A Wonderfully Luminous Sea

Today the Beagle returned to the sea, sailing along with billions of raindrops completing their cyclic journey from ocean to land and back to the ocean again. Darwin makes a few observations:

“All day we have been sailing within two or three miles off the coast.— For 40 miles it has been one single line of sandy hillocks, without any break or change. The country within is uninhabited, & ships never frequent this track, so that it is the most desolate place I have ever visited.— At sunset, before anchoring, we came rather suddenly on a bank, & were obliged instantly to put the ship up to the wind.— This fine weather is of the greatest importance to the surveying & as long as it lasts, sailing slowly along the coast is sufficient for all purposes.” (Aug 22)

More interestingly, in his Zoological Notebook Darwin records a light show in the ocean – one performed by bioluminescent critters.

“Between Points St Antonio & Corrientes: the sea was very luminous: light, pale, sparkling, but not as in Tropics either milky or in flashes.— The Luminous particles passed through fine gauze.— In the water were some minute Crustaceæ of the genus Cyclops. I should not be surprised if these added to the effect.— During the day the sea has abounded with Dianœa.— & I find these when kept in water till they are dead render it luminous.— can this be the cause of the appearance in the ocean.” (Zoological Notebook)

Bioluminescent seas in Hawaii (from Maui Magazine)

He includes several later follow-up notes trying to uncover the cause of this bioluminescence.  For example, on September 4^th he adds:

“I observe that during this night, Crustaceæ of the Schiropodes & some other Macrouris, appear to abound on the surface, whilst during the day few can be taken. … certainly many crustaceæ are luminous may this not explain help to explain the phenomenon of the luminous sea.” (Zoological Notebook)

Part of a page from Darwin’s notebook, showing the entry in his own hand – the (a) and (b) in the margin refer to the additional notes added later (from darwin-online.org.uk)

And on October 23^rd he writes:

“Sea wonderfully luminous; milky when seen in the mass; sparkling in numerous bright spots when seen in a tumbler; but I could not succeed in making by agitation, water in a watch glass show luminous particles, although certainly abundant in it.— The breakers & bows & wake of ship, i.e. when air acts on water, is luminous: this was after a heavy sea — Can this by destroying numbers of small animals be the cause” (Zoological Notebook)

It is this patience that typifies Darwin throughout his career.  His wonder of natural world leads him to make careful (often detailed) observations, sometime slowly over many months or years.  Over time he gradually tries to piece together an answer – a hypothesis – modifying his thoughts as new observations are made.  He truly was a great scientist.

I don’t think he was ever able to fully answer this question in his lifetime, but we now know a bit more about the chemical reaction that leads to bioluminescence.  Basically it is the result of a chemical reaction that releases energy in the visible spectrum of light.  An organism (a plant, animal, fungi, bacteria – there are all types of bioluminescent organisms) releases the chemical luciferin which reacts with oxygen to release energy.  This reaction would be relatively slow, and not very “bright” if it were not for an enzyme called luciferase. Luciferase acts as a catalyst speeding up the chemical reaction (thereby resulting in more energy released, i.e., a brighter glow).  There are some slight modifications, but this is also the reaction that occurs in fireflies (see Darwin and the Shining Ones).

I suppose one might argue that Darwin was less interested in the chemistry of the glowing sea then the general ecological reason why the animals glowed (and what specifically was doing the glowing). Critters glow for a lot of reasons – some to lure prey, others to distract predators, lure mates, or just plain communicate.  And it is not uncommon for surface water marine organisms that glow to be triggered by the wake of a boat or some other animal swimming through the water.  For example, on the night of the big lightning storm back in late July, Darwin saw that the, “the sea was so highly luminous, that the tracks of the penguins were marked by a fiery wake” (Voyage of the Beagle).

I’m sure there would be many more light shows to come, and that we have not heard Darwin’s last hypothesis on the topic. Stay tuned… (RJV)