There are many terrestrial species with a strong reliance on sound, but only the bat comes close to the complete reliance seen in many marine and aquatic species. Just as the bat needs sound to find prey and navigate its nocturnal world, species that inhabit water use sound to overcome the rapid loss of light at depth. The use of sound to locate prey and mates, navigate along migration routes, communicate with conspecifics and defend against predators is seen in a diversity of forms in the ocean. With increasing ocean noise threatening more well-known top predators, like the southern resident killer whales, understanding the impacts further down the food web has never been more important.
While most people know about the songs and sounds made by whales, few people know that other animals in the ocean make sound too! Here’s a look at some of the more interesting examples from shrimp to fish.
In crustaceans, like crabs and shrimp, sound is mainly used to deter predators or stun prey. The snapping shrimp (Alpheus heterochaelis) takes things to another level. These little guys can close their claws so quickly that they produce cavitation bubbles, and when these bubbles collapse they produce sound and light, which was named “shrimpoluminescence” by the researchers who discovered it. In the tropics and subtropics, the snapping noises they produce dominate the soundscape both day and night, interfering with active and passive acoustic efforts and likely shaping the way that other animals in the ecosystem use sound. The displacement of water that the snapping motion creates can be sensed by hairs on the snapping-claws of other snapping shrimp, making it an important mode of communication as well.
When approached by predators, spiny lobsters (Pallinuridae) produce a loud rasping sound as a startling deterrent (Sound 1). While most arthropods use friction between hard surfaces to create rasping sounds, lobsters use a different ‘stick and slip’ mechanisms that allows them to produce sound even when they have a soft shell following molting. This reduces their vulnerability to predation because they are still able to deter predators when their exoskeleton has not yet hardened. The movement spiny lobsters use to produce sound is similar to the motion of a bow across the strings of a stringed instrument; the bow sticks and slips in rapid succession, creating vibrations from the unstable movement. This type of sound production is called stridulation.
Catfish are also known to produce sound through stridulation. An interlocking mechanism at the base of the pectoral fins produces sound through the same ‘stick and slip’ mechanism used by the spiny lobster.
Seahorses also use this mechanism between bones in the cranium to produce a wide variety of sounds, like clicks and growls. For seahorses, sound is mainly used during competition between males, in stressful situations, and during feeding.
Croaking gouramis from the genus Trichopsis produce croaking noises as part of breeding displays and to assert dominance over other gouramis. To produce sound, the fish stretches their pectoral fin tendons and then “plucks” them with the cartilage that supports the fin structure. On the outside, the fish looks like they are just beating their fins back and forth. These sounds are used mostly by male gouramis to maintain their dominance over other males. Interestingly, females use a separate purring sound to entice males and synchronize mating, one of the only instances of this behavior among fishes.
While swimbladders mainly help fish maintain their neutral buoyancy, for some species they are also important for sound production and amplification. The most famous fish that produces sound using the swimbladder is the Oyster toadfish (Opsanus tau). Male toadfish produce courtship boatwhistle calls for hours at a time that are used to attract females to their nests. To produce boatwhistles, the toadfish uses the fastest known vertebrate muscles. Two muscle positioned on either side of the heart-shaped swimbladder are contracted at the same time, producing sound waves in the air-filled swimbladder. Another closely-related species that uses the same mechanism is the Plainfin midshipman (Porichthys notatus). In parts of California these species make the local news for keeping residents awake with their loud and disturbing humming!
In some places, these sounds are drowned out by ship noise and other man-made sound, disrupting the important communication that is occurring. One listen to a hydrophone makes it obvious just how loud the sea can be. While traditional pollution is still a hazard for many ecosystems, scientists are only beginning to understand the impact of noise pollution on sensitive ecosystems around the globe. For many animals, like most species of sea turtle, there is almost no research about their sounds, so judging the potential negative impact we might be having is extremely difficult. Next time you dip your head under the waves, take a closer listen and see who else is sharing the sea with you. You might be surprised at just how much is happening without us even knowing.
Conservation Made Simple
For the especially curious…
Here’s the link to the hydrophone located at Lime Kiln State Park, on San Juan Island, WA. Take a listen and a see which sounds you can identify. Scientists mainly use this hydrophone to keep track of southern resident killer whales, but it also illustrates the impact of ship noise really well. Orcasound has a network of hydrophones around the Salish Sea, and most of them have live streams that you can access on their site.
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