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. These flatfish are distinguished by the prominent dark crossbands on their brown eyed side, black abdominal cavity membrane lining known as the peritoneum, and white blind side. 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.
Taxonomy 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. A cryptic species of Symphurus in the southern Tonga and Kermadec Arcs was previously described as S. thermophilus, however, research has suggested that it has a distinct genetic makeup compared to that of the S. thermophilus in the northern Mariana Arc, despite exhibiting comparable morphology and behavior.
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 might also occur at yet-unexplored vent sites in-between these locations.Tonguefish were associated with three different substrata across the Daikoku, Kasuga-2, and Nikko seamounts. On the Daikoku and Kasuga-2 vents, the tonguefish were found on volcanoclastic and sulfuric sediments. In the Nikko vents, they were found among dense colonies of tubeworms. The fish were also associated with sheeted sulfur flows and conglomerates that were found near high temperature vents and sulfur. On outcrops of andesite and basalt that were untouched of venting, there were no tonguefish found. Of surfaces with dense microbial mats, tonguefish populations were noticeably reduced.
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. The overall distribution of S. thermophilus are relatively dispersed. The highest abundances were found in pits of granular sulfur or volcanoclastic sediments. 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.
Anatomy and morphology
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