Today’s title mashes up not one but two Monty Python bits.
Following up on last week’s interview with geomythologist Tim Burbery, while doing research for a submission to this anthology I watched a fascinating French documentary about one inspiration for the legendary sea serpent. The link above is to the production company, which has a free preview, but you can also watch the whole film and even chat with the director Bertrand Loyer on YouTube. The first comment is from him.
These giant oarfish occasionally wash up on temperate beaches around the world, and in Japan they are seen as harbingers of doom, precursors to earthquakes and tsunamis. Is this an example of Burbery’s useful mythological knowledge, or a common superstition? Seismic science has so far not found any direct link, but the documentary makes much of how far sound, especially low frequencies, travels through water, and how even cleaning the optical sensors on their buoy with a toothbrush would vibrate the long Kevlar cable attaching it to the seabed and draw fish in from the open ocean surrounding it.
This seems like a fine opportunity to geek out neuroscientifically, but first,
A little basic physics
I just learned this morning that there are two different ways to hear sound underwater.
Sound is both a pressure disturbance and a motion of particles, generated by a sound source (basically a structure moving back and forth). These two features of sound, pressure and motion, goes hand in hand: it is the movement of particles towards and away from each other that generate the increased and decreased pressure. When we talk about human hearing, we usually think of the pressure component. It is the sound pressure fluctuations that put the tympanic membrane into motion, driving our middle-ear ossicles and eventually pumping the fluid in the cochlea, where the hair cells are located that detect sound. Like all other mammals, we are thus primarily sensitive to the pressure component of the sound field.
For fish, the situation is radically different. The inner ear of a fish consists of calciferous structures, called otoliths, situated close to hair cells that can feel the movement of the otolith relative the surrounding fish body. When the fish is rocked back and forth in a sound field the heavier otolith lags behind – and the hair cells are bent and register the motion. Thus, the fish ear is primarily sensitive to the particle motion of the sound field. It is possible to create a sound field in the laboratory which only consists of sound pressure, with very little particle motion. In such as sound field, a mammalian ear is still registering the sound very well, whereas many fish cannot detect any signal at all.
The fish ear is fundamentally a motion detector.
So as a refresher,
distance is how far you move,
velocity is how fast you move (distance / time), and
acceleration is how fast your velocity changes (distance / time squared).
Gravity is mathematically an acceleration, because the longer you fall, the faster you fall. Gravity keeps speeding you up until terminal velocity, when the air is pushing back on you hard enough to cancel out the acceleration.
Now the Neuro
In addition to the coiled up version inside their heads (the cochlea) fish also have a lateral line running the length of their bodies, which works somewhat the same way.
Following the general principle that long wavelengths need long detectors, it makes some sense that a 10-meter long fish might be better at hearing very low frequencies like seismic tremors. But wouldn’t that make them better at getting out of the way? It’s not a questions that’s likely to be answered any time soon, because it’s not like we could get a giant oarfish into a lab to test it. Maybe a baby, but that would be smaller and kind of defeat the purpose. Simulation might be the best way to approach the question.
If you search “giant oarfish” online you will find many, many pictures. This one is my personal favorite, drawn by Georges Cuvier in 1828. If you zoom in, you can see the black dots of the lateral line starting on the head up near the eye and sloping down towards the bottom.
In the ocean, the giant oarfish swims vertically, and as shown in the documentary sometimes holds those very long trailing pectoral fins out to the side in a T shape. According to them, those “oars” are where the fish got its name.
They migrate up and down through the water column, following plankton. Up at night, down during the day.
Lights in the Deep
They go to some trouble in the documentary figuring out whether the oarfish can glow like other deep-sea critters. It turns out that unlike a firefly, they are not making their own light. They swallow glowing bacteria, and instead of digesting them, they sequester at least some of them in special organs somewhere on their bodies. In the oarfish these are on the front of its head.
I kept this super-basic for a general audience, but the links go into much more detail, which I’m happy to discuss.
Just to wrap it back to the beginning, here’s a little quote from the above linked interview with Mr. Gilliam.
People are losing their sense of humour, and that, to me, is probably the most important sense. Sense of touch is very important, sense of taste also – but sense of humour is more important. You get to the point where people are frightened to laugh. ‘Oh, no, you’re making fun of somebody!’ No, I’m making fun of humanity, and we are an absurd species of creatures.
More absurd than a shiny ribbon of a fish, a sequined sea serpent with streamers?
Without a doubt.
Oarfish don't have scales, so "sequined" is the wrong metaphor, despite the Marvel-ous alliteration potential. Given the Xmas season, a long strip of tinsel might be more appropriate.