Chasing sea snakes in Australia | Colour and Vision

To coincide with the opening of our Colour and Vision exhibition and #WorldSnakeDay, Museum researcher Dr Bruno Simões tells us about recent fieldwork he undertook in Australia to learn about vision in snakes.

As a vision biologist, I’m interested in how animal vision has evolved and how it functions. The dramatic impact living in an aquatic environment can have on visual systems led me to become particularly interested in sea snakes.

Olive sea snake swimming through a coral reef

Aipysurus laevis is a venomous sea snake found off the coast of Australia and other Indo-Pacific areas © Tchami, licensed under CC BY-SA 2.0

Sea snakes are part of the family Elapidae, along with kraits, mambas, cobras and taipans. The family consists of more than 360 species, including some extremely venomous species that live in aquatic and terrestrial (land-based) habitats in Australasia, among other places.

In 2014 I travelled to Australia to observe sea snakes in their natural environment and to collect DNA samples that would allow me to analyse their visual systems.

Close-up of a sea snake's head and eye

I want to find out how sea snake vision has changed to suit their aquatic lifestyles. This is one of the snakes we caught and collected DNA from in Australia.

Adaption to aquatic environments often includes radical changes in the visual system. Whales and dolphins lost the ability to see colour as an adaption to deep-sea diving, for example, and seals lost blue-light-sensitive vision as a result of the amphibious phase of their evolution. Deep-sea fish exhibit very different visual capabilities compared to species that live closer to the surface.

Humpback whale diving underwater

Living in an aquatic environment can cause dramatic changes to an animal’s vision over time. Whales no longer see colour. © Sylke Rohrlach, licenced under CC BY-SA 2.0

Snake eyes

Molecular studies show that snakes belonging to the sub-families Hydrophiinae (true sea snakes) and Laticaudinae (sea kraits) conquered the sea at least twice (independently) in their evolutionary history. While sea kraits evolved to live in marine habitats around 18 million years ago, true sea snakes only adapted to living in the sea about seven million years ago.

Photograph of a banded sea krait swimming along the ocean floor

A banded sea krait, Laticauda colubrina. Sea kraits evolved to live in the sea 18 million years ago, while other sea snakes did so more recently. © Rich Carey/Shutterstock.com

True sea snakes are more closely related to terrestrial Australian snakes, such as tiger snakes and brown snakes, than to sea kraits.

Photo of a tiger snake

Tiger snakes live exclusively on land, but they are closely related to sea snakes in the group Hydrophiinae – more so than sea kraits. © Ian W Fieggen, licensed under CC BY-SA 3.0

When it comes to vision, the sea snakes studied so far only have cone-like cells in their retinas, which provide colour information in bright light. They apparently lack the other major retinal cells (rods) which animals use to see in dim light. However, our snake vision project recently discovered that some of the cone-like cells of sea snakes are actually modified rod cells and still express genes responsible for vision in dim light. But how the visual system of sea snakes works and how it has changed through time, as well as the reason for such changes, is still largely unknown.

To answer some of these questions, we worked with the University of Adelaide’s sea snake specialist Dr Kate Sanders, and I travelled to the small town of Broome in western Australia to participate in fieldwork to help sample sea snakes.

Photograph of Cable Beach in Broome, Australia

It wasn’t exactly a hardship working in Broome – it is a beautiful place, with a beautiful beach

Snakes in a tropical paradise

Marine snakes show a fascinating range of behaviours and adaptations to their environment.

True sea snakes give birth to live young in the water and some species, such as those in the Hydrophis and Aipysurus groups, live exclusively in the sea. However, other semi-aquatic species such as Hydrelaps and Ephalophis, come to land to forage for food along tidal creek banks or exposed mudflats. Sea kraits return to land to lay eggs.

Photograph of a sea krait on sand at the edge of the water

Sea kraits come on to land to search for food and to lay their eggs (public domain image, via Wikimedia Commons)

As well as one of the best tropical beaches in the world, Cable Beach, Broome has a large mangrove. Here, some of the biggest tides in the world create floodplains that can become dry sand within a couple of hours. This is a very good habitat for semi-aquatic sea snakes. Small pools in the mangrove trap fish and are the perfect place for these snakes to feed. Flooding of the area at high tide, helps the snakes return to the sea.

Photo of mangrove in Broome, Australia

The mangroves at Broome harboured more than just sea snakes – I also saw crocodiles there at night

Photograph of exposed sand and mangrove in Broome, Australia

The huge tides at Broome mean large expanses of land become exposed or submerged in just a couple of hours

Walking the mangrove by day, scanning the sea by night

Each day we spent about four hours walking across the mudflats in baking temperatures well above 35°C. But despite searching the small pools for sea snakes, the only vertebrates we found were mudskippers.

