This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Author's note: This is the latest post in the Wonderful Things series. You can read more about this series here.
One of the most amazing experiences of my life was immediately preceded by one of the scariest: I turned out my dive light in the ocean at night. I was near the top of the reef on the Dutch Caribbean island of Bonaire and I was not alone. My husband and dive naturalist and videographer Anna De Loach floated nearby. Fear gripped me nonetheless. Here I was in the middle of the ocean, purposely disabling the one sense I had to alert me to danger. Never mind that we were probably safer with the lights out (turning the light on in the ocean is a bit like ringing the dinner bell) -- it still felt like a deliberate act of vulnerability, bordering on stupidity.
Indeed, giant silver tarpon nearly as big as me looking to capitalize on our dive lights to feed had been escorting us before we turned out the light, and immediately after the lights went out one swam right in front of me -- I could dimly see its shadow. But there was something strange about this tarpon: it trailed a sparkling cloud of magical pixie dust like a unicorn in a kids' cartoon. I waved my hand in front of me: the ocean lit up with sparkles. I made pixie dust too! I knew enough to know these were tiny marine organisms that light up when disturbed, but it was exhilarating to see in person.
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But magical as they were, sparkling tarpon were not what we were here to see. When I was getting certified to dive five years ago, I overheard one of the dive instructors describe an extraordinary sight she had seen on Bonaire. She talked of a night dive without lights where she witnessed a mating display by animals called ostracods that made the ocean light up like a swirling galaxy of stars. I vowed to see this someday.
When I heard about and researched the Curasub (which I wrote about last month here and here), it occurred to me that Bonaire was nearby -- a mere 30 miles away from Curaçao, it turned out. Could I achieve both dreams in one trip? The answer was yes, thanks to the inter-island airline InselAir. But who could show me how to see the ostracods? I knew of Anna through her excellent marine natural history blog, The Blenny Watcher Blog, where she and her husband have a special fondness for tiny marine organisms. I thought that if any one could tell me how to see this natural wonder, it was her. To my amazement, it turned out that not only were Ned and Anna going to be on Bonaire themselves in September, Anna offered to take me to see the ostracods personally.
Now, at last, I was about to witness the event I'd waited so long to see. I still had not a very clear idea of what I'd see, though. As my eyes adjusted, I found myself not in a galaxy of stars, but in a field of shooting fireworks -- ghostly spirit lights rising from the reef. I had imagined it was the organisms themselves that glowed, like fireflies and most other bioluminescent animals I know of. But it quickly became apparent that some little creature was squirting luminescent blue goo into the water as fixed points of light in a rising, zig-zagging pattern. The points persisted like a string of glowing pearls for several seconds before fading, so that perhaps 8-12 points were visible at any one time in an ephemeral, biochemical constellation.
Occasionally, whatever was making these displays would start doing so right in front of my face, seemingly oblivious to the presence of a large, bumbling human hovering nearby. Sometimes the squirts remained fixed points of light, but sometimes they blurred into a line if the water was moving. If I waved my hand, they blew away like smoke.
After a time during which my jaw would have been on the coral heads had it not been clenched around a regulator, two thoughts came to me:
I cannot believe what I am seeing right now.
and
I wish Darwin could have seen this.
To my amazement, when we turned our lights back on, Anna wrote on her dive whiteboard "Not much tonight". Holy crap! I thought. If that's not much, what does a lot look like?
Evolutionary biologist Todd Oakley, who studies the visual system of arthropods, visited Belize in January to observe a different species of displaying ostracod for only the second time. "I was literally cursing into my snorkel in amazement -- "Holy @#$!!$ I cannot !@#!@ believe this is real"," Oakley told me via email. "And I almost never curse."
I could relate.
In 2008, Trevor Rivers and James Morin wrote, "The luminescent displays of Caribbean ostracods are the most complex found in the marine environment to date, and, based on hundreds of in situ observations of over 65 species (Morin and Cohen, 1991), suggests that they rival or even exceed those of terrestrial fireflies."
When I originally wrote this post, I did not think I would be able to show you what I was talking about. Anna has tried for years to film the ostracods to no avail. But thanks to photographer Elliot Lowndes, who worked as a camera assistant for the production company Ammonite with a special dim-light sensitive underwater camera to help film this National Geographic documentary, you can see a different species lighting up the nighttime Caribbean.
