An international team of scientists, led by Dr. Brooke Flammang from New Jersey Institute of Technology, has identified unique anatomical features in a species of blind fish – the waterfall climbing cave fish (Cryptotora thamicola) — that enable the fish to walk and climb waterfalls in a manner comparable to terrestrial vertebrates.
The waterfall climbing cave fish (Cryptotora thamicola): dorsal view, resting on the bottom of glass tank. Image credit: Daphne Soares.
The waterfall climbing cave fish, also known as the cave angel fish, is a rare species of fish endemic to the Tham Maelana and Tham Susa karst formation in northern Thailand.
This fish can grow to 1.1 inches (2.8 cm). It is depigmented and has no visible eyes.
It can walk up rocks in fast-flowing water and on wet surfaces in air.
“Cryptotora are found only in rapids and not in lentic pools,” Dr. Flammang and co-authors wrote in a paper published in the journal Scientific Reports this week.
“They are commonly observed climbing steep rock surfaces in fast-flowing waterfalls created by basalt or andesite intrusions.”
“While it is anecdotally known that these fish can walk, the rare and protected status of these fish has limited research into the functional morphology of their walking behavior.”
According to the team, the waterfall climbing cave fish possesses morphological features that have previously only been attributed to tetrapods.
“We show that the blind cavefish Cryptotora thamicola walks and climbs waterfalls with a salamander-like diagonal-couplets lateral sequence gait and has evolved a robust pelvic girdle that shares morphological features associated with terrestrial vertebrates,” the scientists said.
“The pelvis and vertebral column of this fish allow it to support its body weight against gravity and provide large sites for muscle attachment for walking,” Dr. Flammang added.
The discovery of this unique capability, not seen in any other living fishes, also has implications for understanding how the anatomy that all species need to walk on land evolved after the transition from finned to limbed appendages in the Devonian period (419.2 to 358.9 million years ago).
The waterfall climbing cave fish (Cryptotora thamicola), also known as the cave angel fish, is a species of troglobitic hillstream loach endemic to Thailand. 2 It reaches a length of 2.8 centimetres (1.1 in) SL. This fish is known for its fins which can grapple onto terrain and is capable of climbing. This fish is the only known member of its genus.
Fishes have adapted a number of different behaviors to move out of the water, but none have been described as being able to walk on land with a tetrapod-like gait. Here we show that the blind cavefish Cryptotora thamicola walks and climbs waterfalls with a salamander-like diagonal-couplets lateral sequence gait and has evolved a robust pelvic girdle that shares morphological features associated with terrestrial vertebrates. In all other fishes, the pelvic bones are suspended in a muscular sling or loosely attached to the pectoral girdle anteriorly. In contrast, the pelvic girdle of Cryptotora is a large, broad puboischiadic plate that is joined to the iliac process of a hypertrophied sacral rib; fusion of these bones in tetrapods creates an acetabulum. The vertebral column in the sacral area has large anterior and posterior zygapophyses, transverse processes, and broad neural spines, all of which are associated with terrestrial organisms. The diagonal-couplet lateral sequence gait was accomplished by rotation of the pectoral and pelvic girdles creating a standing wave of the axial body. These findings are significant because they represent the first example of behavioural and morphological adaptation in an extant fish that converges on the tetrapodal walking behaviour and morphology.