A few days ago I visited my friends at the Centre for Fortean Zoology (for non-Tet Zoo-related reasons), and I particularly enjoyed looking at their amphiumas. Purely because I want to share the photos I took – well, and because amphiumas are weird, little known and really, really neat – I thought I’d say a little bit about them.
As usual, that ‘little bit’ quickly grew into a full-length article… nuts.
There really isn’t much about amphiumas in the literature at all. They have been covered on Tet Zoo before: there’s a brief introduction in one of the caudate articles (The wonder that is the internally fertilizing salamander clade: caudates part II), but I really should devote a full-length article to them some time. Oh yeah, like this one [captive Three-toed amphiuma shown below].
Amphiumas – the members of the caudate clade Amphiumidae – are long-bodied, eel-like, aquatic, predatory salamanders with strongly reduced limbs. They have internal gills even as adults. Long, narrow, highly septate lungs (they can be 30 cm long in an amphiuma 100 cm long) are also present, and are far from relictual: about 56% of the animal’s required oyxgen is obtained via the lungs. Amphiumas ventilate their lungs infrequently, and are very efficient at using the oxygen they inspire (Martin & Hutchison 1979). The importance of lung-breathing means that amphiumas can survive in warm and/or stagnant, poorly oxygenated waters that would ordinarily be inhospitable to an aquatic amphibian.
The limbs are so tiny that they really can’t be much use in swimming, foraging or in crawling (either on submerged or emergent substrates), but they’re larger in juveniles and may play a role in their behaviour. As is typical for salamanders, the limbs can be regenerated when lost (Morgan 1903). Adults are anecdotally reported to use the hindlimbs to manipulate food prior to eating it, and they also (sometimes) constrict prey while subduing it. Lateral line organs are present on the head and body. As is typical for long-bodied vertebrates, the number of vertebrae is very high (there are 65-70 precaudals). Amphiumid vertebrae are extremely distinctive and possess large, parallel crests on their dorsal surfaces and a prominent ventral keel: these features are almost certainly related to the strong, complex trunk musculature that amphiumas use in burrowing (in particular the dorsalis trunci epaxial muscles: Auffenberg (1959)).
Wide gapes, long jaws, rapid bites
Amphiumas have long, narrow snouts compared to most other salamanders, and are able to open the jaws extremely wide. In part, this is because the occipital condyles (which are normally paired convexities on either side of the foramen magnum) extend posteriorly as short stalks, allowing the whole skull to be rotated much further dorsally (and ventrally) than is normally the case (a similar system evolved convergently in plagiosauroid temnospondyls) [adjacent photo by Andri Pogo; from here].
Amphiumas will simply grab prey with a rapid bite, but high-speed photography has shown that they can also use extremely rapid jaw-opening and throat expansion to generate suction (Erdman & Cundall 1984). Suction feeding has evolved on numerous separate occasions within secondarily aquatic tetrapods. I keep saying I’ll elaborate on this subject. One day I will. Amphiumas lack eyelids, as is typical of aquatic amphibians, and also lack a tongue. Their premaxillae are fused (perhaps to reinforce the snout and allow for more powerful bites), and the sides of the snout bones have a distinctive sculptured texture. The teeth have distinct pedicels (flexible zones separating the crowns from the roots) and there are rows of teeth on the vomerine bones on the palate. They are reported to bite readily and nastily and are sometimes referred to as to the only amphibians within their range that can pose any sort of physical (as opposed to chemical) danger to humans [A. tridactylum skull shown below; borrowed from here on flickr].
They’re nocturnal and hide in mud burrows; the small One-toed amphiuma Amphiuma pholeter (restricted to Florida and Georgia) is a specialised denizen of habitats where the substrate is composed exclusively of fine-grained mud; it doesn’t occur in places where there is sand, gravel or rocks. The burrows can be as long as 1 m.
