Study: At Least Eleven Species of Hillstream Loach Fish Have Land-Walking Abilities

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In a study published in the Journal of Morphology, an international team of scientists has pieced together the ancestral relationships that make up the family tree of hillstream loaches (Balitoridae), detailing for the first time a range of unusual pelvic adaptations across the family that have given some of its members an ability to crawl, or even walk as salamanders do, to navigate terrestrial surfaces.

Top left: photograph of the cave angel fish (Cryptotora thamicola) in dorsal view. Bottom left: μCT of skeleton in lateral view. Right: μCT scan of pelvis in transverse view. Image credit: Zach Randall, Florida Museum of Natural History / Brooke Flammang, New Jersey Institute of Technology.

Top left: photograph of the cave angel fish (Cryptotora thamicola) in dorsal view. Bottom left: μCT of skeleton in lateral view. Right: μCT scan of pelvis in transverse view. Image credit: Zach Randall, Florida Museum of Natural History / Brooke Flammang, New Jersey Institute of Technology.

Balitoridae is a family of small fish containing about 100 known species found in South, Southeast and East Asia.

The most famous hillstream loach species is the cave angel fish (Cryptotora thamicola), the only living fish species caught in the act of walking.

Also known as the waterfall climbing cave fish, this rare, blind fish occurs in Thailand and has an uncanny ability to walk on land and climb waterfalls using four limbs that move in salamander-like fashion.

“The cave angel fish’s walk is a key adaptation for surviving fast-flowing cave streams,” said co-author Dr. Zachary Randall, biologist and manager of the Florida Museum of Natural History.

“It can grip rocky streambeds and move between habitats — even up waterfalls — as water levels fluctuate in the dry season.”

“The cave angel fish’s increased mobility could help it access well-oxygenated stream regions with few or no occupants. Still, little is known about the species, including what it eats.”

Dr. Randall and colleagues analyzed the bone structure of 29 hillstream loach species, describing for the first time three categories of pelvic shapes.

Based on the shape of the bone that connects some loaches’ spines to their pelvic fins, they found that 10 other species shared the cave angel fish’s unusually hefty pelvic girdle.

“Fishes don’t usually have any connection between their spine and pelvic fin,” Dr. Randall said.

“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.”

“The modified morphology of these Balitoridae, particularly the enlarged sacral rib connecting the pelvic plate to the vertebral column, is a big part of why studying this family is so exciting,” said lead author Callie Crawford, Ph.D. candidate in the Department of Biological Sciences at New Jersey Institute of Technology.

‘These loaches have converged on a structural requirement to support terrestrial walking not seen in other fishes.”

“What we’ve discovered is three anatomical groupings that have major implications for the biomechanics of terrestrial locomotion of these loaches, and the relationships among these fishes suggest that the ability to adapt to fast-flowing rivers may be what was passed on genetically, more than the specific morphology itself.”

“Now that we have revealed a spectrum of pelvic morphologies among these fishes, we can compare the extent of skeletal support with the walking performance in a species,” said senior author Dr. Brooke Flammang, also from the Department of Biological Sciences at New Jersey Institute of Technology.

“This will allow us to measure the mechanical contribution of robust hips to terrestrial locomotion.”

The scientists used CT scanning and DNA analysis to trace the evolutionary history of Balitoridae and found that, rather than evolving from a single origin, a robust pelvic region appeared several times across the family.

“We were able to use hundreds of genes to trace how pelvic shapes in these fish have evolved over time,” Dr. Randall said.

“Now, we have a much more accurate tree that adds a framework for studying how many species can walk and the extent to which they’re able to.”

“This study brought together a team of researchers with interests and levels of expertise that varied from those of us who do fieldwork and study fishes in their natural habitats to geneticists to comparative anatomists,” said Dr. Lawrence Page, curator of fishes at the Florida Museum of Natural History.

“The result is a greatly improved understanding of the evolution of an extremely uncommon event — the ability of a fish to walk on land.”

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Callie H. Crawford et al. Skeletal and muscular pelvic morphology of hillstream loaches (Cypriniformes: Balitoridae). Journal of Morphology, published online August 13, 2020; doi: 10.1002/jmor.21247

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