Undersea volcano-dwelling fish!

According to this BBC article, scientists have witnessed the extreme lifestyle of tonguefish that like to skip across pools of molten sulphur on the ocean floor. The animals - a type of flatfish - were filmed on three expeditions to undersea volcanoes in the western Pacific. Huge numbers were seen to gather around the sulphur ponds which well up from beneath the seafloor. Researchers are trying to figure out how they survive in such a hostile environment.

There are a lot of toxic heavy metals coming out of these active volcanoes. The water is very warm and can be very acidic, with pH levels as low as two (similar to sulphuric acid).

The area of interest is the Mariana Arc, a 745 mile chain of volcanic seamounts and islands between Guam and Japan. It hosts a number of hydrothermal vents - rock systems that draw water through cracks in the seafloor, heat it to temperatures which can be well above boiling, load it with dissolved metals and other chemicals, and then eject the hot fluid back into the ocean. This type of habitat will support a range of specialized animals such as crabs, shrimp, mussels, and worms - but very few fish. And the flatfish seen on the Mariana Arc seamounts are a first.