Listening to Snapping Shrimp

I’m working on setting up a citizen scientist project to document where snapping shrimp (family Alpheidae) are active pre- and post-contamination by the oil spill in the Gulf of Mexico. In this post, I just want to introduce the basic concepts and provide an example sound file.

Snapping shrimp comprise a number of species, mostly distributed in tropical to temperate waters. They live in near-shore structured environments, including seagrasses, rocks, and coral reefs. They are predators on small, live prey, and they kill or stun their prey using a snap from a disproportionately large claw. The snap of the claw generates a cavitation event and, by the way, a high-amplitude, broadband transient sound that is also called a snap. The combined noise from the local population of snapping shrimp is a familiar feature not only to bioacoustics researchers, but to anyone who snorkels or SCUBA dives in areas with snapping shrimp.

Because of this noise and the role snapping shrimp play in the marine food web, they are an excellent candidate as an “indicator species”, a species that can be easily monitored and which provides a measure of the health of that part of the marine food web. Better yet, the monitoring and assessment can be done acoustically, by sound recording, to get a measure for a local population.

If I had a chunk of money to throw at this, a sophisticated way to do this would be to make a baseline of calibrated sound recordings and be able to characterize tidal and daily cycle effects on snapping shrimp sound activity, and thus be able to statistically determine a reduction in activity post-contamination. I estimate somewhere around $10K would be needed to set up a portable data collection system from scratch with that kind of capability. Not having that in spare change in my pocket, I’m looking at a somewhat different approach that a lot more people can get into with minimal outlay of funds and just a bit of do-it-yourself drive.

Because snapping shrimp noise is broadband, you can hear it even in plain audio recordings, though the peak frequencies are actually ultrasonic. This means any sort of audio recorder can be used to find out if snapping shrimp are present in a location: cassette tape recorder, digital recorders, and even video cameras. The thing that any of those will need is a microphone input. What to plug in for that recording? A hydrophone would be great, but most people don’t have those lying around. But one can also make a normal microphone water-resistant and use it. It is best to think of such a microphone as disposable, since better sensitivity also corresponds to the water-resistance being more fragile, and saltwater is great at destroying electronics. In another post, I’ll describe making your own hydrophone or water-resistant microphone. If you already have a recorder, the additional cost is under $50 to be able to record underwater sound. I’m not looking for this sort of recording to do as much, simply to say whether a snapping shrimp population is active or not.

Below is an example of a simple recording I made last night that demonstrates the presence of an active population of snapping shrimp at one location and time. I’m still working on what additional information should be noted along with the recording, but I think what I provide here may be sufficient.

File: s_sunshine_skyway_201006241851_WS_30006.wma
Recorder: Olympus WS-320M, ST HQ mode, CONF mic sensitivity
Transducer: Salvaged hydrophone from a sonobuoy
Transducer depth: Approximately 2 feet
Recording made by: Wesley R. Elsberry
Date: 2010-06-24
Time: 18:51 EDT
Latitude: 27.586371°
Longitude: -82.620388°
Location description: South Sunshine Skyway Bridge on road to south fishing pier, at overpass over water, north side, toward east end.

I’ll be posting more on this topic later.