Expert disputes smallest fish claim

Last week's announcement of a new species of fish that was the world's smallest has been challenged by an ichthyologist who works on parasitic anglerfishes.

Ted Pietsch of the University of Washington says that the tiny 7.9mm long cyprinid described last week is much larger than a species he's found, in which he's found sexually mature males of just 6.2 and 6.5mm in length.

Pietsch says that Photocorynus spiniceps, a deep sea angler fish found in the Gulf of Panama, should be considered the world's smallest, even though the females growing to a substantial 4.5-5 cm long.

Pietsch described his tiny species back in September, but Britz and Kottelat who found the tiny Paedocypris progenetica cyprinid, hadn't seen it when they submitted their paper which came out last week.

EctoparasiteUnlike Paedocypris progenetica, which is a free living member of the carp family found in acidic pools in Sumatra, Photocorynus spiniceps is what's known as an ectoparasite and a sexual parasite.

Males permanently latch on to the middle of the back of females, which are almost ten times their size, by biting and then fusing to the fish to become a permanent fixture.

Depending on the species up to eight parasitic males will attach to a female to become sexual parasites. The female gathers food and the parasitic males drink blood and produce sperm using testes which fill up most of their body, effectively producing a sort of hermaphrodite female.

Unlike other anglerfishes, which have a lure on the top of their head called an illium, parasitic anglerfishes instead have massive nostrils and a heightened sense of smell to track down females to parasitise.

Who's the smallest?Dr Ralf Britz, one of the authors of the Paedocypris paper who works in the Fish Division in the Department of Lower Vertebrates told the Natural History Museum: "The whole exchange is quite amusing and in the end, what is really important is that we appreciate that there are still many areas on this globe that are unexplored, containing vast numbers of new species, among them very unusual ones that need to be discovered and their biology and anatomy explored.

"For some habitats we are facing a race against time, because they disappear faster than we can survey them."