The cave angel fish was first observed walking on rocks in a Thailand cave four years ago and now, scientists have identified nearly a dozen species of fish with the same ability.
The team analyzed pelvic structures of about 30 hillstream loach species and found 11 match that of the rare, blind Cryptotora thamicola identified in 2016.
Using CT scan, the results described three categories of pelvic formation for the first time and allowed researches to determine the structure that provides walking capabilities.
These species have more robust sacral rib connections between an area of cartilage that supports the fins and the spinal column, enabling them to walk out of water with a tetrapod‐like lateral‐sequence.
The discovery was made by researchers from the Florida Museum of Natural History, the New Jersey Institute of Technology, Louisiana State University and Maejo University in Thailand.
Biologist Zachary Randall, manager of the Florida Museum's imaging lab and one of the study's co-authors, said: ‘Fishes don't usually have any connection between their spine and pelvic fin.’
‘But before, the idea was that the cave angel fish was totally unique. What's really cool about this paper is that it shows with high detail that robust pelvic girdles are more common than we thought in the hillstream loach family.’
Cryptotora thamicola, also known as the cave angel fish, is a blind cavefish first discovered lurking in the caves of northern Thailand in 2016.
The fish was seen climbing up rocks in a waterfall, with similar movement of a salamander.
While Cryptotora thamicola is not the only fish known to leave the water, researchers say it's unique in its skeletal structure, giving it a salamander-like gait.
Other types of climbing fish, including the Hawaiian waterfall climbing gobies, use undulation or suction to move out of the water.
Cryptotora thamicola, however, walks with a gait known as a 'diagonal-couplets lateral sequence,' which is only attributed to tetrapods – all animals that descended from shared four-footed ancestor.
The team notes that there are more than 100 species of hillstream loach living throughout Southeast Asia and the cave angel fish has been the only one observed with walking abilities.