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 from several widely dispersed locations in the western Pacific Ocean and occurs in great numbers. They are tolerant of harsh conditions and are often found in close association with elemental sulfur, including molten sulfur pools that exceed 180 °C in temperature. 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.
These fish were first observed in nature in 1988, and were provisionally assigned to the species Symphurus orientalis before being recognized as a new species. The species name thermophilus is from the Greek thermos meaning "heat" and philos meaning "lover", referring to its association with hydrothermal vents.
Distribution and habitat
This species has a wide, disjunct distribution in the western Pacific, from the Kaikata Seamount near the Bonin Islands off southeastern Japan, to the Rumble 3 and Macauley Submarine Volcanos on the Kermadec Ridge off northern New Zealand, including the Nikko Seamount near Minami-Iohjima Island, the Minami-Ensei Knoll in the Mid-Okinawa Trough, the Kasuga-2 and Daikoku Seamounts in the Marianas Islands arc, and the Volcano-1 and Volcano-2 Seamounts in the Tonga arc. S. thermophilus likely also occurs at yet-unexplored vent sites between these locations.
S. thermophilus occurs only within relatively shallow active hydrothermal vent sites at a depth of 239–733 m, with most found between 300–400 m. Both adults and juveniles are found in the same habitats. Unusually for a vent fish, S. thermophilus favors environments that are rich in sulfur; they have been observed oriented vertically on solid sulfur walls, resting on beds of newly congealed sulfur adjacent to a rivulet of molten sulfur, and even on a thin crust of consolidated sulfur pebbles overlaying a molten sulfur bed with a temperature of 187 °C (though the crust is considerably cooler).
While many flatfish species prefer a fine substrate to burrow in, S. thermophilus frequents coarse substrates and is sometimes found over solid surfaces. At the Kaikata Seamount, S. thermophilus was observed on coarse sand bottoms where water of 19-22 °C was percolating through the sediment. At the Minami-Ensei Knoll, this species was found on white metachromatic sediments in water 5-10 °C warmer than the ambient seawater. At the Kasuga-2 Seamount, it occurred on a variety of dark- and light-colored gravel sediments and on bacterial mats.
Where it occurs, S. thermophilus is often extremely abundant; it is the most numerous obligate vent vertebrate known to date. At the Kaikata Seamount, they are found in such numbers that the fish overlap one another on the bottom. Point densities at the Daikoku Seamount have been recorded as high as 392 individuals per square meter; these densities are an order of magnitude higher than flatfish densities reported anywhere else.
Description
Like other tonguefishes, S. thermophilus has a laterally flattened body with united dorsal, caudal, and anal fins. Both eyes, which are fairly large and rounded in this species, are located on the left side of the head in adults. The pectoral fins and lateral line are absent, and there is only a single, right-side pelvic fin. The head is moderately long, with a blunt snout and long, broadly arched jaws. There are 4-5 rows of teeth on the upper and lower jaws of the blind side, and 2-3 and 1 rows on upper and lower jaws respectively of the eyed side. The teeth are sharp and recurved, and better developed on the blind-side jaws.
The body is notably deep compared to other Symphurus species. The origin of the dorsal fin is located above the eyes and contains 88-94 rays. The dorsal fin pterygiophores and neural spines have a 1-2-2-2-2 interdigitation pattern. The pelvic fin is moderately long, contains 4 rays, and is connected to the body by a delicate membrane. The anal fin contains 74-80 fin rays. The caudal fin is relatively long and contains 14 rays. The scales are small and strongly ctenoid in shape, numbering 47-56 rows transversely and 100-112 rows longitudinally.