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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