Scientists have described the first ever species of dragonet known to live entirely in freshwater.
The new species, which is a member of the family Callionymidae, is so unusual that it has been placed in a new genus of its own.
Hiroyuki Motomura of the Kagoshima University Museum and Takahiko Mukai of Gifu University in Japan have just described the new tropical fish as Tonlesapia tsukawakii in a paper in the journal Ichthyological Exploration of Freshwaters.
The weird fish, which is related to the marine Scooter blennies of the genus Synchiropus, were purchased from a wet fish stall in Phsar Krom, Cambodia, and were subsequently collected from the north west of Lake Tonle Sap near Siem Reap in Cambodia.
Although Tonle Sap is a lake, it is connected to the sea at one end via the Tonle Sap and Mekong rivers, which flow into the South China Sea.
As a result of its marine connections, Tonle Sap contains several species of fish that have evolved from ancestors that entered the lake via influent rivers.
However, what's remarkable about Tonlesapia tsukawakii is that it was collected at the opposite end of the lake, 310 miles/500 km from the sea, in completely freshwater.
Since the study found several fish with well-developed gonads, the authors believe that the fish is a species that lives and breeds completely in freshwater, rather than migrating down the lake and rivers and back out to salt water to spawn.
Two other species of euryhaline callionymid are known, but this is the first species recorded that is believed to spend its entire life in freshwater.
The fish is believed to reach around 2-3cm in length, has a flattened body and upwardly facing eyes, and lacks a swimbladder as an adaptation for life on the bottom.
The species is named in honour of Dr Shinji Tsukawaki of Kanazawa University.
For more details on the discovery see the paper: Motomura H and T Mukai (2006) - Tonlesapia tsukawakii, a new genus and species of freshwater dragonet (Perciformes: Callionymidae) from Lake Tonle Sap, Cambodia. Icthyological Exploration of Freshwaters, Vol. 17. No. 1. pp 43-52.