Night swims are often accompanied by head torches, beach campfires or lanterns. But at this time of year, marine life lights the way along the coast at locations including Jersey, Anglesey, County Cork and Bournemouth.
Bioluminescence is the biochemical emission of light by organisms that might live along the shoreline or in the sea. Concentrations of creatures such as dinoflagellates and pelagia noctiluca, or nightlight of the sea, can result in an otherworldly, sparkly swim. In seas and oceans, thousands of living creatures glow with light they produce themselves. It is a language developed in dark ocean depths: a way of catching prey by deception, with a little glowing lure attached to a big fish with a hungry mouth, or like a flashing neon sign indicating that they are looking for a mate.
Up in surface waters where wild swimmers swim, the most likely glowing creature to be encountered is invisible to the naked eye but collectively wonderful. Dinoflagellates are microscopic algae that predominantly dwell in seawater. Like plants, they capture light from the sun using photosynthesis, while a few of them augment their diet by eating microscopic animals. Some of the energy they harvest from the sun is converted to luciferin – the light-bearing protein. When disturbed at night they trigger a chemical reaction that breaks down luciferin and releases light. It is thought that this is a defence against predation – a cluster of dinoflagellates creating a small burst of light might be enough to startle the tiny creatures that eat them.
Dinoflagellates are in the sea year-round. But in summer calm seas allow them to aggregate, and warmth makes them breed more rapidly. They can be numerous enough that on a night swim, as your body moves through the water creating swirls and eddies, little sparks appear. It is worth wearing goggles so that you can see these flickers of light underwater too. A duck dive down is like jumping into the Milky Way.
Dinoflagellates. Credit: Billy Huynh on Unsplash
“Wear goggles so that you can see flickers of light underwater. A duck dive down is like jumping into the Milky Way.”
There are also larger, glow-in-the-dark creatures that swimmers can meet. Pelagia noctiluca, whose scientific name means nightlight of the sea, are possibly not the bioluminescent beauty you want to swim with because they sting. Their vernacular name is mauve stinger, which precisely describes how they look in daylight, and feel night or day. Less beautiful, but enjoyable without the risk of a sting, are a type of bristly marine worm caulleriella bioculata. It lives in sand or soft mud, so walking across the intertidal area between dry land and sea on a night swim is when you might see them as they glow in response to your passing.
The Romans had a way to enjoy bioluminescent creatures, though being squashed and rubbed across the face and hands of a person is presumably not fun for a piddock (pholas dactylus). Pliny the Elder described their glittering juice as a marvel we should wonder at, while writing about how Romans delighted in the way piddocks shone when they were chewed – in a persons mouth and when drops fell from them on to clothes. Romans left piddock shells in the heap of edible shellfish shells at the Roman villa near Holcombe, Somerset and in Pliny’s words “…it is the nature of these fish to shine in darkness….”.
Perhaps we owe more thanks to seas, oceans and the dinoflagellates that inhabit them as every other breath we take is supplied with oxygen from these hardworking but little-recognised organisms. Those tiny bioluminescent sea sparkles that are a joy to swim in are a visible part of chemical-energetic pathways from the sun via living organisms that give us air to breathe.
Bioluminescence and night sky in the Maldives. Credit: Kevin Wolf
“Those little bioluminescent sea sparkles that are a joy to swim in are a visible part of chemical-energetic pathways from the sun via living organisms that give us air to breathe.”
Where and how to see bioluminescence
Dark nights are best as you are better able to detect the low-level sparkle when you aren’t desensitised by brighter artificial sources of light- so away from urban areas, and wait a while in the dark without torchlight on before getting in the water. Bright moonlight can also diminish the ease of seeing bioluminescence in the sea, so cloudy nights can be perfect. However, I have seen bioluminescence swimming off the beach in Bournemouth where there is lighting along the promenade. Try Jersey beaches for waking up a glow from bristly marine worms in the sand. Look out for piddocks glowing in rock pools. North Anglesey is worth a visit around autumn equinox for a chance to combine Northern Lights with bioluminescence. Lough Hyne and Castlehaven Bay in County Cork in Ireland are reliably bioluminescent. Lough Hyne was more bioluminescent than when I went for a sparkly swim in Colombia.
As per going for a night swim to enjoy the Northern Lights, take the time and do the extra planning staying safe at night requires: a night swim should not be your first swim in that location. Remember to check the ease of entry and exit in daylight first or go with a swimmer who has swum there before and is familiar with your swimming ability. Be prepared to just enjoy a night-time walk if you reach the sea and conditions are not suited to the swim you have planned.