Species of amphibian
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The hellbender (Cryptobranchus alleganiensis), also known as the hellbender salamander, is a species of aquatic giant salamander endemic to the eastern and central United States. It is the largest salamander in North America. A member of the family Cryptobranchidae, the hellbender is the only extant member of the genus "Cryptobranchus". Other closely related salamanders in the same family are in the genus "Andrias", which contains the Japanese and Chinese giant salamanders. The hellbender is much larger than any other salamander in its geographic range, and employs an unusual adaption for respiration through cutaneous gas exchange via capillaries found in its lateral skin folds. It fills a particular niche — both as a predator and prey — in its ecosystem, which either it or its ancestors have occupied for around 65 million years. The species is listed as Vulnerable on the IUCN Red List of Threatened Species due to the impacts of disease and widespread habitat loss and degradation throughout much of its range.
Etymology.
The origin of the name "hellbender" is unclear. The Missouri Department of Conservation says:
The name 'hellbender' probably comes from the animal's odd look. One theory claims the hellbender was named by settlers who thought "it was a creature from hell where it's bent on returning." Another rendition says the undulating skin of a hellbender reminded observers of "horrible tortures of the infernal regions." In reality, it's a harmless aquatic salamander.
In a study conducted in Indiana, informing the public about the rarity and locality of the hellbender resulted in more positive attitudes toward this species than were previously held. Other vernacular names include snot otter, lasagna lizard, devil dog, mud-devil, mud dog, water dog, grampus, Allegheny alligator, and leverian water newt.
The generic name, "Cryptobranchus", is derived from the Ancient Greek (hidden) and (gill). The subspecies name "bishopi" honors the American herpetologist Sherman C. Bishop.
Description.
"C. alleganiensis" has a flat body and head, with beady dorsal eyes and slimy skin. Like most salamanders, it has short legs with four toes on the front legs and five on its back limbs, and its tail is keeled for propulsion. Its tail is shaped like a rudder, but it is rarely used for swimming; these salamanders use pads on their toes instead to grip rocks and walk up and down streams instead of swimming. The hellbender has working lungs, but gill slits are often retained, although only immature specimens have true gills; the hellbender absorbs oxygen from the water through capillaries of its side frills. The frills run from their neck down to the base of their tail on each side of their body. The frills’ function is to increase the surface area of the hellbender and to help the hellbender breathe. Only occasionally leaving the water, the hellbender makes little use of these lungs and the juveniles lose their external gills after around 18 months or about 125 mm in length. Hellbenders use their lungs for buoyancy more than breathing. It is blotchy brown or red-brown in color, with a paler underbelly. Hellbenders can also be described as having a gray, or yellowish-brown, to even black coloration.
Both males and females grow to an adult length of from snout to vent, with a total length of , making them the fourth-largest aquatic salamander species in the world (after the South China giant salamander, the Chinese giant salamander and the Japanese giant salamander, respectively) and the largest amphibian in North America, although this length is rivaled by the reticulated siren of the southeastern United States (although the siren is much leaner in build). While males and females grow at similar rates, the females tend to live longer and therefore grow larger. An adult weighs , making them the fifth heaviest living amphibian in the world after their South China, Chinese and Japanese cousins and the goliath frog, while the largest cane toads may also weigh as much as a hellbender. Hellbenders reach sexual maturity at about five years of age, and may live 30 years in captivity.
The hellbender has a few characteristics that make it distinguishable from other native salamanders, including a gigantic, dorsoventrally flattened body with thick folds travelling down the sides, a single open gill slit on each side, and hind feet with five toes each. Easily distinguished from most other endemic salamander species simply by their size, hellbenders average up to 60 cm or about 2 ft in length; the only species requiring further distinction (due to an overlap in distribution and size range) is the common mudpuppy ("Necturus maculosus"). This demarcation can be made by noting the presence of external gills in the mudpuppy, which are lacking in the hellbender, as well as the presence of four toes on each hind foot of the mudpuppy (in contrast with the hellbender's five). Furthermore, the average size of "C. a. alleganiensis" has been reported to be 45–60 cm (with some reported as reaching up to 74 cm or 30 in), while "N. m. maculosus" has a reported average size of in length, which means that hellbender adults will still generally be notably larger than even the biggest mudpuppies.
Taxonomy.
The genus "Cryptobranchus" has historically been considered to contain only one species, "C. alleganiensis", with two subspecies, "C. a. alleganiensis" and "C. a. bishopi". A recent decline in population size of the Ozark subspecies "C. a. bishopi" has led to further research into populations of this subspecies, including genetic analysis to determine the best method for conservation.
Crowhurst et al., for instance, found that the "Ozark subspecies" denomination is insufficient for describing genetic (and therefore evolutionary) divergence within the genus "Cryptobranchus" in the Ozark region. They found three equally divergent genetic units within the genus: "C. a. alleganiensis", and two distinct eastern and western populations of "C. a. bishopi". These three groups were shown to be isolated, and are considered to most likely be "diverging on different evolutionary paths".
Distribution.
Hellbenders are present in a number of Eastern US states, from southern New York to northern Georgia, including parts of Ohio, Pennsylvania, Maryland, West Virginia, Virginia, Kentucky, Illinois, Indiana, Tennessee, North Carolina, South Carolina, Alabama, Mississippi, Arkansas, Missouri, and extending into Oklahoma and Kansas. The subspecies (or species, depending on the source) "C. a. bishopi" is confined to the Ozarks of northern Arkansas and southern Missouri, while "C. a. alleganiensis" is found in the rest of these states.