I should emphasize these are *not* the same ostracods or ostracod display pattern I saw in Bonaire, but this movie does do a good job of showing the general effect. Look for signalling ostracods from :30 -- :53. The video is worth watching for a while after that (they also show the same sparkling water phenomenon I observed) but it eventually wanders down a hunt-for-the-giant-squid fetish path, and to my dismay, never once mentions that it was humble ostracods that made "one of the little known wonders of the world" at the beginning.
So just what sort of creature is a bioluminescing ostracod?
This sort of creature.
Composite exposure of low light exposure and flash photography to reveal ostracod squirting light. Photo by Elliot Lowndes, www.mrlowndes.com; Used with permission.
If you don't find it adorable, I am going to revoke your Mammal Card.
The little creatures -- smaller than a tomato seed, I was told by Anna -- are called cypridinid (sip-ri-DIN-id) ostracods. Ostracods, also called seed shrimp, are tiny crustaceans enclosed within hinged valves. Cypridinids are only a few millimeters long and their valves bear a notch through which they stick their second antenna and row it like an oar to swim, crawl, or burrow. They have a simple eye in the middle of their heads, two compound eyes on either side, and seven pairs of appendages (NOTE: since I wrote this, Prof. Morin has informed me that technically, the "penis" is a pair of copulatory limbs, so technically ostracods have 8 pairs of appendages), though you wouldn't know it to look at them, since only the first two (one for sensing, and the aforementioned one for swimming) protrude. In this scanning electron micrograph (a high resolution picture taken by coating an ostracod in gold and bombarding it with electrons), you can see these two antennae and the approximate location of their compound eyes inside. Note also the delicate nubbly texture of the valve.
Scanning electron micrograph of a male Vargula contragula, showing texture of the valve and swimming antenna in valve notch. Although the valve looks opaque in this image, it is transparent in life. E indicates position and size of lateral compound eye, A-1 = first (sensory) antennae, A-2 = second antenna, M = mandible, CO= copulatory apparatus (i.e. penis). Fig. 2 from Morin 1985. Click for source.
In this image, all seven ostracod appendages are numbered and various other bits are labelled:
Major morphological features in the ostracod genus Vargula. Numbers 1-7 indicate the paired limbs. Excerpt of Fig. 1 from Morin 1986. Click for source.
Cypridinid ostracods that produce bioluminescent mating displays occur in only one place on Earth: the Caribbean Sea.
"Nearly every evening of the year across the entire Caribbean, toward the end of twilight, shallow water cypridinid males leave their benthic haunts, enter the water column, and become demersal plankters for about an hour as they attempt to attract and mate with females." -- Morin and Cohen, 1991.
Something very similar occurs nearly every evening of the year in the bars of the Caribbean as well.
Amazingly, it wasn't until 1980, when James Morin and a colleague caught them at St. Croix in the Virgin Islands, that it was even known what creature was responsible for these displays. "In order to detect the displays, one must be in the proper underwater habitat in the dark, at the precise time of signaling; this is probably why the displays went unnoticed for so long," Morin wrote in 1991. "Once we knew where and when to look, we discovered signaling ostracods on every major reef system we examined in the Caribbean; they are abundant and ubiquitous."
Morin and his colleagues have so far found over 65 species of cypridinid ostracods in the Caribbean, but only about a third have been formally described. I contacted him to see if I could find out more about the species that I saw on Bonaire. He has visited Bonaire, but according to him it was only a quick trip and he only saw two species: an upward displaying species in just a few meters of water, and a downward displaying species in the reefs to about 40 feet. Both of those species belong to an as-yet undescribed genus. Since the description of the species I saw differs from these two, he said it's possible I observed a new species. If I'd known that when I was diving, I might have passed out from excitement, so it's probably better I found out later.
Ominously, Morin notes that bioluminescent ostracods are threatened by increasing shoreline development, since they will only display when the sky is nearly dark. Morin has been unable to find ostracods who display in shallow water off coasts brightly lit by cities -- even in places where they were formerly collected like Montego Bay, Jamaica. Yet in areas that were sufficiently dark, Morin and his colleagues could see and collect displaying ostracods while snorkeling with ease.
