Photo: An ostracod releasing a bioluminescent mixture. Elliot Lowndes
I just saw some tiny flashing lights in the water at Carrie Bow. This phenomenon is known as bioluminescence which is when an organism produces and emits light (e.g. fireflies).There are many bioluminescent marine organims including fish, squid (even my past study species, the dumpling squid), crustaceans, jellyfish and tiny single celled algae. The light is produced via the chemical reaction of luciferin (a pigment) with oxygen. An enzyme (luciferase) is required to speed up the reaction.
Most marine species produce blue or green light via this reaction. This is because these are the wavelengths that travel the greatest distance through seawater. If you’ve ever had a chance to dive, you’ll notice that red is the colour that disappears first. However, there are some species that produce red, yellow or even infrared light such as some fish, worms and jellyfish.
But what are they using this light for? Some species (including dumpling squid), use it for counter illumination. This means, that when they are swimming around at night, they emit a some light downwards as camouflage. This diminishes their shadow so that any predator looking up won’t see (and try to eat) a squid shadow above them. If you’ve seen Finding Nemo you will be aware of another use: prey capture. Anglerfish lure prey towards their mouth with a glowing bulb.
The final use I want to mention (although there are other uses such as distraction or use as a warning signal), is courtship. Now, I am not sure what was in the water tonight, but it could have been ostracods, a small marine crustacean which is found around here. Male ostracods squirt out a bioluminescent mixture into the water. They do this about an hour after sunset over shallow seagrass beds (which is where I saw it tonight). They’re doing this to attract females.
Usually males produce several pulses of light, the pattern of which varies among species. The cool part is when he swims upwards in a spiral, releasing light pulses a regular intervals. The female uses the position and the timings of the light pulses as a way to determine the males position, intercept him and mate with him. Of course, she only does this if she likes his light pulses. It amazes me that such a tiny organism can use these light pulses to calculate the future position of a male, the distance she is from him, and then the direction she should swim in to intercept him. Pretty awesome!
Even though I’m not sure that it was ostracods in the water tonight, I would like to think that it was and that I was watching some little male love songs in the seagrass below.