Snapping Shrimps are very unique creatures that generate high intensity sound which is louder than the big whales who are much larger in size. Snapping shrimps are also in large numbers in a shrimp bed (a few thousands), rather than single digits as in the case of big whales.
When shrimps snap their claws, they generate sound. This mechanism is cavitation, and the bursting of cavitation bubbles generate sound in the frequency band from 2 to 250 kHz. Such sound originating from an entire shrimp bed is so loud that it can impact multiple underwater systems, including sonars deployed onboard submarines and for coastal surveillance.
Snapping shrimps are warm water creatures that live in tropical and sub-tropical littoral waters. Their habitat has temperatures more than 11oC and is at depths less than 55 metres, with bottoms of corals, hard rocks, or sponges to allow hiding place from their prey. They can tolerate a wide variety of salinity levels ranging from the mesohaline (5-18 ppt) to hyperhaline (>40 ppt), but are found abundantly in the range of 35 ppt.
Snapping shrimps use their high intensity sound to stun and capture their prey. The size of their population in any colony depends on the habitat being favourable to the above conditions. The colonies have sufficient adult population as their breeding is spread across the entire year, so the sound generated is reasonably stable.
The Indian Ocean Region (IOR) presents very favourable conditions for their breeding. The average sea surface temperature in the IOR ranges between 27oC and 32oC, and the depth is largely less than 70 metres in most parts of the region. The salinity levels vary in the range of 32 to 37 ppt with large local variations within; especially, salinity in the Arabian sea is high due to high evaporation, whereas in the Bay of Bengal it is low due to high influx of freshwater from various river systems. The seabed provides shelters like crevices and caves in corals, rocky seabed, and shells, making it favourable for these species to flourish. Thus, the IOR is a hotspot for snapping shrimps.
The loud noise made by these snapping shrimps is a major impediment in establishing effective underwater domain awareness (UDA) in the IOR. Ongoing geopolitical developments make the IOR extremely critical from a geostrategic perspective. Submarine proliferations in the IOR are on the rise and the snapping shrimp noise can significantly limit their deployment in the tropical littoral water. Consequently, not only do maritime domain awareness (MDA) and UDA become important, but so does mapping the soundscape, given the noise of the snapping shrimp.
The study starts with identifying hotspots based on habitat conditions. Since the shrimps’ sensitivity to their habitats is well established, we now need to monitor diurnal and seasonal changes in the habitat and then correlate them to the impact on the soundscape. Also, shrimps are active during the night, searching for food, so we must do a spatio-temporal analysis of their habitat before soundscape mapping.
The diurnal variations are very interesting. Night-time noise levels are approximately 3-6 dB higher than daytime and they peak at dawn and dusk due to predatory activities. Seasonal variations are less profound, however snapping noises get loud during warm climate and can certainly get louder with global warming. Habitat mapping leads us to the next step of soundscape mapping.
There are two methods which provide a spatio-temporal distribution of the noise made by the snapping shrimps. The first is based on a geometric consideration and assumes that individual shrimps snap independently. The second is based on behavioural consideration and assumes that individual shrimp snapping is interdependent.
The noise peaks in the frequency band of 2-15 kHz, but this band has multiple other competing sources as well. Wind noise, heavy precipitation, industrial activities in shallow waters, and the sounds of certain other oceanic animals such as the whales, dolphins and porpoises overlap the same frequency band and need to be accounted for.
Snapping shrimp noises are not always a nuisance. They can be used as a sound source for underwater surveillance with passive sonars deployed in the region. They can also be used to illuminate the region and to detect present of underwater targets in the tropical littoral waters. Active sound sources could be a giveaway in terms of covert deployment, and they can help optimize battery power in buoy-based sonar deployment. Given that the IOR is getting very sensitive due to substantial submarine proliferation, it could consider this to its advantage.
While carrying out extensive research on this topic, the Maritime Research Centre, Pune discovered that there is ample scope to take this study forward. It can be translated into a tool for defence applications, environment monitoring, effective UDA framework realization, blue economic policy formulation, oceanographic studies in the tropical littoral waters, and a lot more.
(This post first appeared here in The Tilak Chronicle.)
Dr Arnab Das
Dr Arnab Das is a former Naval Officer and has obtained a PhD on Underwater Technology from IIT Delhi. He is Founder & Director of Maritime Research Centre (MRC), Pune and also runs a startup called M/S NirDhwani Technology Pvt Ltd (NDT). Both MRC and NDT focus on the Underwater Domain Awareness (UDA) Framework.
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