Halosaurs are eel-shaped fishes found only at great ocean depths. As the family Halosauridae, halosaurs are one of two families within the order Notacanthiformes; the other being the deep-sea spiny eels, Notacanthidae. Halosaurs are thought to have a worldwide distribution, with some 17 species in three genera represented. Only a handful of specimens have been observed alive, all in chance encounters with Remotely operated underwater vehicles. The term "halosaur" refers to the type genus, '' Halosaurus'', which is a Greek compound word, ''hals'' meaning "sea" and ''sauros'' meaning "

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...

". Halosaurs have a spotty fossil record, the oldest known genus being '' Echidnocephalus'' from the Late Cretaceous strata of Westphalia, Germany, and the second-oldest known genus, '' Laytonia'', from Miocene strata of California and Oregon. The fossil specimens already bear strong resemblance to the modern genera. The halosaurs' greatly elongated bodies end in whip-like tails; their scales are large. One small dorsal fin is close to the sharply pointed, mostly scaleless head. The tail fin is greatly reduced, with the anal fin being the largest fin. Their pectoral fins are slender and greatly elongated. Their mouths are somewhat large, with the lower jaw shorter than the upper jaw. The

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled Organ (anatomy), organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their curren ...

is absent. The largest species, the 90-cm (3-ft) long abyssal halosaur (''Halosauropsis macrochir'') is also one of the most deep-living fish, recorded at depths of 3,300 m (11,000 ft). Halosaurs have developed certain adaptations to life at these extreme depths, where no light penetrates. Their

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

system enabling the detection of vibrations in the water is highly developed; the pores run the length of the fish's body. Some species are also known to hold their elongated pectorals erect and forward, possibly providing a further means of detection. Halosaurs are

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

fish, spending their time cruising over or resting on the sea floor, where temperatures may be just 2-4 °C. They propel themselves with rhythmic, lateral undulations of their bodies, not unlike sea snakes. Halosaurs are thought to prey mainly on benthic invertebrates, such as polychaete worms, echinoderms, and

crustacea Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

ns such as

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...

s, but they may also consume small fishes and

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...

. In life, most halosaurs are grey to bluish-black in colour. Like other notacanthiform fish, halosaurs are able to regenerate their tails easily if lost. This adaptation can be likened to certain terrestrial

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s, such as the glass lizard, which sacrifices its tail to evade predators.

References

External links

Video footage of a halosaur from NOAA's Ocean Explorer
{{Taxonbar, from=Q1193126 Halosauridae Deep sea fish