invaded fresh water and marine environments multiple
times. Independent invasions of the ocean occurred in the
ancestors of acrochordids, homalopsines, natricines,
hydrophiids, and laticaudids.
As a result of their long, thin physique, snakes evolved a
single (right), elongated lung, providing a natural
buoyancy and flotation device. Sea snakes’ lungs have
been further modified to increase buoyancy and in
hydrophiines the lung extends to occupy up to 100% of
the trunk. Snakes moving through water employ a
swimming style resembling that of long, thin fish, known
as anguilliform locomotion, in which alternative waves
pass down the body, propelling the animal forwards. The
lateral undulation inherited from their terrestrial ances-
tors formed a good basis for this technique, however the
biomechanics are very different. Unlike the terrestrial
application, where force is applied at fixed points and the
waves travelling down the body dampen towards the rear,
the regular waves swimming snakes use increase in
amplitude posteriorly. And in terrestrial lateral undula-
tion, the propulsive force is generated by lateral surfaces
of the body pushing against irregularities in the substrate,
whereas in swimming, snakes move forward by their
movement accelerating portions of the surrounding
water. As a further adaption for swimming, aquatic
snakes evolved features for increasing the surface area
against which their body pushes against the water. Sea
snakes (hydrophiids and laticaudids) have laterally-
compressed, paddle-like tails which generate consider-
able lift. Swimming snakes also are highly streamlined,
with reduced ventral scutes and small, narrow heads not
demarcated from the body. It may be that the already
streamlined, buoyant bodies of snakes explains why
hydrophiines have adapted to an exclusively marine
lifestyle; despite lizards having a higher diversity than
snakes (5,600 species versus 3,400), there are no
completely marine lizards.
Various snakes move on both land and water; for
example, Australian Tiger Snakes (Notochis scutatus)
often forage in water. Depending on the habitat baby
snakes experience early in life - from areas lacking any
bodies of water, to permanently swampy habitats - they
exhibit different locomotory abilities. Different con-
straints on optimal morphology and physiology create a
trade-off between locomoting with maximum efficiency
on land versus water, such that improved swimming/
diving abilities correspond with reduced terrestrial
performance. The ability for juvenile snakes to optimize
their locomotory abilities according to the habitat they
grow up in is highly adaptive; it means their bodies are
The Turtleheaded Sea Snake (Emydocephalus annulatus) has a paddle-
like tail which generates considerable lift. Image by Paul Cowell.