Cryptobranchids are giant salamanders which inhabit cool, flowing freshwater in Japan, China and North America. They are an ancient group and are regarded as living fossils that have remained little changed for over 160 million years (Gao & Shubin, 2003). There are just three known species within two genera, all of which are classified as Threatened or Near Threatened. The Chinese giant salamander (Andrias davidianus) is the largest in the world growing to 1.7 m in length and weighing up to 60 kg (Murphy et al., 2000) (Figure 1). This species is Critically Endangered due to threats from over-harvesting and habitat destruction. The Japanese giant salamander (Andrias japonicus) is a closely related species and is slightly smaller than A. davidianus. Although classified at Near Threatened by the IUCN, its habitat is highly fragmented and populations are declining in many parts of its range (Kaneko & Matsui, 2004). The North American hellbender (Cryptobranchus alleganiensis) is the largest salamander in North America, reaching up to 74 cm in length (AmphibiaWeb, 2019). All giant salamanders live secretive lives but have similar ecology and biology, which includes extreme longevity (up to 60 years), parental care by males, and large larvae. However, there are differences in their habitats, diets, reproductive behaviour, egg size and mating strategies (Lou et al., 2018). Although various studies have been carried out on these species, their behaviour and ecology is still poorly understood, and often published research is in national languages which is not readily accessible. Also, much evidence of their behaviour is through occasional observations and not systematic studies (Lou et al., 2018). In this article we highlight some recent research findings of these unusual and important species.
All three species of giant salamander exhibit paternal care with the male guarding eggs, spending considerable time tail fanning and agitating eggs (Okada et al., 2015) (Figure 2). The first systematic study providing data on paternal from a North American hellbender nest was carried out by Settle et al. (2018). The researchers used unique underwater infra-red video cameras to the record behaviour of a male hellbender in his aquatic nest for a period of five weeks during the breeding season. In this study the male spent 98.4% of the 41.7 hours of observable footage guarding the eggs, rarely leaving them for the whole five week period (Settle et al., 2018). Interestingly, the male was observed to consume its own eggs (filial cannibalism) on seven occasions. The reasons for this are unknown but may either be due to the nutritional value of eggs to the male, or removing eggs that had been contaminated by a fungal infection (Settle et al., 2018). As in other giant salamander species, the male hellbender spent considerable time (60%) fanning the eggs. This is thought to increase oxygen levels in the water and prevent embryo mortality. Overall, this study highlights that male guarding of eggs in hellbenders increases survival of the eggs through decreased predation, increased oxygen levels and potentially removing diseased or dead eggs from the clutch. However, this high level of male guarding comes with several costs including low rate of feeding by the male and increased energy expenditure. Population declines of this North American hellbender could alter the relative costs and benefits of male parental care and impact on survival of populations (Settle et al., 2018).
Recently, a new species of leech (Placobdella appalachiensis) and an unknown species of parasitic trypanosome (Trypanosoma sp.), have been found in a population of hellbenders from eastern North America (Hopkins et al., 2014). Research by DuRant et al. (2015) showed that hellbenders infected by leeches had altered physiology compared to unaffected hellbenders. Leeches dampened the immune response of the salamanders, increasing levels of trypanosome infection. In addition, infection with leeches altered the circadian rhythms of the salamanders which impacted on metabolic processes, foraging behaviour and social interactions, particularly during the time of the year when hellbender activity increases as a result of the onset of breeding (DuRant et al., 2015). It is thought that leech saliva directly impacts on the salamander’s physiology since it is known to contain many different hormones and neurosignalling molecules that are believed to directly affect the salamander’s responses to infection. Understanding the dynamics between the leech species to hellbenders is important for conservation of hellbenders which are declining throughout its range. The findings from this study increase our understanding of the physiological effects of leeches on hellbenders and future research needs to examine the possible impacts on reproduction which may influence conservation efforts.
Understanding the longevity of declining species is crucial and provides important information for conservation, such as total lifespan, age of sexual maturity, and age structure of populations in the field (Yamasaki et al., 2017). However, research on longevity in the Japanese giant salamander is lacking since it is not possible to determine age from weight or length. The only reliable method to determine age in salamanders is through skeletochronology. This method is based on the presence of growth layers recorded in cross sections of long bones (Halliday & Verrell, 1988). By taking cross sections of toes, it is possible to count the layers and thus calculate the age of an individual (Figure 3). Yamasaki et al. (2017) are the first researchers to successfully use skeletochronology to estimate age in the Japanese giant salamander. In this study hellbenders lived up to 11 years. Anecdotal observations suggest that this species may live up to 60 years, but this is based on captive specimens. Long-lived species have slow growth rates and long generation times which means they are slow to adapt to environmental change. Therefore further studies on this species will help understand maximum life span and will have implications for conservation of this declining species.
References
AmphibiaWeb (2019) Cryptobranchus alleganiensis: Hellbender <http://amphibiaweb.org/species/3861> University of California, Berkeley, CA, USA. Accessed 21 June, 2019.
DuRant, S., Hopkins, W. A., Davis, A. K. & Romero, M. (2015) Evidence of ectoparasite-induced endocrine disruption in an imperilled giant salamander, the eastern hellbender (Cryptobranchus alleganiensis). The Journal of Experimental Biology, 218: 2297-2304. doi:10.1242/jeb.118703.
Gao, K. & Shubin, N. (2003) Earliest known crown-group salamanders. Nature, 422: 424-428.
Halliday, T. R. & Verrell, P. A. (1988) Body size and age in amphibians and reptiles. Journal of Herpetology, 22 (3): 253-265.
Hopkins, W. A., Moser, W. E., Garst, D. W., Richardson, D. J., Hammond, C. I. & Lazo-Wasem, E. A. (2014) Morphological and molecular characterization of a new species of leech (Glossiphoniidae, Hirudinida): implications for the health of its imperilled amphibian host (Cryptobranchus alleganiensis). ZooKeys, 378: 83-101.
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Luo, Q., Tong, F., Song, Y., Wang, H., Du, M. & Ji, H. (2018) Observation of the breeding behavior of the Chinese giant salamander (Andrias davidianus) using a digital monitoring system. Animals, 8: 161. doi:10.3390/ani8100161.
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Okada, S., Fukuda, Y., & Takahashi, M. K. (2015) Paternal care behaviors of Japanese giant salamander Andrias japonicus in natural populations. Journal of Ethology, 33: 1–7.
Settle, R. A., Briggler, J. T. & Mathis, A. (2018) A quantitative field study of paternal care in Ozark hellbenders, North America’s giant salamanders. Journal of Ethology, 36: 235–242 https://doi.org/10.1007/s10164-018-0553-0.
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