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Dive Dry with Dr. Bill

#422: Pistol Shrimp: "Crack" Shots

I considered delaying this column since the tragic events in Tucson, Arizona, are still fresh in our minds. However, I know my readers are intelligent and discerning (why else would they be reading this?) and can handle this week's topic, the pistol or snapping shrimp. Although it is said that children should be seen and not heard, pistol shrimp are usually heard but not seen. In my research I was amused to find that the now deceased marine biologist Dr. Joel Hedgpeth made a similar comment in his revision of Ed "Doc" Ricketts' classic Between Pacific Tides.

The impulse for writing about this species of arthropod came from a request to me by a diver in Australia asking what the "Rice Crispies" background sound in some of her underwater video footage was. I watched her video and listened to what was actually a familiar sound from my thousands of dives here in southern California. I'm afraid my icon, Jacques Yves-Cousteau, fibbed a bit when he referred to the deep as "the Silent World." During my earliest days diving in our kelp forests back in the late 1960s, I learned that the sounds I heard came from these small, 1-2 inch shrimp which lived in the crevasses of rocky reefs and under rocks. National Geographic states that the sound from these shrimp can actually mask a submarine from detection by sonar, or a targeted ship from the sub's sonar, a phenomenon first noticed during World War II!

There are about 600 species of pistol shrimp known throughout the world. Most live in tropical and temperate coastal waters. Many live in burrows they dig, while others frequent coral and rocky reefs as well as seagrass beds. Some species from coral reefs have an interesting relationship with fish known as gobies. The two share the shrimp's burrow. When they are outside of it, the pistol shrimp maintains contact with the goby using its antennae. The fish has better vision and can alert the shrimp to signs of danger, causing both to retreat into the burrow. The species I'm familiar with in our waters, the twist claw pistol shrimp, used to be known scientifically as Crangon dentipes when I arrived on the island, but has now been assigned to a different genus initially as Alpheus dentipes and currently as Alpheus clamator.

Wikipedia states that these shrimp compete with the sperm whale for the title of "loudest animal in the sea." I was surprised to find I wasn't even in the running. The sound is produced by one of their two claws which is much larger and different in structure than the other. I had always thought it was simply the mechanism of the claw's "thumb" snapping against the base that created the sound. During my research I discovered a quite different, and highly intriguing mechanism.

When this specialized claw snaps shut, it sends out a burst of water that is followed by a cavitation bubble of air traveling at speeds of 60 mph. This in turn releases a sound rated at 218 decibels (db) in one species. For comparison, a pneumatic jack hammer registers 125 db, and a jet engine 100 feet away registers 140 db The human eardrum may rupture from sounds of a "mere" 150 decibels. When the cavitation bubble collapses, it is said to reach temperatures over 4,700 degrees Celsius (about 8,500 F). The sun's surface temperature is about 5,500 C. (nearly 10,000 F). This collapse produces a flash of faint light (invisible to the human eye) through a process known as sonoluminescence (sono = sound). It is actually the collapse of this bubble rather than the snapping of the claw that creates the sound we hear. The mantis shrimp, with its formidable snapping claws, is another marine critter that can do this.

So WHY do pistol shrimp create such a noisy environment? Apparently there are several reasons. The first is for hunting. The shrimp senses the presence of small fish or invertebrates using its antennae, then creeps slowly out of its burrow and... BANG! The prey is stunned, captured and carried inside to be munched on. Of course since these are pistol shrimp, rather than rifle shrimp, the range of their weapon is fairly short. Some state the snapping of the claw is also used for defense. Apparently the sound is also used to communicate with other shrimp. I can hear you loud and clear... but I have no idea what you're trying to say! Of course, I'm no shrimp... quite the contrary!

A study just released as I was writing this indicates that juvenile fish tend to settle in areas with the most noise, often created by pistol shrimp as well as other fish and invertebrates! This was discovered on the Great Barrier Reef where scientists played the sounds recorded from different types of habitat over newly created artificial reefs. The youngsters chose reef sites with more noise, especially ones that were associated with their preferred habitats. I guess they'd prefer living on "The Flats" in Avalon then. The ability to discriminate suitable habitat using sound adds another selection factor in addition to chemical and visual senses. The scientists speculated on the effect human generated noise such as that from boats, drilling or shore development might have on this ability.

If you would like to hear this "snap, crackle and pop" sound for yourself, there is a BBC video available on YouTube at http://www.youtube.com/watch?v=XC6I8iPiHT8 and another http://www.youtube.com/watch?v=ONQlTMUYCW4 detailing the production of the sound.

© 2011 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Image of a species of pistol shrimp showing snapping claw (inset), courtesy of Sea Life, Tavernier, FL.

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia