Symphurus thermophilus is a species of tonguefish notable for being the only flatfish known to be an obligate inhabitant of hydrothermal vents.

It is known to inhabit several widely dispersed locations in the western Pacific Ocean and occurs in great numbers.

[3] As they are not significantly different in appearance and feeding habits from other tonguefishes, they are thought to be relatively recent colonizers of vent ecosystems.

At the Minami-Ensei Knoll, this species was found on white metachromatic sediments in water 5-10 °C warmer than the ambient seawater.

On the Daikoku vents, the abundance of fish was found to be 80 fish/m2, which was calculated from around 150 non-overlapping images collected through remotely operated vehicles (ROVs) from 2005-2006.

[5] Like other tonguefishes, S. thermophilus has a laterally flattened body with united dorsal, caudal, and anal fins.

Some individuals have a white patch over two-thirds of the abdominal cavity, sometimes with bluish-green tints and bordered posteriorly by a black blotch.

[8] S. thermophilus spend most of their time on the sea floor, moving forwards or backwards by undulating their bodies and sometimes burying themselves completely in the substrate.

The morphology of S. thermophilus does not show any differences from other deepwater Symphurus species that would suggest adaptations to its unique habitat.

However, S. thermophilus likely possesses extensive physiological and biochemical adaptations for coping with the harsh conditions around hydrothermal vents, such as temperature and pH fluctuations, and exposure to heavy metals.

In particular, they must have high hemoglobin oxygen affinities and efficient respiratory systems to deal with the toxic hydrogen sulfide in venting fluid.

[3] Individuals of S. thermophilus often show skeletal abnormalities such as undeveloped fin rays or fused bones, likely attributable to the vent environment.

To create space in their stomachs for additional food intake, they expel any ingested sediment either via their gills or gut upon spotting their prey.

[5] Based on the fatty acid composition and stable carbon isotope ratios in tissues of S. thermophilus found on the Mariana Arc, there is evidence that the fish ingest material that originated in the photic zone.

The fish at these sites appear to be "sit and wait" predators, preying on slow-moving shrimp that wander too close.

The large numbers of S. thermophilus found on sulfur crusts where there are no obvious prey items may feed directly on filaments of chemosynthetic bacteria.

The newly hatched fry are initially sustained by a yolk sac, and have a completely developed mouth, eyes, and digestive system by 7 days of age.