I’ve related this in part elsewhere on the site, but I thought it deserved its own post, especially since it was one of the more memorable experiences from a few years ago.
When I lived in Florida, I started “maintaining” a small saltwater aquarium to house photo subjects and interesting marine critters. Being close to both the ocean and the Indian River Lagoon, a large isolated saltwater sound inside the barrier islands facing the Atlantic, I had lots of access to the aquatic residents. I could bicycle down to the lagoon to snorkel, and did this frequently. Mostly, what I saw were crabs, barnacles, and oysters, but I had encounters with dolphins, manatees, stingrays, and jellyfish too.
Now, I put “maintaining” this aquarium in quotes above because I did not manage this as most people manage saltwater aquariums with their exotic tropical fish. There was no careful measuring of pH levels, no purchases of salt and mineral mix, no consideration of compatible species, and no filtering. Instead, I obtained water directly from the lagoon a few times a week, simply aerated or circulated, and whatever I caught that I had an interest in had to fend for itself. I had a basic heater to maintain the water at roughly the same temperature as the sound, and not much else. I couldn’t filter, since many species that I caught were filter feeders themselves and relied on microorganisms in the water.
I had no underwater photography gear, so the tank allowed me to photograph various small catches under controlled conditions, where lighting and setting, as well as water clarity, were not up for grabs. While most critters stayed only a few days at best, several of my subjects became long-term residents, as keeping them required little more than fresh water and seaweeds, and they thrived surprisingly well. I’m pretty ambiguous about fish, but I like crustaceans, so I had several porcelain crabs, a handful of fascinating little grass shrimp, numerous small anemones, and a tiny flounder that I caught by chance, slightly larger than a quarter.
One catch stymied me, though. Basically, it was a 8 cm (3 in) “lobster” in deep green fading to blue-white on the underside, with one pincer being huge and misshapen, tumorous-looking. It bore some resemblance to a crayfish, but slimmer, and I was pretty sure there were no saltwater crayfish. Web searches turned up nothing – what do you search on, especially when, like me, you have no knowledge of marine biology? It was very shy and remained hidden, so I kept it for a few weeks as I worked out ways to photograph it in a decent setting.
Some nights, down at a dock collecting water and whatever nocturnal denizens I could spot, I heard a very sharp clack! nearby, like a small stone hitting a rock at high velocity. Absolutely no one was around, and the area I would hear it from, less than a few meters away, was often devoid of rocks above the surface. I found it hard to believe that anything beneath the surface would produce a sound in this pitch (usually water reduces the pitch,) so this remained a mystery. I was keeping a loose journal of observations at the time, and in that I speculated on things like archer fish, the species that hunts insects above the surface by spitting water droplets at them, but even then, this seemed implausible.
Sometimes, while snorkeling, I’d hear it too, and one day I got a series of them while trying to extricate a small crab from a crevice in a piling. Aha! But the pitch seemed a little off, and certainly not as loud.
Then, one night when I was almost asleep, the tank produced a sound like a marble hitting the side, and I immediately got up to see if one of the glass sides had fractured. But no cracks, leaks, or chips were visible in the slightest, and nothing that could fall over to strike the side. I had several crab species in the tank at this time, but able to produce so sharp a noise? Naaahhh…
You have, of course, already made the connection since I didn’t structure this tale as a murder mystery, but this was an ongoing curiosity to me for a while – until I stumbled across an illustration for a pistol shrimp, sometimes called snapping shrimp, while doing some online research. And suddenly realized just how cool one of my residents really was.
Pistol shrimp (Alpheus heterochaelis) have a unique way of obtaining food. That misshapen claw (chelae) houses a simple mechanism that allows the thumb (dactylus) to open and lock in place, cocked like a gun, to be released under great muscular force when a food source is nearby. The shape of the claw and the velocity of it slamming closed produces a shock wave that stuns nearby prey, whereupon the shrimp can amble over and eat it leisurely – and of course, the shock wave was what I was hearing. The sound has to be heard to be believed, since it’s surprisingly loud. Bear in mind that this method of hunting is not a contact system but works at short distances, most especially on prey that can be affected by pressure. I cannot attempt to explain this as “non-aggressive,” since this is predation and not scavenging or foraging, but the shy pistol shrimp doesn’t have to throw down with its prey in full-contact sport, but opens its can of WhupAss (well-shaken it would seem) from a safe distance, bringing a gun to what is almost always a knife fight throughout the environment.
Even more interesting, the force actually causes a cavitation bubble to appear (cool video,) momentarily creating a region of vacuum in the water within the immediate vicinity of The Claw, and this can at times even produce a tiny flash of light called sonoluminescence This is not visible to the naked eye so I never witnessed this myself, and in fact only got to see the behavior in action once. This occurred when I introduced a fist-sized thinstripe hermit crab (Clibanarius vittatus) into the tank for the afternoon, to obtain some images. While the crab sat, minding its own business atop a rock, the pistol shrimp obviously had some difficulties with its presence. I watched as, several times, the shrimp eased out from under a rock, slipped up to the hermit crab from behind, and placed The Claw right against one of the crab’s legs, firing off a shot at point-blank range before darting back under cover. The crab, perhaps with smug condescension, showed absolutely no reaction at all to this treatment. No, I didn’t get any pics of this happening, since my observation angle was poor and setting up the camera and lights likely would have halted the action.
A perfectly legitimate question is to ask how something like this could have evolved (yet it is illegitimate, when unable to fathom this, to assume it could not have.) But grasping appendages and claws to break apart food are common, so the significant difference here is the speed and shape of such producing a shock wave. Yet almost all chelae would, to some extent, and sounds can be used for communication and threat displays as well, so have multiple reasons to be refined by evolution. Anything that produces a sound (which is really just pressure waves in the medium, in this case water) that can affect something else at short distances can obtain prey without contact, and even a tiny distance is an advantage. It is also fairly likely that, long ago, the prey species were easier to stun too, and both the defensive pressure resistance of the prey, and the sonic ability of the pistol shrimp, increased in competition with each other over thousands of generations of each. I’m just sorry I can’t be around in a few thousand more generations to watch blaster shrimp nailing their food with sonoluminescent laser bolts…
If you were sharp-eyed above (or have seen the image in the photo gallery of the site,) you noticed the eggs carried in the pleopods (swimming appendages under the tail) of the pistol shrimp in my tank. Nothing ever came of this, possibly because there was no male handy to fertilize them. Below, a family portrait from the aquarium, showing a transparent grass shrimp (atop the rock and facing the camera), two tiny hermit crabs, and even the flounder, mostly buried in the crushed shell at bottom – directly under the larger hermit crab can be seen one of the googly eyes, and the darker region to the right of that is the top surface of the fish.