1993 | Carrie Bow Cay, Belize
Snapping shrimp (Synalpheus regalis) bubble stunning
Southwest of Belize City, near the border with Guatemala, sits the ancient Maya city of Caracol. Its towering temples and numerous houses and fields sprawl over hundreds of square kilometres, marking it as one of the most important regional centres of the Classic Maya period, reaching its peak more than a thousand years ago.
Like all larger Maya cities, Caracol engaged in a series of wars, especially with their rival Tikal (which itself stood in for the rebel jungle base in the original 1977 Star Wars film). To do so, they maintained a central governing power and an army, as well as a host of craftspeople, scribes, farmers and other defined social roles—over 100,000 people in all.
Why am I telling you this? Well, now head due east from Caracol. You’ll soon cross the coast, but just keep going until you reach the edge of the Belize barrier reef, the point where the shallow shelf drops down into the deeper ocean. There you’ll find Carrie Bow Cay, and a field research station maintained by the Smithsonian Institution on a tiny, idyllic sand island.
Which is where marine biologist J. Emmett Duffy first found and described a millimetres-long crustacean, with a social life oddly similar to that over at Caracol. And tool use of astronomical power.
Sponge monarchs
In the early 1990s, Duffy methodically collected sponges growing in the warm waters around Carrie Bow Cay. Keeping them submerged until reaching his field lab, he would then systematically identify and count all the little creatures that had made a given sponge their home.
In particular, he was interested in the shrimp that occupied and moved through the nooks and crannies. Collectively, members of the same species in the one sponge made up a self-contained shrimp city, interacting with all their fellow shrimp-folk on a daily basis.
What Duffy didn’t expect, however, was that at least one of these species—which he named Synalpheus regalis—wasn’t just a colony of unrelated animals. Instead, each sponge contained a single shrimp queen, who gave birth to all the colony members; hence the ‘regal’ part of the species name. In turn, those other city-dwelling shrimp took care of maintenance, food gathering, and defence of the group. (As yet there are no signs of scribes, but the parallels with Caracol are there. Plus, caracol means ‘shell’ in Spanish, surely that’s worth something?)
This kind of highly-organised living arrangement—eusociality—was unknown in any marine animal at the time Duffy announced it in 1996. You may recognise it as the kind of thing that social insects like ants, bees and termites do, as well as some mole rats. Yet here it was, independently evolved in tiny crustaceans off the Belize coast.
And this is where we reach the tool-use part. Because those non-breeding shrimp that are tasked with patrolling their sponge city are literally armed with a massive gun.
Snap judgement
There are hundreds of species of snapping shrimp in the family Alpheidae, and most are not eusocial. But they do all have an extraordinary claw. Technically it’s the first pereiopod—if you’ve read my piece on carrying crabs you'll recall that the pereiopods are the animal’s walking legs. In the image at the top of the post it’s that unmissable, ridiculously large claw. For S. regalis, Duffy found that this claw is usually as long as, or longer than, the male animal’s main shell or carapace.
Here’s a selection of other Alpheidae. Note that the main claw can be on the left or right side, and in fact the animal can swap sides if it loses the claw in an accident:
Comparison of related species has shown that the large claw evolved just once in shrimp history. That feature then prompted an adaptive radiation, which is when changed conditions allow for expansion into new ecological niches, leading eventually to new species.
The claw has two main parts, a stable ‘anvil’ section known as the propus, and a moveable hammer called a dactyl. The dactyl is attached by a hinge, and the shape of the base of that hinge means that it can be pulled back and held in position, just like a cocked gun. You can see that cocked position in the central shrimp at the bottom of the image above.
The dactyl has a protruding bump—or plunger—that matches a corresponding socket on the propus, so that when it’s released, the plunger snaps into the socket with astonishing speed, and abruptly pushes out the water that used to be there. To help you visualise what’s going on, this image from Michel Versluis and colleagues shows the set-up for an Alpheus heterochaelis shrimp. This species is larger than the eusocial Synalpheus regalis, but you get the idea:
But wait—as cool as this might be, this isn’t tool use is it? We’ve established that using parts of your own body doesn’t qualify. So have we just gone through Maya temples and picked apart sponges for nothing? Ok, I hear you. Let’s get to the final twist in our tale.
A stunning collapse
Until a few decades ago, scientists thought that the sudden collision between the two parts of the claw caused the loud snapping sound that gives the animal its common name. (The sound of thousands of these little castanet players is common in shallow tropical waters around the world, to the point where it interferes with human sonar equipment.)
