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Four beguiling organisms from beneath the waves

Comb jelly (beroe cucumis).
Comb jelly (Beroe cucumis).
Credit: ALEXANDER SEMENOV / Getty Images

1.    Comb Jellies

Lights playing across the surface of the sea at night may seem otherworldly. They’re not: they are the work of single-celled organisms.

In fact, more than three-quarters of marine species emit light – far more than anybody suspected, according to a study published in April by Séverine Martini and Steve Haddock from the Monterey Bay Aquarium Research Institute in California. For 17 years the researchers have plumbed the depths to watch the light show using remotely-operated subs fitted with high-definition video.

At depths down to 100 metres, comb jellies steal the show.

Lion’s mane jellyfish (Cyanea capillata).
Credit: ALEXANDER SEMENOV / Getty Images

2.    Lion’s Mane Jellyfish

Beguilingly beautiful though it may be, the Lion’s Mane jellyfish is not a species you want to encounter on a night swim. The biggest jellyfish in the sea, its ‘mane’ of stinging tentacles can reach 30 metres in length. Like the comb jelly, the Lion’s mane is a near-surface swimmer, but 500 to 1500 metres below its jellyfish relatives still dominate the light show. Why do they glow? We can’t be sure. Theories include to attract prey, startle a predator or to camouflage themselves against a moonlit sky from predators below.

Ghostly Seadevil (Haplophryne mollis).
Credit: Peter David / Getty Images

3.    Ghostly Seadevil

The deeper they sent their submersibles, the less bioluminescence Martini and Haddock saw. But that’s because there are simply fewer creatures at these food-sparse depths. The proportion of all marine species that can bioluminesce remains a virtually constant 75% at all depths, the researchers report.

Among fish, the new survey found that almost 55% of  deep sea species can light up. The ghostly seadevil, found at depths of about 2,250 metres, is actually a pale species of  anglerfish – famous for its built-in bioluminescent fishing rods. In the case of the seadevil – also called the soft leftvent angler – the fishing rod is reduced to a small skin flap held in front of the mouth.

A larvacean of the genus Oikopleura.
Credit: WIM VAN EGMOND / SCIENCE PHOTO LIBRARY

4.    Larvaceans

Below 2,250 metres, small tadpole-like creatures called larvaceans are the most common glowing creatures captured in Martini and Haddock’s high-sensitivity footage. About a centimetre in length, they are actually tunicates that never moved on from their larval stage.

At depths where no light penetrates from the surface, creatures don’t have to glow very brightly to light up their surroundings. Under the bright lights of a deep sea submersible, their bioluminescence can be drowned out. The new study was the first detailed survey to use video cameras sensitive enough to pick out this low-level bioluminescence. The results are illuminating, showing the ocean is a much brighter place than we ever imagined.