Halosaurs are

eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 20 families, 164 genera, and about 1000 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...

-shaped

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

es found only at great ocean depths. As the family Halosauridae, halosaurs are one of two families within the order

Notacanthiformes The Notacanthiformes are an order of deep-sea ray-finned fishes, consisting of the families Halosauridae and Notacanthidae ( spiny eels). ''Fishes of the World'' (2006) lists it as the suborder Notacanthoidei of Albuliformes. The notacanthif ...

; the other being the deep-sea spiny eels,

Notacanthidae Notacanthidae, the deep-sea spiny eels, are a family of fishes found worldwide below , and as deep as . Their bodies are greatly elongated, though more tapered than in true eels. The caudal fin is small or nonexistent, while the anal fin is len ...

. 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 vehicle A remotely operated underwater vehicle (ROUV) or remotely operated vehicle (ROV) is a free-swimming submersible craft used to perform underwater observation, inspection and physical tasks such as valve operations, hydraulic functions and other g ...

s. The term "halosaur" refers to the type genus, ''

Halosaurus ''Halosaurus'' is a genus of fish in the family Halosauridae. This genus currently contains the following recognized species: * '' Halosaurus attenuatus'' Garman, 1899 (Galapagos halosaur) * '' Halosaurus carinicauda'' (Alcock, 1889) (Andaman ...

'', which is a

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

compound word In linguistics, a compound is a lexeme (less precisely, a word or Sign language, sign) that consists of more than one Word stem, stem. Compounding, composition or nominal composition is the process of word formation that creates compound lexemes. C ...

, ''hals'' meaning "sea" and ''sauros'' meaning "

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

". Halosaurs have a spotty fossil record, the oldest known genus being ''

Echidnocephalus ''Echidnocephalus'' (Greek for " Echidna's head") is an extinct genus of prehistoric halosaur known from the Late Cretaceous. It is the earliest known definitive member of the order Notacanthiformes. It contains one definitive species known fr ...

'' from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

(

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

) strata of

Westphalia Westphalia (; ; ) is a region of northwestern Germany and one of the three historic parts of the state of North Rhine-Westphalia. It has an area of and 7.9 million inhabitants. The territory of the region is almost identical with the h ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

, and the second-oldest known genus, ''

Laytonia ''Laytonia'' is an extinct genus of prehistoric halosaur that lived in deep water off the North American Pacific Coast from the Zemorrian Epoch (comprising either the latest Oligocene or lower Miocene) until during the Upper Miocene subepoch, whe ...

'', from

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

strata of

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

and

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

. 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 in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift ...

is absent in all known species, except for ''Aldrovandia oleosa'', which has a very small bladder.Aldrovandia oleosa, a New Species of the Halosauridae, with Observations on Several Other Species of the Family
/ref> The largest species, the 90-cm (3-ft) long

abyssal halosaur The abyssal halosaur (''Halosauropsis macrochir'') is a species of bottom-dwelling, deep-sea fish in the family Halosauridae that is found in all oceans at depths of 1100 to 3500 meters. It is the only member of its genus. Abyssal halosaur can r ...

(''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 epithelia ...

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 the Ancient Greek word (), meaning "the depths". ...

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 snake Sea snakes, or coral reef snakes, are Elapidae, elapid snakes that inhabit Marine (ocean), marine environments for most or all of their lives. They belong to two subfamilies, Hydrophiinae and Sea krait, Laticaudinae. Hydrophiinae also includes ...

s. Halosaurs are thought to prey mainly on benthic

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s, such as

polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...

worms,

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

s, and

crustacea Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

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 (living in the water column), some are benthos, benthic (living on the sedimen ...

s, but they may also consume small

es and

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

s. 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 commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s, such as the

glass lizard Anguinae is a subfamily of legless lizards in the family Anguidae, commonly called glass lizards, glass snakes or slow worms. The first two names come from the fact their tails easily break or snap off. Members of Anguinae are native to North A ...

, which sacrifices its tail to evade predators.

Genera

Halosurs are classified into the following genera: The genus '' Enchelurus'' is sometimes considered a halosaur, but has more recently been found to either be

sister A sister is a woman or a girl who shares parents or a parent with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to ref ...

to the

Anguilliformes Eels are ray-finned fish belonging to the order (biology), order Anguilliformes (), which consists of eight suborders, 20 Family (biology), families, 164 genus, genera, and about 1000 species. Eels undergo considerable development from the earl ...

and

as a whole, or a stem-member of the Anguilliformes.

References

External links

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