Diving in the dark: Discovering deep-sea bioluminescent animals

Cécile Debitus-Vauglin, a marine biochemist with a PhD in natural product chemistry, guest blogs today to share her experience during off-shore ‘black-water dives’, conducted in deep channel located between Tahiti and Mo’orea islands, French Polynesia. Cécile has provided us with us expertise and knowledge on sea jellies, which has contributed to the development of our new sea jelly exhibit at S.E.A. Aquarium.

Descending into the inky darkness of night to experience the nocturnal wonders of the ocean can seem like an intimidating prospect. However, it can also open you up to a world of rich and unusual encounters, as sea creatures that typically spend the day at great depth, migrate to the surface waters after sunset.

Black-water dives, or night dives, provides an insight into the rarely seen curiosities of the deep. For my team of researchers exploring the deep channels of Tahiti and Mo’orea islands, French Polynesia, night dives are an invaluable way to observe animals in their natural surroundings.


Cécile’s team was the first to photograph this species of sea jelly, Nausithoe punctata, in their natural environment.

Surrounded by darkness and suspended 1500 metres above the sea floor, flashes of light surround us. The surreal scene arises not from our torches, but from plankton that have evolved to glow in the dark creating natural light, also known as ‘bioluminescence’.

In the deep ocean, where sunlight cannot reach, many animals have developed the ability to produce and emit light for defence, warning or camouflage. On these dives, it is common to encounter several species of bioluminescent animals such as sea jellies, comb jellies and siphonophores.

Bioluminescent Thimble sea jelly (Linuche unguiculata)

The spectacular bioluminescent Thimble sea jelly (Linuche unguiculata)

Why deep-sea creatures surface at night

Most of the ocean’s living space is not on the sea floor but in the water above it. Salt water provides the nutrients that allow many microalgae and animals, commonly referred as plankton, to live suspended in the water.

Plankton needs sunlight to make their food which limits them to the surface waters. They in turn become food for other animals, ranging from microscopic organisms to some of the biggest creatures in the ocean.

For the plankton-eating bioluminescent animals, foraging during the day would make them visible to predators, so they spend the daylight hours in deeper waters and only feed at night. As night approaches they move towards the surface, presenting us with the perfect opportunity to study these fascinating creatures of the deep.


Sea jellies drift with the ocean currents and many will make a daily vertical migration to the surface. Pulsating through the water these sea jellies can be strikingly bioluminescent, emitting a very bright light, and evidence suggests that this could be a defence against predators.

comb jelly

The Comb jelly is one such deep-sea dwelling animal that surfaces at night. Scientists have likened Comb jellies to alien life forms, because their structure is so different from all other animals.

Comb jellies have rows of cilia (tiny hair-like structures that look like combs) running up and down their bodies. These cilia act like tiny oars that propel them through the water.

When light is shone at a Comb jelly, a rainbow effect is seen as the light scatters off the moving combs. Most Comb jellies are also bioluminescent and emit blue and green light that can only be seen in darkness.

A Siphonophore colony

Another interesting group of animals are the Siphonophores; a spectacular colony creatures with highly specialised working parts. Some parts catch prey, others digest food, some parts reproduce and others direct the action by swimming. These mesmerizing floating colonies can reach remarkable lengths – up to 40 metres – but most are far smaller and less conspicuous.

A Siphonophore (Agalma okeni)

A Siphonophore (Agalma okeni)

Siphonophores are predators that feed by casting a wide net of venomous tentacles and waiting for prey to swim into them. For me, a wetsuit is a good form of protection against these stings- but for some reason they always manage to find bare hands and lips.

Challenges of black-water dives

The incredible opportunity to view deep-sea animals in their natural environment comes with its challenges. The animals we are studying often swim away from our torchlight, and we have to be careful not to follow them down too deep.

We are never alone; dolphins are frequent companions on these dives, hunting the animals we illuminate with our torch light. We can hear them well before we can see them, as they communicate with one another through a series of clicks and whistles.

How ocean exploration helps science

Crystal sea jelly (Aequoera victoria).

The Crystal sea jelly (Aequoera victoria).

Ocean exploration often leads to new ideas, new theories and discoveries. The Green Fluorescent Protein (GFP), first found in the bodies of the Crystal sea jelly (Aequoera victoria). The GFP has been used to track how cancer cells spread, how HIV infections progress and even which male ends up fertilising a female fruit fly’s egg.

For scientists studying deep-sea creatures, nocturnal feeding sessions present unrivalled opportunities to observe the fascinating functions that many of these animals have evolved over time.

About the writer

Cécile Debitus-Vauglin

Cécile Debitus-Vauglin is a marine biochemist with a PhD in natural product chemistry. Cécile has been involved discovering products derived from marine invertebrates. She has been an avid SCUBA diver since 1985 and spent many years working in South Pacific Islands.

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