Those firey bodies

Strange beings lurk in the deep sea. Weird life, unlike anything found near the ocean’s surface, and bizarre enough to make anything lucky enough to be dwelling on land spit out its morning coffee in disgust.

A pretty high proportion of this deepwater life glows. In areas of the ocean termed ‘mesopelagic’ (between 500 and 1,000m deep) there are no obstacles to hide behind, no floor to hide on, and yet some weak downwelling sunlight ensuring that complex beings need to retain some level of vision to survive. This combination of characteristics makes bioluminescence a particularly valuable tool, both for concealment, communication and combat (Young 1980). All known bioluminescence is either produced by bacteria housed within the animal in question, or produced by specialised organelles of the animal itself.

My current research involves just this; looking into how Pyrosomes bioluminesce, working out what makes them glow.

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Pyrosomes are a colonial animal, meaning what you see in the image above is a colony of tiny individual animals, all stuck together. Each of these animals is called a zooid.

There are eight known species of Pyrosomes, all of them forming colonies shaped like long tubes, closed at one end. Each individual zooid sucks in water from the outside, at the top of each zooid you see in the image below.

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This water is then passed through the feathery structure you see underneath, where they filter out any food particles, before expelling it into the hollow inner of the colony. The combined force of the water from each zooid being ejected into this cavity and so out of the colony’s rear thus propels it along, in a wonderfully simple coupling of both feeding and movement.

Pyrosomes are known to produce one of the most spectacular bioluminescent displays of any animal; upon seeing them in 1849 the great Thomas Huxley was moved to write “I have just watched the moon set in all her glory, and looked at those lesser moons, the beautiful Pyrosoma, shining like white-hot cylinders in the water.”

Despite the beauty of the blue-green flashes that these animals produce, little is known about how it comes about, perhaps simply due to their obscurity (every time I tell another biologist what I’m studying they inevitably say ‘what? What’re they?!’), and so they are little-studied.

Each zooid has a light organ flanking the siphon on their exterior, but strangely these are not neurally connected to the brain. Instead, when the zooid is stimulated either by light or physical movement, the tiny hairs in its gill basket cease moving. This reduces blood flow to the light organ, which then begins to luminesce in response. As each zooid luminesces in response to light, a wave of light emission quickly travels through the colony, sustained for many seconds.

Because the gills of all the zooids in the colony have now stopped sucking in water, the colony begins to sink. Pyrosome colonies are often found in groups, and so any nearby will begin to luminesce and sink in turn, in a signal passing through the group. This is both incredibly simple and very effective at helping them either avoid areas with lots of sediment in that would clog their gills, or deter predators-your prey suddenly glowing blindingly all around you is enough to scare most away, having been lit up to any larger predators with a flash that may as well sound ‘dinner!’

However we still don’t know how Pyrosome light organs actually work, simply that when given the signal, they light up. This is what my research is looking at-trying to define if they house symbiotic bacteria that bioluminesce, or if the light organs contain organelles that luminesce; as seen in many fish.

The light organs have been found to use enzymes similar to those found in Photobacterium; a common light-emitting bacteria, but they have very different appearance to any known bioluminescent species of bacterium. Attempts to grow these cells outside of the light organs have failed, which may indicate that either they are bacteria so specialised that they can only live inside their host, or that they are in fact organelles.

If they are bacteria, that they look very different to all known bioluminescent bacteria is unsurprising-as they can only live in Pyrosomes they will likely have been isolated from the outside world for a very long time.

Most of my research is looking at the genetics of these cells, using their DNA to identify if they are in fact bacteria, and if so to try and identify them.

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So why am I looking at this subject? Perhaps one of the main answers, as with most science, is sheer curiosity: there’s a mystery to be solved, and it’s interesting discovering how this works for curiosity’s sake alone. The method Pyrosomes use to control bioluminescence is unknown in any other groups of animals, and so to discover just what it is that they are controlling in this way would help fill a tiny part of the great gap that is humanity’s current ignorance of the natural world.

However such faux-noble reasoning aside, bioluminescence is important to humanity. So important that in 2008 a team of researchers was awarded the Nobel Prize for chemistry for their pioneering series of work on a bioluminescent jellyfish. Through their subsequent work on the protein this jellyfish species uses to luminesce, a wide variety of laboratory techniques have been developed, helping scientists to discover many new phenomena, and help develop treatments for many diseases. Though it would be quite optimistic to expect this research to spawn such a grand legacy, this sort of curiosity-driven work is at the root of innumerable scientific advances.

So who knows, perhaps these bizarre creatures from the deep may save us all.

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First photo by Nick Hobgood, all other photos by myself.

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About hammerheadbat

A conservation biology PhD, I spend my days studying tropical deforestation, bats, and wider ecological questions.
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