the hind feet, as opposed to the Hellbender, which has
five. There is also a significant difference in size, with
N. maculosus rarely exceeding 40 centimetres in length,
whereas the average size for an adult Hellbender is
between 45-60 centimetres.
complement of DNA; roughly 25 times that of a human
being.
So what selective pressures have driven this remarkable
assemblage of amphibians to stay trapped in adolescence?
What are the forces that favour neoteny? The largest
Another addition to this bizarre collective, amphiumas
salamanders would undoubtedly be clumsy and vulnerable
(Amphiuma spp.) or ‘Conger Eels’ share a similar
on land, and could be expected to have a higher degree of
distribution with sirens, but are not closely related. With
difficulty in hunting prey items. These species are
an extremely elongated body, capable of measuring more commonly restricted to fast-flowing streams at some
than a metre, and tiny, vestigial legs, it is no surprise that elevation, due to their requirement for cool, highly-
they are sometimes mistaken for eels or snakes.
oxygenated water. Neoteny is also often associated with
Amphiumas also undergo incomplete metamorphosis,
montane environments, where the energy expenditure
losing their external gills but retaining a pair of gill slits.
necessary for metamorphosis may constitute an unneces-
Unusually, considering they are rarely found outside
sary risk to survival, especially in circumstances where
water, they appear to rely primarily upon their lungs for
the terrestrial environment is relatively inhospitable, and
respiration. Amphiumas also have an extraordinarily large subject to seasonal fluctuations and extremes. This is
often coupled with low levels
of iodine, which is instrumental
in producing the thyroid
hormones required to initiate
metamorphosis.
1.
The Axolotl (Ambystoma
mexicanum) or Mexican
Walking Fish, the species
originally responsible for the
concept of neoteny, is a
celebrated example. Endemic
to several permanent lakes
around Mexico City (only one
of which now remains), at an
average elevation of around
2,250 metres, Axolotls exist
permanently in the larval state,
complete with external gills,
Salamanders
1. Grotto
(Eurycea spelaea)
native to the Ozark
Mountains. Image
by Matt Jeppson.
Cave
2. Tennessee
Salamander
(Gyrinophilus
palleucus). Image
by Matthew L.
Niemiller.
Texas Blind
3. The
Salamander
(Eurycea rathbuni)
was first discovered
in a well. Image by
Ryan Hagerty.
2.