During mating, sperm is transferred directly into the female cloaca and there is no production of the unusual sperm packets (spermatophores) seen elsewhere among salamanders. The eggs (as many as 150 or 200) are laid in strings, and the female guards them by coiling her body around them [image below, from here, shows female Two-toed amphiuma and egg clutch]. They take about 20 weeks to hatch and the female stays with them for all of that time (not a typo: 20 WEEKS), so you might consider parental care to be extended. The more we learn about animals, the more difficult it becomes to make generalisations, and one of the generalisations I recall from childhood – that amphibians and reptiles are inferior to mammals and birds because they don’t practise parental care – looks nowadays like ill-informed nonsense. Because female amphiumas lay their eggs when water levels are high, they sometimes become stranded in isolated puddles or hollows when the water levels fall again, and the newly hatched babies then have to crawl to the water (Halliday & Verrell 1986). All amphiumas can crawl across land when they have to.
For some reason, amphiumas have enormous amounts of DNA: about 25 times more than we do. This clearly demonstrates that they’re the most complex creatures in existence and are destined to evolve into millions of species and take over the planet (I’m joking, by the way. We actually have no idea why some organisms have much more DNA than others).
There are three extant amphiumas: the Two-toed amphiuma Amphiuma means*, Three-toed amphiuma A. tridactylum and One-toed amphiuma A. pholeter. Two- and Three-toed amphiumas have sometimes been regarded as subspecies of the same species, but this (a) isn’t helpful given that the two can be readily and consistently distinguished, and (b) is contradicted by some studies on phylogeny (see below).
* Biologist D. Bruce Means has published on amphiumas: I bet he’s tired of being the butt of constant jokes.
All are native to the south-eastern USA (from Texas in the west to Virginia in the east) where, confusingly, they’re sometimes called ‘congo eels’ or ‘conger eels’. Needless to say, they shouldn’t be confused with real conger eels (these being marine, err, eels). Fossil amphiumids are known from the Cretaceous onwards; all are North American, with a German record of the group (A. nordica from the Pleistocene) being a misidentified teleost according to Gardner (2003) (it was named for a parasphenoid: one of the bones from the bottom of the braincase). A. jepseni from the Paleocene of Wyoming seems to have had a shorter, broader snout than living species. Paleoamphiuma (why did they go for this spelling, and not Palaeoamphiuma?) from the Eocene of Wyoming was described as a particularly well-preserved fossil amphiuma (Rieppel & Grande 1998), but its identification was doubted by Gardner (2003) and it might be a sirenid [Three-toed amphiuma shown here; photo by Brad Moon].
Gigantism and rapid evolution
Two- and Three-toed amphiumas can be more than a metre long (the One-toed amphiuma doesn’t exceed 40 cm in total length) and are thus giants in the salamander world. A record-holding Two-toed amphiuma was 116 cm long. The fossil species all seem to be have relatively small (less than 40 cm), which is a shame as I really like the idea of a 2-m-long, eel-like Paleocene swamp-monster amphibian. There is a fossil species from the Miocene of Texas – A. antica – that might have been large, but it doesn’t seem to have been larger than A. means, and its identification as an amphiuma was also doubted by Gardner (2003).
Some molecular studies have found Two- and Three-toed amphiumas to be close relatives (Karlin & Means 1994), and some morphologists have agreed with this (Gardner 2003). However, more recent molecular phylogenetic work indicates that the large Two-toed amphiuma is more closely related to the small One-toed amphiuma than it is the large Three-toed amphiuma (Bonett et al. 2009), and the Two-toed and One-toed lineages seem to have diverged as recently as the Late Pliocene. At the moment it isn’t clear if gigantism evolved once on the Amphiuma stem – in which case the One-toed amphiuma is a specialised dwarf – or if it evolved independently in Two- and Three-toed amphiumas (Bonett et al. 2009). Whatever happened, rapid evolution must have occurred given the Late Pliocene divergence between the One-toed and Two-toed lineages [image below shows amphiumid phylogeny from Bonett et al. (2009)].
