Behaviour

Experimental project

Introduction

Alpheid shrimp are a major source of noise in the ocean and are capable of amazing feats. By a rapid contraction of the muscles in their major chelae these shrimp produce a cavitation bubble that collapses to produce a deadly snapping sound (Versluis et al, 2000). This cavitation bubble produces acoustic pressures up to 80 kPa, travels at 97 km/h and can extend to 4 cm away from the shrimp (Versluis et al, 2000). The sound caused by the collapse of the cavitation bubble can reach 218 decibels and reaches over 4000 °C (Lohse et al, 2001).  A. strenuus is one of the larges in the alpheid family suggesting it too has these capabilities (Banner & Banner, 1982).
In this study, specimens of A. strenuus of different sizes were collected to see if the size of the shrimp affects their ability to produce a cavitation bubble of such effect.

Method

A total of twelve samples of A. strenuus were collected and sorted by size into three categories: Adult (5-6.5 cm), sub-adult (3-5 cm) and juveniles (<1-3 cm). Measurements of length were taken from the tip of the telson to the rostrum at the end of the carapace. They were then added to a tank complete with coral rubble for comfort.
Snapping was then induced by tapping the shrimps with a wooden skewer shown in the video below. Each shrimp was recorded individually using the microphone from an underwater digital camera from ~30 cm away. The sound file was then extracted from the video and imported into a program called Audacity©. As the shrimp produced a sound greater than the recording capabilities of the camera, the sound in decibels was deduced by comparing the spike produced by the shrimp with quieter background noise (construction workers using a concrete breaker ~5m from the tank). The wavelength of the background noise was proportionate to the amplitude therefore, by using the wavelength of the shrimp’s snap the amplitude could be determined by extrapolation.

Results

Discussion

The results show a clear positive correlation between the size of the specimen and the sound intensity with the smallest shrimp of 1.6 cm producing a sound of 162 dBs to the largest shrimp of 6.3 cm with a snap of 187 dBs. However, there do appear to be some anomalies as the smallest shrimp did not produce the lowest decibel count nor did the largest produce the highest. This could be due to the quality of the recording device and the errors associated in retrieving the results.
Au & Banks (1998) found similar results with snaps ranging between 183 to 189 decibels depending on the size of the shrimp and claw. Their results also show clear correlation between size and the intensity of the snap produced. However, the minimum decibel count of 183 decibels relates approximately to the size of a sub-adult A. strenuus, this could be due to the differences in size of species and possibly the equipment used.
In conclusion, the size of the shrimp and claw has a positive correlation on the intensity of the sound produced by the snap as a larger claw/shrimp produces a larger cavitation bubble.