Some hellbender populations—namely a few in Missouri, Pennsylvania, and Tennessee—have historically been noted to be quite abundant, but several man-made threats have converged on the species such that it has seen a serious population decline throughout its range. In Missouri, it is estimated that the populations have declined by 77% since the 1980s. Hellbender populations were listed in 1981 as already extirpated or endangered in Illinois, Indiana, Iowa, and Maryland, decreasing in Arkansas and Kentucky, and generally threatened as a species throughout their range by various human activities and developments.
Ecology.
Hellbenders are found in clear, clean water and their presence is an indicator that the water is good quality. The hellbender salamander, considered a "habitat specialist", has adapted to fill a specific niche within a very specific environment, and is labeled as such "because its success is dependent on a constancy of dissolved oxygen, temperature and flow found in swift water areas", which in turn limits it to a narrow spectrum of stream/river choices. As a result of this specialization, hellbenders are generally found in areas with large, irregularly shaped, and intermittent rocks and swiftly moving water, while they tend to avoid wider, slow-moving waters with muddy banks and/or slab rock bottoms. This specialization likely contributed to the decline in their populations, as collectors could easily identify their specific habitats. One collector noted, at one time, "one could find a specimen under almost every suitable rock", but after years of collecting, the population had declined significantly. The same collector noted, he "never found two specimens under the same rock", corroborating the account given by other researchers that hellbenders are generally solitary; they are thought to gather only during the mating season. If rocks are lacking, hellbenders have been known to use holes in stream banks as habitat. On average, their home range is estimated to be 198 square meters as of 2005. The ideal habitat for a hellbender has a large amount of gravel, low pH, cool water temperatures, and low specific conductivity. The large amounts of gravel enable the hellbender to hide, cool water temperatures allow for more efficient cutaneous gas exchange, and low specific conductivity may indicate an undisturbed stream. Hellbender communities may be more concentrated in undisturbed areas.
Both subspecies, "C. a. alleganiensis" and "C. a. bishopi" undergo a metamorphosis after around a year and a half of life. At this point, when they are roughly long, they lose the gills present during their larval stage. Until then, they are easily confused with mudpuppies, and can be differentiated often only through toe number. After this metamorphosis, hellbenders must be able to absorb oxygen through the folds in their skin, which is largely behind the need for fast-moving, oxygenated water. If a hellbender ends up in an area of slow-moving water, not enough of it will pass over its skin in a given time, making it difficult to garner enough oxygen to support necessary respiratory functions. A below-favorable oxygen content can make life equally difficult.
Hellbenders are preyed upon by diverse predators, including various fish and reptiles (including both snakes and turtles). Particularly, largemouth bass is a predator that can consume a hellbender 1–3 years old. Cannibalism of eggs is also considered a common occurrence. One study found that in areas with increased deforestation, the likelihood of filial cannibalism increases. 
In another study by Kenison &amp; Wilson (2018), researchers found that young, captive hellbenders showed altered behavior in response to predatory fish nearby. Because of their altered behavior, it was observed and concluded that hellbenders are capable of detecting kairomones, which are chemical cues emitted by predatory species. This suggests that hellbenders can recognize kairomones as stressful stimuli and identify potential predators.
Life history and behavior.
Behavior.
Once a hellbender finds a favorable location, it generally does not stray too far from it—except occasionally for breeding and hunting—and will protect it from other hellbenders both in and out of the breeding season. While the range of two hellbenders may overlap, they are noted as rarely being present in the overlapping area when the other salamander is in the area. The species is at least somewhat nocturnal, with peak activity being reported by one source as occurring around "two hours after dark" and again at dawn (although the dawn peak was recorded in the lab and could be misleading as a result). Nocturnal activity has been found to be most prevalent in early summer, perhaps coinciding with highest water depths. Adult hellbenders can live up to 25–30 years. 
Diet.
"C. alleganiensis" feeds primarily on crayfish and small fish, but also insects, worms, molluscs, tadpoles and smaller salamanders. A study conducted in 2017 found that larval hellbenders eat mayfly and caddisfly nymphs. One report, written by a commercial collector in the 1940s, noted a trend of more crayfish predation in the summer during times of higher prey activity, whereas fish made up a larger part of the winter diet, when crayfish are less active. There seems to be a specific temperature range in which hellbenders feed, as well: between . Cannibalism—mainly on eggs—has been known to occur within hellbender populations. One researcher claimed perhaps density is maintained, and density dependence in turn created, in part by intraspecific predation. When feeding on large prey items relative to themselves, it has been found that they use suction feeding.
Reproduction.
The hellbenders' breeding season begins in late August or early- to mid-September and can continue as late as the end of November, depending on region. They exhibit no sexual dimorphism, except during the fall mating season, when males have a bulging ring around their cloacal glands. Unlike most salamanders, the hellbender performs external fertilization. Before mating, each male excavates a brood site, a saucer-shaped depression under a rock or log, with its entrance positioned out of the direct current, usually pointing downstream. The male remains in the brood site awaiting a female. When a female approaches, the male guides or drives her into his burrow and prevents her from leaving until she oviposits.
Female hellbenders lay 150–200 eggs over a two- to