A whole lot of shakin’ going on

Benthic Habitat – Scottish Natural Heritage photo.

The Aquatic Noise conference in Berlin earlier this July, Louise Roberts delivered a presentation on “bio-tremulation” – a term she uses to express the way that various animals communicate through substrate vibration. We typically think of acoustical communication in terms of conveying information through oscillations invoked in air or water and into hearing organs. But bio-tremulation, by definition, is transmitted through other materials, like soil, wood, leaves, branches, even mud.

It turns out that a lot of animals use these mechanical communication channels – elephants communicating seismically through the earth, termites communicating through their wooden warrens, and moles and grubs communicating through their surrounding soil.

There are undoubtedly a lot of mechanisms behind this, because unlike the incredibly complex organs of hearing, vocalization, and phonation, tremulation can be generated and read by body parts. Toads use their toes, elephants use their feet, mole-rats use their foreheads. Pretty much any body part in contact with the surrounding substrate can transmit and receive substrate-borne vibrations.

There was an interesting anecdote told by friend and bioacoustician Ron Kastelein,[1] which illustrated how nuanced biotremulation can be. His firm was asked to find a way to diminish sea-floor damage caused the commercial flounder fishery. The current practice involves drag-trawling for this bottom-dwelling fish, a practice that rips up the benthic habitat, and brings up a lot of mud with the fish. If they could figure out how to get the fishes off the bottom and up into the water column, they could avoid damaging the seafloor, and bring up clean fish.

Their first cut was to expose the flounders to loud noise – to sort of “scare them up” out of the mud. But in the presence of noise, flounders “scare down;” that is, they wiggle a bit and bury themselves a bit deeper into the sand and mud. They then tried to “torment the bottom” with powerful a mechanical vibrator, but this just stimulated more “wiggle response” from the flounders who just buried themselves a little bit deeper.

They then introduced a little lobster onto the tank, and just due the bio tremulation of the lobster’s poiky little legs into the muddy bottom, all of the flounders lifted up off the bottom and into the water column – trying to escape their lobster predator.

So this illustrates the phenomenological experience of the flounders: Gargantuan substrate noise is not really a problem for them. What is a problem is the delicate scratching from the leg-tips of a predator.

So where do we take this when it comes to our inquiry into ocean noise? I have always had problems with how U.S. regulatory agencies have defined the metrics that trigger regulatory thresholds. They have always been defined in terms of how humans synthesize sound – missing so much of how various animals use sound – or in the case of the flounders, vibration. It wasn’t how loud it was, but what it sounded like.

Loud acoustical energy can damage and compromise an animal’s hearing organs, but most ocean animals – predator and prey alike, are more interested in the proximity of a sound source than how loud it is. So the primacy we give to the notion of “masking” – of an unwanted sound eclipsing the reception of a wanted sound, may not be so important to some ocean animals.

You need look no further than inter-tidal habitats, where waves come crashing in, rearranging rocks, and creating sheets and walls of noisy bubbles. There are undoubtedly a lot of communication channels at play in this chaotic habitat that are not masked by all of the crashing surf. I suspect substrate vibration plays pretty boldly in this habitat.

As Dr. Roberts concluded in her presentation, “The vibrascape is an undiscovered country…”

[1] Ron took a cue from a paper I wrote in 2002 to devise a way to cleanly harvest wild scallops.