But the introduction of high-speed cameras has overturned that idea. Filming at over 40,000 frames per second, we can see that it’s not the click of shell on shell. Instead, water jets out of the claw socket at such a rate that it produces an area of extreme low pressure just ahead of the claw tip. Within a fraction of a second, tiny bubbles in the water expand into the low pressure zone, and then collapse again to generate a local shock wave, a process known as cavitation.
It’s that collapsing bubble that we hear, and it’s loud. With the claw shutting in less than a millisecond, the water jet emerges at several metres per second (remember the animal is just centimetres long). The bubble collapse happens only a few millimetres from the end of the claw, and it has been measured at around 200 decibels—roughly equivalent to the loudest sound that NASA ever recorded from its Saturn V rocket (the ones that took people to the moon). Temperatures inside the bubble momentarily reach around 5000 degrees Kelvin, similar to the surface of the sun or the centre of the Earth’s core.
This video, from the team that first discovered shrimp cavitation, includes their original high-speed footage, as well as (if you’re interested) some of the calculations they did:
This, then, is the tool use we’ve been waiting for. The shrimp actively manipulates the water around it to generate a shockwave that can stun or kill its prey. Control is achieved by the fact that the water jet is directed out of a thin channel in the giant claw. The material used (air/water) is only temporarily configured into a tool, but that doesn’t rule it out as tool use, since many plant tools likewise naturally decay after use, albeit on a slightly longer scale.
Researchers have found that the shrimp also use this mechanism to communicate with members of their own species, although when doing so they generally stay outside the zone of cavitation damage to avoid death or serious injury. Other species, including the crabs, fish and other crustaceans that these shrimp may consume, are not given that courtesy.
The same tool comes in handy when excavating a burrow, with the animal using the repeated shockwave to dig into the substrate. And the claw’s owner is itself protected somewhat by an evolved ‘orbital hood’, a forward projection of its shell that partially or completely covers its eyes. Of the two features—hood and claw—the hood evolved first, and it reaches its greatest diversity of forms in the Synalpheus and Alpheus genera we’ve been discussing. Evolution is blind, but the shrimps needn’t be.
For the armed guard social class back at the Synalpheus sponge in Belize, though, the weapon is more than just a handy communication or digging tool. It helps encapsulate the animal’s life purpose. Duffy describes an encounter between a Synalpheus regalis guard and another snapping shrimp species:
contact between a resident and a ‘foreign’ intruder…generally resulted in an intense battle, with both individuals repeatedly snapping at one another with their powerful major chelae (claws). This continued sporadically until the intruder was killed.
The defensible, enclosed nature of the shrimp home sponge is analogous to the nests of eusocial insects, and Duffy posits that the snapping claw plays a central protective role, similar to the sting of bees and wasps. Or perhaps the might of a Maya army.
Further viewing: a few years ago the BBC produced this short video, in which you can watch a snapping shrimp first stun its prey, then drag it back into the shadows…
And if you’re ever in Belize, why not visit the beautiful city of Caracol, and take in the view from the Sky Palace?
Sources: Duffy, J.E. (1996) Synalpheus regalis, New Species, a Sponge-Dwelling Shrimp from the Belize Barrier Reef, with Comments on Host Specificity in Synalpheus. Journal of Crustacean Biology 16: 564–573. || Duffy, J. E. Eusociality in a coral-reef shrimp. Nature 381: 512-514. || Rocha, L. et al. (2014) Specimen collection: An essential tool. Science 344: 814-815. || Anker et al. (2006) Morphological phylogeny of alpheid shrimps: parallel preadaptation and the origin of a key morphological innovation, the snapping claw. Evolution 60: 2507–2528. || Versluis, M. et al. (2000) How Snapping Shrimp Snap: Through Cavitating Bubbles. Science 289: 2114-2117. || Herberholz, J. & B. Scmitz (1999) Flow visualisation and high speed video analysis of water jets in the snapping shrimp (Alpheus heterochaelis). Journal of Comparative Physiology A 185: 41-49. || Lohse, D. et al. (2001) Snapping shrimp make flashing bubbles. Nature 413: 477-478. || Chak, S. et al. (2017) Evolutionary transitions towards eusociality in snapping shrimps. Nature Ecology & Evolution 1: 0096. || Allgood, D. (2012) A Brief Historical Survey of Rocket Testing Induced Acoustic Environments at NASA SSC, https://ntrs.nasa.gov/citations/20120003777.
Main image credit: National Geographic, https://www.nationalgeographic.com/news/2018/03/animals-shrimp-oceans-queens-evolution || Second image credit: Rocha et al. (2014) || Third image credit: Versluis et al. (2000) || Caracol image credit: Pgbk87, via Wikimedia Commons