and status in the wild and, hopefully, more carefully monitor and manage them.

It is not easy to return with or store such gigantic fish in collections. This, and the fact that no one suspected more than one species existed, accounts for the paucity of museum specimens. Although the Purus river is being aggressively fished to meet the demand of local markets, few of these find their way to the halls of science and the number of specimens from the area where the new species was discovered can be counted on one hand and none is from areas farther upstream. This advance in taxonomic knowledge combines with the encouraging effectiveness of limiting numbers of mature fish allowed to be caught to offer hope for these amazing river giants.