Cypridipids glow the same way other bioluminescent organisms do -- better living through chemistry. The chemicals that produce light are synthesized in an organ made of long secretory cells, each cell spanning the whole length of the organ. The cells end in "nozzles" at the upper lip of the ostracod. Far from having light-up butts, these little crustaceans have glands that squirt glowing goo from their face. Some of these glands secrete a protein called luciferin, others secrete the enzyme luciferase. Muscles surrounding the gland seem to physically squeeze the compounds out of the gland cells like toothpaste from a tube. When luciferin and luciferase mix in seawater with oxygen, light appears.
Because cypridinids secrete their glowing chemicals into the water, multiple ejected stars can simultaneously shine, greatly expanding the possibilities for pyrotechnic display and messaging. In many places in the Caribbean, several species may display at the same time over different habitats at different heights in the water. Each species has its own "fingerprint" of habitat, height, time of day, length of display, spacing, and the particular pattern of glowing spots it makes. Some males swim up; others down. Some swim vertically, diagonally, or horizontally.
In one species found in the San Blas Islands of Panama, males signal synchronously with extremely brief flashes as a group. "The effect is dramatic: huge numbers of tight clusters of brief flashes repeatedly pulsing upward toward the sea surface in approximate synchrony with other clusters across vast expanses of grass beds."
You can see a graphical representation of this pattern -- and several other cypridinid osctracod patterns -- at far left in the figure below. The zig-zagging display in midwater over the sand most closely resembles what I saw on Bonaire, although the displays I witnessed seemed to ascend vertically, not diagonally.
Courtship display pattern of 11 luminescent species of Caribbean cypridinid ostracods in the nominal genus Vargula from Panama. Adapted from Morin and Cohen 1991 by Todd Oakley. Used with permission.
The males swim very fast while displaying -- an average of 40 body lengths per second -- but come to a near-screeching halt to eject bioluminescent pulses, possibly to keep each squirt a neat fixed point. Amazingly, at least some species also seem to swim in a spiral, ejecting light at regular intervals along their dizzy path. The spiralling swim effectively slows the male down and apparently allows females to catch up to them if they like what they see and want to inspect the goods. They may also employ a two part display, with the first pulses slower and later pulses faster, perhaps helping the female triangulate the male's location.
All these efforts may be foiled by sneaking males who swim upward with the displaying male and may attempt to make a play for any females that swim their way. Scientists know about these cheaters because when they sweep driftnets through displays or shine lights on them, they find not just one male, but several.
Cypridinid ostracods don't just use their luminous powers for attracting mates and dazzling science bloggers; they also use them for repelling predators. When attacked by fish or crustaceans, males and females alike secrete a "brilliant and massive (several cm) bomb-like cloud of luminescence that persist for many seconds, often more than a minute, around and within the predator." Morin wrote in 1986 (italics mine). "It sometimes results in the predator regurgitating the ostracod and probably startles ('boo' effect) and temporarily blinds (flashbulb effect) the predator." The glowing cloud around and in the predator may attract a meta-predator, thus giving the cloud a burglar alarm function as well. During this secondary attack, the first predator may expectorate the ostracod, which survives the whole ordeal, magical cuteness intact.
References
Morin J.G. (1986). Firefleas of the Sea: Luminescent Signaling in Marine Ostracode Crustaceans, The Florida Entomologist, 69 (1) 105. DOI: http://dx.doi.org/10.2307/3494749
Morin, J.G. and Cohen A.C. (1991). Bioluminescent Displays, Courtship and Reproduction in Ostracodes. In: Crustacean Sexual Biology (R. Bauer and J. Martin, eds.), Columbia Univ. Press, pp. 1-16.
Rivers T.J. & Morin J.G. (2008). Complex sexual courtship displays by luminescent male marine ostracods, Journal of Experimental Biology, 211 (14) 2252-2262. DOI: http://dx.doi.org/10.1242/jeb.011130
Cohen A.C. & Morin J.G. (2010). It's All About Sex: Bioluminescent Courtship Displays, Morphological Variation and Sexual Selection in Two New Genera of Caribbean Ostracodes, Journal of Crustacean Biology, 30 (1) 56-67. DOI: http://dx.doi.org/10.1651/09-3170.1