The large size of the biggest amphiumas is made all the more remarkable by the fact that amphiumas are members of the IFS (= internally fertilizing salamanders) clade, more properly known as Salamandroidea, Salamandriformes or Diadectosalamandroidei (I have yet to work out whether these names properly overlap, as the definitions given for each are somewhat different). Furthermore, within this clade, amphiumas are apparently the sister-taxon to the lungless salamanders or plethodontids (Larson 1991, Larson & Dimmick 1993, Frost et al. 2006). Intuitively, this is surprising given how weird amphiumas are… but, then, I suppose they might seem weird wherever they’re placed in Caudata. More conventionally, they’ve been allied with mole salamanders, or placed in their own group: Amphiumoidea (Estes 1981). Frost et al. (2006) decided that the amphiumid-plethodontid clade was robust enough for a name, and called it Xenosalamandroidei.
As I said at the start of this article, there really isn’t much on amphiumas in the literature at all, and most herpetology books only mention them in passing. Hopefully this little review can act as a useful synthesis.
For previous Tet Zoo articles on salamanders start by looking at the two introductory pieces…
- Giants and sirens: caudates part I
- The wonder that is the internally fertilizing salamander clade: caudates part II
And for specific groups, see…
- Spiky-frilled, lek-breeding amphibious salamanders… or ‘newts’
- Coprophagy and the giraffe-neck program: more on plethodontids
- Axolotls on the EDGE!
- When salamanders invaded the Dinaric Karst: convergence, history, and reinvention of the troglobitic olm
- The USA is still yielding lots of new extant tetrapod species
Still SO much more to do.
Refs – –
Auffenberg, W. 1959. The epaxial musculature of Siren, Amphiuma, and Necturus (Amphibia). Bulletin of the Florida State Museum, Biological Sciences 4, 253-265.
Bonett RM, Chippindale PT, Moler PE, Van Devender RW, & Wake DB (2009). Evolution of gigantism in amphiumid salamanders. PloS one, 4 (5) PMID: 19461997
Erdman, S. & Cundall, D. 1984. The feeding apparatus of the salamander Amphiuma tridactylum: morphology and behaviour. Journal of Morphology 181, 175-204.
Estes, R. 1981. Handbuch der Paläoherpetologie. Teil 2. Gymnophiona, Caudata. Gustav Fischer Verlag, Stuttgart.
Gardner, J. D. 2003. The fossil salamander Proamphiuma cretacea Estes (Caudata; Amphiumidae) and relationships within the Amphiumidae. Journal of Vertebrate Paleontology 23, 769-782.
Halliday, T. R. & Verrell, P. 1986. Salamanders and newts. In Halliday, T. & Adler, A. (eds) Animals of the World: Reptiles and Amphibians. The Leisure Circle (Wembley, UK), pp. 18-29.
Karlin, A. A. & Means, D. B. 1994. Genetic variation in the aquatic salamander genus Amphiuma. American Midland Naturalist 132, 1-9.
Larson, A. 1991. A molecular perspective on the evolutionary relationships of the salamander families. Evolutionary Biology 25, 211-277.
– . & Dimmick, W. W. 1993. Phylogenetic relationships of the salamander families: A analysis of congruence among morphological and molecular characters. Herpetological Monographs 7, 77-93.
Martin, K. M. & Hutchison, V. H. 1979. Ventilatory activity in Amphiuma tridactylum and Siren lacertina (Amphibia, Caudata). Journal of Herpetology 13, 427-434.
Morgan, T. H. 1903. Regeneration of the leg of Amphiuma means. Biological Bulletin 5, 293-296.
Rieppel, O. & Grande, L. 1998. A well-preserved fossil amphiumid (Lissamphibia: Caudata) from the Eocene Green River Formation of Wyoming. Journal of Vertebrate Paleontology 18, 700-708.
Salthe, S. N. & Kaplan, N. O. 1966. Immunology and rates of enzyme evolution in the Amphibia in relation to the origins of certain taxa. Evolution 20, 603-616.