Japanese Giant Salamander

The Japanese giant salamander is the third largest salamander species in the world, after its two close relatives, the Chinese giant salamander, and the South China giant salamander. Being entirely aquatic, these flat-headed, mottled brown creatures are found wriggling in the freshwater streams of central and southwestern Japan.

One of the world’s six giant salamanders, these cold-blooded endemic species of Japan are classified as ‘Special Natural Monuments’ and ‘Treasures of Japan’ by the federal government. Locally known as Hanzaki, Hanzake, or Ankou, they remain camouflaged against the rocky backdrop of streams during the daytime and forage for prey at night.

Description

Size: Length: 2 m (5 ft)

Weight: 25 kg (55 lbs)

Body and Coloration:

Japanese giant salamanders have flat heads, elongated bodies, and long, broad tails like their North American cousins. They have heavily wrinkled skin, varying from gray to dark brown, depending on their age and habitat. The younger ones are slightly yellowish and more spotted, while the adults are almost reddish brown with mottled bellies. 

Their heads are broad and dorsoventrally flattened, having small (almost vestigial) eyes without eyelids due to incomplete metamorphosis. An arched pattern of vomerine teeth starts between the choanae and passes parallel to the maxillary and premaxillary series.

They can be easily distinguished from their Chinese counterparts by their larger, more numerous, and regularly arranged tubercles on their heads and throats. These salamanders also have relatively rounded snouts and slightly shorter tails.

Although they have no visible sexually dimorphic characteristics, an adult male typically has a larger and broader head than an adult female. During the breeding season, the cloacal lips are swollen in males but remain flat in females.

Distribution

They are found in the central mountainous regions and southwestern parts of Japan, particularly in Okayama, Hyogo, Shimane, Tottori, Yamaguchi, Mie, Ehime, Gifu, Ōita, and the islands of Honshu, Shikoku, and Kyushu.

Habitat

They are found in clean, cool, fast-flowing freshwater bodies like large rivers, tributaries, and small streams at elevations between 180 and 1,350 meters, where oxygen is abundant. Studies reveal that larval and juvenile forms primarily inhabit small headwater streams, while adults prefer living in streams surrounded by paddy fields for the constant supply of frogs (their primary prey) that inhabit the areas. There is an overlap in home ranges between males and females, and they tend to be more or less sedentary except during the breeding season.

Diet

These salamanders are generalist carnivores, feeding mainly on frogs, worms, crayfish, freshwater crabs, snails, bony fish, and small mammals. 

Behavior

• Unlike other species, they are wholly aquatic and do not migrate to land after losing their gills through metamorphosis. Due to the lack of gills and large body size, they prefer staying close to flowing water where oxygen is plentiful. Also, they quickly bob their heads out of the water to obtain oxygen.
• Being mostly nocturnal, they primarily rest under the rocks of the streams during the daytime. At night, they crawl on the bottom of the water bodies, making undulating waves while covering short distances. They also shake their bodies sideways to keep deoxygenated water away from their skin and recirculate fresh, oxygenated water.
• When attacked, these salamanders expel a toxic, foul-smelling secretion that quickly hardens into a gelatin-like consistency on the water’s surface. This secretion almost smells like the Japanese pepper plant and helps them ward off their enemies, though it may also have some role in communication.
• As with other Cryptobranchidae salamanders that suck asymmetrically, Giant Japanese salamanders drop one side of their jaw 10 to 40 degrees to suck their prey. They also plant their teeth on the prey’s body with significant pressure from their well-developed jaws, thus preventing them from escaping their grasp.

Lifespan

They are a long-lived species that may live up to 80 years in the wild. However, most individuals do not live this long. In captivity, they can live more than 50 years, with the oldest recorded individual that lived in the Natura Artis Magistra being 52 years at the time of death.  

Reproduction and Life Cycle

Breeding in Japanese Giant Salamanders occurs annually, beginning around late August or early September. During this period, the salamanders gather near spawning or nesting pits in the sandbed, having long burrows (100-150 cm), caverns, and hollow impressions. The large males are highly territorial and competitive, fighting other males for autonomy over these nesting sites. Once a ‘den master’ has established his monopoly over a particular nesting area, he vehemently patrols the site and guards it against intruders, including other males who try to steal their authority.

After a sexually active female enters the den, she approaches the male and spins in a swirling motion to attract her potential mate. After mating, she releases 400 to 500 eggs in the spawning pit. More than one female can lay eggs in a single spawning pit, glued together with a sticky material in a beads-on-thread pattern. The unfertilized eggs are then externally fertilized by the den master and protected from potential predators until they hatch in about 12 to 15 weeks, around late October. The fertilized eggs that show the slightest signs of fungal infection or mortality are immediately consumed by the males (hygienic filial cannibalism).

Like most amphibians, these salamanders undergo three primary developmental stages, including the egg, larva, and adult forms.

Predators

Being apex predators of their food chain, these salamanders have almost no natural enemies for predation. However, humans often hunt them for flesh, and large fish and other salamanders are found to consume their eggs.

Adaptations

• Their dark, wrinkled epidermis helps them camouflage with their surroundings and thus avoid potential predators. 
• These salamanders primarily breathe through their skin by diffusion. Due to the absence of gills, the thick epidermal fold around their neck facilitates the exchange of carbon dioxide and oxygen in water. The presence of a single lung further helps them to maintain buoyancy. 
• They have specialized neurosensory cells (neuromasts) that extend from head to toe and act as the lateral line system. The hair-like extensions of these neuromasts are sensitive to water movements and pressure changes, allowing the salamander to detect the presence of nearby objects, water currents, and even predators and prey.

Conservation Status

Although biologists are unsure of the exact number of Japanese giant salamanders left in the wild, the recent assessment (2021) by the IUCN categorized Japanese giant salamanders as ‘Vulnerable’ (VU). They are also included in Appendix I of CITES. 

Several factors affect the population of these salamanders, with the most critical being habitat loss and fragmentation, mainly due to the construction of dams and concrete streambanks. These developmental activities are predicted to increase in number, and considering the increased frequency of rainstorms in Japan, these factors might further decrease their existing population.

In response to rising food demands, chemical fertilizers and pesticides are used more frequently in agriculture, causing dissolved oxygen levels in streams to decrease and water turbidity to increase, making the streams unsuitable for salamander larvae growth.

Furthermore, hybridizing the Japanese giant salamander with the introduced Chinese giant salamander might lead to inbreeding depression and a loss of genetic heterozygosity in the native species.

Despite national conservation status, the government is yet to undertake steps to conserve these salamanders. However, a few non-profit organizations, like the Japanese Giant Salamander Society and the Hanzaki Research Institute of Japan, have volunteered for population assessments in specific locations of their geographical range. The Hiroshima City Asa Zoological Park of Japan is the first domestic organization to have successfully bred viable Japanese giant salamander offspring in captivity and released them into the wild.

Interesting Facts

1. Japanese giant salamanders, like all giant salamander species, exhibit impressive capabilities to regenerate lost body parts, including limbs, tails, skin, and bones.
2. They have low metabolic rates and can easily survive without eating for weeks.