Photo of a mudskipper on land

The retreating tides create puddles that trap fish – dinner for semi-aquatic sea snakes. Despite hours of searching we mostly saw mudskippers – fish that are able to walk on land using their fins.

In the evenings we boarded boats and headed out to sea. We used powerful lanterns to scan the ocean’s surface for sea snakes, using nets to quickly scoop them out of the water. The ship’s crew took GPS coordinates so we would know what type of habitat the snake came from, including factors such as depth, salinity, water temperature and turbidity.

Bruno Simoes holding a net containing a sea snake, and Jenna Crowe-Riddell, on a boat at night

Me with Jenna Crowe-Riddell from the University of Adelaide continuing our snake search out to sea. If we spotted one we had to be quick to scoop it up – they’re fast!

If the snakes dived they were impossible to capture. They are incredibly fast and graceful in the water, thanks to flattened bodies and paddle-shaped tails that help them swim.

In comparison, out of the water the snakes are very clumsy and slow.

Close-up of Hydrophis peronii in a net

In this Hydrophis peronii specimen we caught, you can see the paddle-shaped tail that makes sea snakes adept at swimming

We photographed each animal we caught, and cut pieces of scales from the edge of the tail for later DNA analysis, before releasing the unharmed snake back into the sea.

Close-up photo of researchers cutting small pieces from a sea snake's tail

We cut small pieces of scales from the tail of each snake, in a way that didn’t harm them

We had to be extremely cautious when dealing with the snakes, since most sea snakes are highly venomous. We wore very thick gloves and someone always had to hold the snake’s head.

Close-up photo of Hydrelaps darwiniensis being held in a gloved hand

Although sea snakes aren’t aggressive, some are very venomous, so the person holding the head wore protective gloves to make sure the snake couldn’t bite them

The search is on

One of the main goals of the expedition was to capture semi-aquatic sea snakes from the genera Hydrelaps and Ephalophis. Both are divergent lineages from other sea snakes and have poorly known ecologies and evolution.

Biologists had rarely reported the black-ringed sea snake, Hydrelaps darwiniensis, in recent years, but local fishermen told us they frequently observed this semi-aquatic species.

Photo of Bruno Simoes and team standing around the amphibious vehicle they used to explore mangroves

We used an amphibious vehicle to explore the mangroves at high tide, by day and by night

Finally, one evening, we spotted one crawling in the mud. That night, knowing where would be best to find this species, we returned to the mangroves at high tide in an amphibious vehicle, and were able to catch several other specimens. It was rather unnerving to discover some of the other animals present: our lanterns showed the glowing eyes of crocodiles.

Close up of Hydrelaps darwiniensis on a gloved hand

The black-ringed sea snake, Hydrelaps darwiniensis, was one of the semi-aquatic species we found in the mangroves and mudflats. There’s still a lot to learn about this species.

In total, we took scale samples from more than 10 species of sea snake, covering much of the evolutionary history and ecological diversity of sea snakes, from shallow and coastal water to the deep sea, and semi-aquatic to fully marine. This will allow us to understand how different marine habitats impact vision in these fascinating animals.

Photo of Aipysurus laevis in a net

We collected scale samples from more than 10 species in Broome, including this olive sea snake, Aipysurus laevis

Photo of Hydrophis peronii

…and this horned sea snake, Hydrophis peronii

Photograph of a sunset over Broome beach

Then it was time to say goodbye to this beautiful location and its fascinating animals, and get to work extracting the DNA from the scale samples

Next steps

Back at the Natural History Museum, DNA was extracted from the scale samples with the help of colleague Filipa Sampaio and vision genes sequenced using advanced high-throughput sequencing technologies.

Bruno Simoes working in the lab at the Natural History Museum

Back at the Museum, DNA was extracted and the genetic code of the vision genes sequenced. I compared the genes from terrestrial, aquatic and semi-aquatic snakes to look for differences.

The results will allow me to answer questions such as:

  • How has the visual system evolved to allow snakes to see in the sea? How have their vision genes changed?
  • What kind of colour vision do these reptiles have? Have their visual pigments changed to become sensitive to particular light wavelengths, such as the red-shifted light of turbid coastal environments?
  • How did the rod cells of sea snakes change into cone cells, a process seemingly rare among vertebrates? What is the impact on their ability to see in low light conditions?

I already have the data to answer these questions and with the processing power of super-computers I assembled the thousands of gene sequences and calculated the statistics that will shed light onto sea snake vision.

This trip and research was funded by a Leverhulme Trust grant awarded to Museum Herpetology Researcher Dr David Gower and collaborators to study wider aspects of snake vision.

I’m looking forward to publishing the findings soon with colleagues in the Museum and Australia.

Dr Bruno Simões
Vision biologist