89=. Aquatic False Brook Salamander (Pseudoeurycea aquatica)
The aquatic false brook salamander is the only known aquatic lungless salamander in Mesoamerica (Central America between Northern Mexico and Panama), all the others being terrestrial (or ground dwelling). Only three individuals of this species have ever been collected and studied, so very little is known about the ecology and behaviour of the aquatic false brook salamander. It is thought to be closely related to another top 100 EDGE plethodontid: Smith’s false brook salamander.
Order: Caudata
Family: Plethodontidae
The Plethodontidae is by far the largest family of salamanders, comprising nearly 70% of all living species. In total there are 378 known plethodontids divided between four subfamilies and 24 genera. The plethodontids are united by the fact that they do not possess lungs and breathe entirely through their skin and mouth lining. They are often referred to as the lungless salamanders, although they are thought to have evolved from highly aquatic, lunged ancestors in the streams of the Appalachian Mountains in eastern North America. The earliest plethodontids were hypothesised to have lost their lungs because individuals with reduced, or absent lungs were less likely to float away in the swift mountain streams where they lived. The vast majority of other salamanders possess lungs, making the lungless salamanders an unusual and fascinating group of animals.
They are thought to have diverged from all other amphibian species 145 million years ago at the boundary between the Jurassic and Cretaceous periods. They are as different from all other amphibian lineages as wombats are from whales, evolving at a time when dinosaurs were still dominant. Overall, plethodontids are the most evolutionary advanced salamanders, so it may at first appear odd that they should have lost their lungs, which are one of the most basic features of all vertebrates living on land. Lacking lungs and being dependent on their skin for respiration places a size restriction on these salamanders because large animals have a relatively small surface area of skin compared to their body’s volume, and have greater difficulty in supplying their body tissues with oxygen compared to smaller animals (which have a large surface area to volume ratio). The long, slender form of the lungless salamanders maximises the surface area available for gas exchange, and some species grow to lengths of over 300 mm.
Plethodontid salamanders occupy a great diversity of habitats, ranging from strictly aquatic to strictly terrestrial, exploring niches as diverse as caves, trees, mountain streams, and they are also found burrowing through the earth. Dependence on their skin for breathing places limitations on where and how lungless salamanders can live. Their skin must be kept moist at all times in order for oxygen to be taken up by the blood in capillaries beneath the skin. This means plethodontids are either confined to humid areas, or must find damp hiding places and only emerge in wet weather, typically at night. The life of a lungless salamander in less humid areas, like Europe and temperate North America, therefore comprises brief periods of activity interspersed with inactive phases that are often very long. They are able to survive the periods of inactivity because they have a very low metabolic rate and low energy requirements. Able to store much of what they eat as fat, they do not need to feed very often.
A further adaptation, present among many species of the lungless salamander subfamilies named “Plethodontinae” (from East and West North America) and “Bolitoglossinae” (from tropical Central and South America), is “direct development”. This is a method of amphibian development where the larval stage (e.g. the tadpole stage in a frog’s life history) has been eliminated. Early development takes place in eggs, which may be laid in moist places away from water, and the young hatch out as miniature adults. The well known amphibian metamorphosis, most commonly appreciated in the transition from tadpole to adult frog, does not occur outside of the egg. This means that certain lungless salamanders in these two subfamilies may live away from water bodies, allowing them to expand their ranges to new areas.
The history and characteristics of the lungless salamanders go some way to explaining their range. They are mostly found in the New World, where they are widely distributed in eastern and western North America, as well as Central and South America. However, continental drift over millions of years has also brought them to the Old World, where they are found in parts of Europe (e.g. Sardinia) and Korea. The existence of the Korean crevice salamander was unknown until 2005, when its discovery was a shock to science, indicating a long history of lungless salamanders in Asia. This is the only known species in Asia, suggesting that the rate of species generation in this part of the world is very low, especially compared to the huge radiation of lungless salamander species in the New World.
Comprising 50 known species, the genus Pseudoeurycea (commonly known as the “false brook salamanders”) is one of the largest genera in the Plethodontidae family, second only to the Bolitoglossa genus (the “mushroomtongue salamanders” – 93 known species). The false brook salamanders are present within the subfamily “Bolitoglossinae” (from Central and South America) and its members are very wide ranging in size, with some of the smallest and largest lungless salamanders included (total lengths from about 65 mm to 325 mm).
The false brook salamanders have recently been reorganised and expanded to absorb a number of other genera that are now considered to fit within the Pseudoeurycea genus. The taxonomy of these salamanders may not be finally resolved just yet, but it is known that the whole group diverged from all other salamanders in the Late Eocene period, at least 34 million years ago. This is around the same time that humans and monkeys shared a common ancestor.
The aquatic false brook salamander is the only known aquatic lungless salamander in Mesoamerica (Central America between Northern Mexico and Panama), all the other being terrestrial (or ground dwelling). It is thought to be closely related to another top 100 EDGE plethodontid: Smith’s false brook salamander.
Description
The aquatic false brook salamander, like all lungless salamanders in the Bolitoglossinae subfamily, possesses a slender body, long tail and prominent eyes. A distinctive feature of the plethodontid family is a narrow groove (the nasolabial groove) running from each nostril to the upper lip: its function is to carry waterborne odours from the ground into the nasal cavity. Another curious trait of the lungless salamanders are mental (from the Latin “mentum”, meaning chin) glands. These are modified mucus glands that release pheromones (chemicals produced by an animal to influence the behaviour of other members if its species, often with regard to breeding receptivity). Mental glands are sometimes visible in males as raised bumps below their lower lip.
Lungless salamanders are very small to medium in size, usually measuring between 25 to 250 mm from the tip of the nose to the end of the tail, which salamanders retain throughout their life. They are unusual among the salamanders in that some species can detach from their tail as a predator-defence mechanism (also known as tail or caudal autotomy). It is therefore not unusual to see individuls missing part or all of their tail, which they may regererate later. Lungless salamanders may have bold patterns on their skin as adults, or they may have a colouration more similar to their environment to aid camouflage. They have well-developed “costal” grooves (successive vertical grooves in the skin along the sides of the body), generally numbering between 12-15. Their limbs are slender and often have largely or completely webbed digits. Species, like the aquatic false brook salamander, in the genus Pseudoeurycea (the “false brook salamanders”) are very similar in form to those in the genera Chiropterotriton (the “splayfoot salamanders”) and Bolitoglossa (the “mushroom-tongue salamanders”).
The aquatic false brook salamander is moderately large in size, with a total length of around 152 mm, the tail being generally longer than the body at 82 mm. Only three specimens have ever been collected and measured, although it has been noted that several individuals that escaped capture by researchers were considerably larger than the ones collected. The tail of this species is long, slender and marked by a constriction at its base. The body is robust and the jaw is very muscular. This species exhibits numerous adaptations to an aquatic life style. The head is broad and triangular with a bluntly pointed snout, small eyes and nostrils that are closed by flap-like structures to keep out water. The “nasolabial” groove is clearly visible, extending down to the upper lip from beneath the nostril flap. The limbs are very long and there are four unwebbed slender and well-formed digits, plus a prominent but short fifth toe.
Dorsal (or upper) surface colouring is reddish-brown with distinct dark brown mottling. The top of the head has a slightly more reddish hue than the back and the upper eyelids are lighter in colour and only have a few brown spots. The ventral surface (or underside) is much paler, possibly grey in colour with yellowish to cream coloured spots. Only three individuals of this species have ever been collected, but it appears that some slight colour intensity variations exist in this species and females seem to be more darkly coloured than males.
Ecology
Only three individuals of this species have ever been collected and studied, so very little is known about the ecology and behaviour of the aquatic false brook salamander. This species appears to be strongly nocturnal (or night active) since all individuals were seen between the times of 22:00 and 03:00. The temperature of the water in which this species has been collected was 10�C and all individuals were seen in plunge pools of waterfalls along this stream. The water in the plunge pools is not only deeper than in other areas of the stream, but the constant falling water oxygenates these pools, possibly enabling this lungless salamander to take up more oxygen from the water through the gas-exchange surfaces of its body.
This species has been seen resting on sand or gravel at the bottom of plunge pools. Individuals moved swiftly towards hiding places beneath large boulders at the back of the waterfall when disturbed. In the water, the aquatic false brook salamander is quick and agile. Out of water, individuals appeared lethargic and slow-moving in comparison to terrestrial false brook salamanders collected in the area. Like all other species within its subfamily (Bolitoglossinae), the aquatic false brook salamander presumably breeds by direct development, meaning that the eggs hatch into miniature adults that bypass the free-living larval form and therefore the necessity for metamorphosis in order to achieve the adult form.
Like other false brook salamanders, the aquatic false brook is probably active all year round and mating is thought to occur throughout the year. Some species in this genus are known to display courtship rituals. The pheromone releasing mental gland in males plays an important role in mating to influence the receptivity of females. During amplexus (the mating embrace), the male clasps the female with both his arms and legs, and rubs pheromones across the female’s snout. Fertilisation is internal and eggs will generally be laid at the beginning of the dry season in November. Female false brook salamanders have been found to guard the eggs throughout their development in many species, often in special hides, until hatching occurs at the beginning of the rainy season. Coordination of egg laying and hatching with dry and wet seasons may not be relevant to the aquatic false brook salamander because it lives in streams and has constant access to water. Very little is known about this species but it is presumed that direct development of the young occurs within the eggs and they hatch as miniature adults. This species is the only known aquatic lungless salamander from Mesoamerica (Northern Mexico down to Panama), it is know to breed around water unlike the rest of its subfamily (the 248 other species of the “Bolitoglossinae”).
