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