This video is sponsored by Endel.
The marine world is a vast and diverse ecosystem. The many different creatures that call it
home have all adapted to certain ecological niches, finding their place amidst the endless
blue. But the boundless opportunities that the oceans present means that even some unexpected
organisms have sought to colonise its sprawling expanse.
Reptiles, which despite their physiological
constraints as air breathers, have repeatedly invaded marine environments. Though they may
not be as successful as their gigantic ancestors were in the Mesozoic era, they still endure
today. Let’s take a closer look at the wonderful world of marine reptiles.
Reptiles are a diverse and successful group. With over 8,000 species found worldwide, only
around 100 species can be considered marine reptiles. They retain the defining characteristics
of reptiles, including tough scaly skin that covers their bodies. But in addition, marine
reptiles have developed a number of additional anatomical and physiological adaptations to
survive in the oceans.
Let’s examine the case of marine iguanas, a reptile species endemic to the Galapagos
Islands. They evolved from land iguanas in order to make the most of the abundant of
food found off the shores of the islands. Each day, they enter the waters, using their
long sideways-flattened tails to move, and their long sharp claws to grip onto rocks
so as to not be swept away by heavy currents. Their short, blunt snouts and strong three-cusped
teeth are excellent for scraping algae off the rocks. But on their own, these anatomical
adaptations are not enough to ensure the survival of the marine iguana. Like many reptiles,
they are ectothermic, meaning cold-blooded,
so their body temperature depends on the temperature
of their surroundings. The waters around Galapagos are cold, posing a significant problem to
marine iguanas. So, to regulate their temperature and stay alive, they use behavioural mechanisms
like basking in the sun for considerable periods after feeding at sea. Their dark coloured
skin helps absorb as much warmth as possible.
The cold conditions of the oceans are not the only challenge marine reptiles have had
to overcome. Perhaps the greatest problem they face is that of salt and water balance
- how to get rid of excess salt. An issue not faced by most terrestrial reptiles. Sea
turtles cry away the salt through their eyes via modified tear glands, producing gooey
tears that stream down their faces. The large amount of salt they extract via this method
means that sea turtles can survive by drinking seawater alone, never needing to visit freshwater
lakes or rivers to drink like crocodiles or marine iguanas do. This adaptation suits their
lifestyle as wanderers of the open ocean that return to land only to lay eggs.
The sea snake,
on the other hand, is able to get sufficient freshwater when it rains as the lighter freshwater
briefly floats on the surface.
Sea snakes live their entire lives at sea where they feed, rest and reproduce. They
have become wholly reliant on the oceans, to such an extent that they have lost the
broad belly scales that allow most terrestrial snakes to move over the ground. On land, sea
snakes are helpless. This also highlights the distinction between sea snakes and sea
kraits, the latter of which are also adapted to life in the oceans but retain the belly
scales and the ability to move on land. Both have flattened paddle-like tails for swimming
through open water. But even this does not allow them to move fast enough to pursue prey
in the open. Instead, many sea snakes feed on bottom-dwellers and burrowing fish like
eels.
In fact, their interactions with eels has possibly led to an evolutionary arms race.
Sea snakes use venom to subdue prey, but the immunity of some eels means snakes require
stronger venom. This explains the the extreme toxicity of sea snake venom, making them among
the most venomous of all snakes. The deadly nature of sea snakes has been taken advantage
of by another organism known as the banded snake eel - a fish species that has adapted
to look almost exactly like the sea krait. So, despite it being harmless, the eel will
be avoided by predators, giving it a major advantage. This phenomenon is known as Batesian
mimicry.
Sea snakes are also excellent at diving. Their thin skin allows them to absorb some oxygen
from the water, helping them overcome the challenge of accessing sufficient oxygen while
submerged. Turtles are less proficient at this, but like diving mammals, they can slow
their heartbeat, and direct the flow of blood away from non-essential organs and towards
the brain to conserve oxygen. The lungs of sea turtles are far more complex than those
found in most reptiles, and are able to store sufficient oxygen for longer dives.
Sea turtles travel far and wide during their lives, but once mature the females will migrate
thousands of miles to lay their eggs at the site where they themselves were hatched. They
are thought to rely on sophisticated means of navigation, making use of biological compasses
to maintain direction in the open ocean.
Once they arrive, they haul their heavy bodies
above the high tide line, laboriously dig a metre-deep hole in the sand using their
front flippers, before laying up to 100 eggs in to the chamber they dug.
After concealing
them beneath the sand, the turtles return to the open ocean. These mass nesting events
show how well-adapted turtles really are to life in the oceans. Their long flippers and
flattened shells may prove cumbersome on land, but the success of marine turtles is due to
the way these adaptations allow them to move quickly and efficiently through the water,
and even take part in long-distance migrations.
While sea turtles will not return to their eggs once laid, another kind of marine reptile
is far more attentive of its brood. In Florida, the saltwater crocodile will lay her eggs
in a mound of mud along the banks of a river or estuary. Unlike turtles, lizards and all
other reptiles, she will not abandon the nest but remain nearby to fend off predators. Highly
sensitive ears pick up the calls of hatchlings, signalling the crocodile to excavate the nest,
gently pick up her young in her mouth, and carry them to the water.
The highly varied anatomies and behaviours of marine reptiles demonstrates the many different
ways in which they have sought to colonise the oceans.
In direct contrast to their land-dwelling
relatives, marine reptiles appear cumbersome on land, and at ease in the marine world with
their streamlined bodies. This highlights the extraordinary ability of life to adapt
to very different habitats and conditions, by adopting just a few small changes.
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