False brook salamanders are well adapted for moving around their habitat and have prehensile tails, meaning they can use their flexible tail as a fifth limb to grip with and hang from. This may prove useful in an aquatic habitat, and aquatic false brook salamanders are known to be good swimmers. False brook salamanders may at first appear very vulnerable to predators but a number of defense mechanisms have been found among the members of this genus. These include: noxious skin secretions; poisonous gland on the back of the head (parotoid glands); warning colours on the skin of the back (also termed aposematic colouration) or camouflage colouration; and caudal autotomy (tail detachment). Behaviourally, false brook salamanders have been found to orchestrate many defensive methods, including immobile posture, coiling and flipping of the body, tail undulation displays, and biting. When even slightly threatened, the aquatic false brook salamander has been seen to angle its head downward to make its large jaw appear even more muscular – a possible defense mechanism peculiar to this species.
Habitat
The aquatic false brook salamander is the only known aquatic plethodontid from Central America (or “Mesoamerica”), all the other species being from terrestrial or arboreal (tree associated), ground-level, subterranean (underground) or cave habitats. This species is only known from one location in a stream tributary of the Río La Lana flowing through cloud forest at an altitude of over 2,100 metres above sea level. This stream was less than 1.5 m deep in most places, but some of the plunge pools at the base of waterfalls were about five metres wide and one metre deep. All individuals of this species that have been seen in the wild were found in these plunge pools that were at a temperature of 10�C.
Distribution
The aquatic false brook salamander is known only from Totontepec on the northern mountain slopes of central-eastern Oaxaca, Mexico in streams at an altitude of 2,103 metres above sea level.
Population Estimate
There is no current population data for the aquatic false brook salamander as it has not been seen since 1978. Following the complete destruction of its only known habitat by the mid 1980s, it is thought that this species may have gone extinct over two decades ago.
Population Trend
Thought to be in decline by the IUCN Red List of Threatened Species, if not already extinct.
Status
Listed as Critically Endangered on the IUCN Red List of Threatened Species because its extent of occurrence is less than 100 km sq. and its area of occupancy is less than 10 km sq., all individuals are in a single location, and there is continuing decline in the extent and quality of its habitat around Totontepec, Mexico.
Threats
This species has not been seen since 1978, when it was originally collected from a stream running through pristine cloud forest. The following year, scientists returned to the locality of the aquatic false brook salamander and found that the forest had been cleared to the edge of the stream on one side and the water was full of burned logs and ashes. In the three nights of careful searching that ensued, no aquatic false brook salamanders were seen. When researchers returned to the area again in 1983 and 1984, they discovered that the entire area had been cleared of forest and that the stream had become badly silted. The habitat is now so badly degraded that is seems impossible that this species would be able to survive in the area.
The possible extinction of the aquatic false brook salamander therefore dates back over two decades ago. The clearing of forest and the resulting degradation of streams due to burned debris has led to a disappearance of this species in the Río La Lana. The cloud forest habitat of this species has been completely destroyed at its only known locality due to wood extraction, farming, and human settlement. The resulting degradation of streams due to burned debris has also likely contributed to the species' disappearance.
Conservation Underway
There are currently no conservation measures underway for the aquatic false brook salamander and it is not known from any protected areas.
Conservation Proposed
Further survey work is urgently needed to establish whether this species might survive in the wild outside of its only known locality, which has now been subjected to habitat destruction so severe that no aquatic false brook salamanders have been seen since 1978.
If the species is found to survive elsewhere, it is essential that its habitat is protected to prevent further population extinctions of this species. In addition to conserving any remaining wild habitat for this species, the IUCN Technical Guidelines for the Management of Ex situ Populations, part of the IUCN Red List of Threatened Species, recommend that all Critically Endangered species should have an ex situ population managed to guard against the extinction of the species. An ex situ population is ideally a breeding colony of a species maintained outside its natural habitat, giving rise to individuals from that species that are sheltered from problems associated with their situation in the wild. This can be located in the species’ range or in a foreign country that has the facilities to support a captive breeding programme for that species. Since the aquatic false brook salamander is currently categorised as Critically Endangered, the possibility of a captive breeding programme for this species should be investigated if future surveys discover any remaining populations.
Links
AmphibiaWeb
Global Amphibian Assessment
Tree of Life Web Project
References
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