Leaf-cutting Snapping Shrimps

Snapping shrimps, also known as pistol shrimps make more underwater noise than any other marine animals. Look up why snapping shrimps make loud popping noises and you will almost certainly find some dazzling claims such as “While hunting, pistol shrimps open the hammer part [of their nippers] and then release it, allowing it to snap into the other part. This release is extraordinarily fast and results in an enormously powerful low-pressure cavitation bubble. The bubble that is created serves the purpose of stunning a prey. How does that work? The bubble shoots out at a speed of 62 miles an hour, reaching a temperature of 4700 degrees Celsius for a very brief period of time. This temperature is actually close to the temperature of the Sun.” Indeed, it is the holy grail of videographers to capture this ultra high speed phenomenon as the clips listed below show.

BBC Earth Unplugged - Slow motion pistol shrimp attack hits 4000⁰C! - Slo Mo #14

National Geographic - Amazing Pistol Shrimp Stun "Gun" | World's Deadliest

However with my $400 dollar superzoom camera, I have filmed a completely different reason for snapping. I can find no trace of what the video below shows in the scientific literature and this may be the first time that this information has ever been reported.

If mangrove snapping shrimps use their large nippers to cut up mangrove leaves so that the leaves can be pulled down into their burrows and consumed, this makes the mangrove snapping shrimp the marine equivalent of leaf-cutter ants. A radical claim such as this must be supported by a lot of evidence.

I became suspicious of the standard explanation for snapping shrimp behaviour when it occurred to me that having a bright orange warning flag in the middle of an ambush predator’s lethal weapon does not make sense.

Giant snapping shrimp are large and powerful but not very aggressive

Ecological observations did not add up either. There were simply too many snapping shrimps in some swamps. On some nights, the noise from snapping shrimps is a dull roar, like giant raindrops falling on a tin roof. At low tide, these areas are completely cratered with snapping shrimp holes. As for preying on shrimps, crabs and fish, I have watched the tide flow into snapping shrimp areas and it carries with it very few potential prey. Other types of shrimps have eye-shine which is easily seen in torch light and even the tiniest fish can be seen when they move.

Stilt mangrove swamp near the seaward fringe

The entire floor of this area is pitted with snapping shrimp holes

The mangrove swamps that fringe the open mudflats of Trinity Bay are flooded by seawater that crosses at least one hundred metres bare sand and mud flats. Not that many small shrimps and juvenile fish want to cross such a space - so they hide in creeks. Many mangrove areas are more than 500 m from the seaward fringe of the mangroves and are flooded by seawater for less than 6% of the time, yet snapping shrimps are still abundant. The idea that snapping shrimps are mainly predators was starting to fall apart.

Snapping shrimp holes in stunted Ceriops backswamp near a salt pan

So far, I have discovered that snapping shrimps rarely leave their holes or even come to the entrances of their burrows. At low tide, there are occasional snaps. Snapping is most intense in the evening and when the outgoing tide has exposed the mangrove floor. Waters continue to drain from the forest floor for minutes to hours when the tide recedes and leaves are sometimes transported in the thin film of receding water. Snapping shrimps can be observed grabbing these drifting leaves and pulling them into their burrows. Very occasionally, it is possible observe a leaf being cut up on the surface. 

Snapping shrimp capturing a mangrove leaf

Observing the snapping shrimps is difficult as during the day, the sky reflects on the waters surface, making visibility through the surface quite low. At night, a powerful light is needed for photography and this usually suppresses snapping shrimp activity.  Once I turned my light out after scanning an area for several minutes and was greeted with roar of snapping shrimp pops. Snapping shrimp may also occasionally respond to snapping noises by making a pop themselves. Popping up my camera flash makes enough noise to trigger the occasional response.

The one time that snapping shrimps are quiet is at high tide. Bream and other snapping shrimp predators can be seen swimming through the milky waters and it is too dangerous for shrimps to be active. On the incoming tide, the shallow film of moving water that snapping shrimps need to catch floating leaves becomes too deep and too swift only sixty seconds after the incoming tide has arrived and they fall silent seconds later.

In the soft muds of the seaward edge of an accreting stilt mangrove swamp, snapping shrimp appear to outnumber crabs. Only in the infrequently flooded and very saline Ceriops swamps do numbers of snapping shrimp fall. The video in this post was filmed in a Ceriops swamp. Sometimes a few snapping shrimps even persist around the edges of salt pans. Middle areas which have a mix of mangrove species seem to have equal numbers of snapping shrimp and crab burrows. This unreported and extraordinary abundance would make snapping shrimps one of the most important components of the mangrove ecosystem.

Two tiny crabs can be seen but the glossy mud surface indicates that crab grazing is insignificant

My work in identifying the species of mangrove snapping shrimp is ongoing. They belong to the genus Alpheus. Alpheus strenuus and A. cf. lobidens are the leading contenders and are found from Australia to at least South East Asia. American mangroves have similar species.

Mangrove snapping shrimp live in burrows that resemble a the branches of a stag horn coral. Several sloping burrows that radiate out from a central chamber. The central chamber can be as much as 0.5 m below the surface and possibly even deeper. Tracing tunnel systems with ones fingers in semi-liquid mud has its limitations. The burrows also sometimes intersect the burrows of animals that I would not like to grab such as mantis shrimp. I have traced several burrows in newly colonised mangrove swamps where the snapping shrimp burrows are first generation and have not become part of an interconnected network of burrows. Most burrow systems have approximately a dozen entrances. Craters may form where part of the branched burrow system has a roof collapse. Originally, I thought the craters were by design and were to increase the surface area water within the burrow mouth to improve oxygenation.

Collapsing tunnels in sandy areas reveal the tunnel structure created by a single shrimp

Usually one or two small snapping shrimp is recovered together with a goby. It appears that even mangrove snapping shrimp have gobies as friends.

Goby and shrimp are rarely observed, the shrimp was touching the goby before the photo.

This goby was restricted to this hole for at least consecutive five days due to very low high tides.

When the tide goes out the goby become a prisoner within the snapping shrimps tunnel complex. In Cairns, which has two tides per day and tides are moderately large, six days can pass between tidal flushing of the seaward zone. That is a long time to be confined to a muddy hole the size of a tea cup. At night, a goby can be seen in one hole in about 50. Only once have I seen a snapping shrimp antenna running over the back a goby and I wonder if the relationship between shrimp and goby is as tight as the relationship is for reef living species.

Goby and shrimp obtained from tunnels in a mangrove creek bank.

There is much more to this discovery that I can report and many of the details remain shrouded in mystery. There is much more work to be done