For many years, scientists have thought that the oceans are a unique world of silence. During the last few decades though, research findings revealed that oceans are a rather noisy environment. Noises, like waves, rain, earthquakes and biological sounds from vocalizing animals always occurred underwater and lately since the industrial revolution, anthropogenic noise began to dominate the oceans, though biological sounds still remain a significant contribution to the ocean soundscapes.
But what kind of animals are contributing to these soundscapes?
Marine mammals are well-known vocalizing creatures. Through the evolutionary pressure, they inherited traits to adapt to underwater life. Their active lifestyle and predatory success required a mechanism of underwater communication and navigation that would envy of even the most innovative human technology. Therefore they have been for decades the main ambassadors of an impressive underwater oral culture. But furthermore marine mammals are considered as worthy delegates of an underwater communication culture, which can have linguistic variations like human languages, such as complex phrases and local dialects. Sperm whales use different dialects and their “clicks” (vocalization produced for echolocation and communication) could have accents. In addition, dolphins use a complicated whistle vocabulary which could have similarities with whistled language invented within human societies to deliver messages over long distances. Examples of ancient whistled languages still exist in regions like La Gomera in the Canary Islands, some parts of Southern Africa and Eastern Africa, the Greek village of Antia, and villages within the mountains of north-east Turkey.
And what about other marine organisms?
Considering the fact that sound is an important component of the marine environment, then it is expected that during the evolutionary process, marine organisms have developed the ability to sense important sounds for their survival. The apex predators use sound to locate their prey and it is therefore expected that a link should occur between the use of echolocation signals from the predators and the perception of sound by their potential prey. Oceans are characterized by a combination of acoustic habitats and a lot marine organisms seem to share the ability to receive and produce sound. It has been proven that other animals like marine turtles, fishes and invertebrates produce noise and react when exposed to sound, though our knowledge on such biological sounds is rather limited.
Studies have shown that fish can hear (Popper and Hastings, 2009). Fishes, which have no swim bladder or an underdeveloped bladder (e.g. Elasmobranchs or benthic fishes), are not sensing the pressure like mammals but their hearing ability is mainly a reaction to particle motion. Their inner ear responds directly to acoustic particle motion. Fishes with swim bladders can additionally detect the pressure component of sound. For example, Cods have been shown to use their swim bladder to sense sound and due to this fact, they have increased significantly their hearing sensitivity. Fishes during their younger life stages, eggs, larva’s, fry and juveniles are more sensitive to loud noises (Popper and Hastings, 2009). It has been proven that they are mainly sensitive to low frequencies. The majority of fish detect sounds from below 50 Hz up to 500 or even 1,500 Hz. Some species can sense sound to over 3,000 Hz and a few can detect sounds to well over 100 kHz. Surprisingly it has been shown that Cod (Astrup,1999) and Herring can sense, like most of the species the low frequencies, but they can also hear ultrasound like the echolocation clicks of their predators, the dolphins. The scientists have used non-invasive electrophysiological methods to measure the hearing sensitivity of several fish species and assess their audiograms. Through this method it has been proven, that species like Japanese anchovy (Engraulis japonicus) have hearing sensitivity at 300 Hz (Akamatsu eta al., 1996) and the Spotlined sardine (Sardinops melanostictus) on the other hand have a hearing sensitivity of 1,000 Hz (Akamatsu et al., 2003).
Fish can also talk (Fish et al., 1970)!
Sound production is used by the family of Gadidae to attract mates during spawning season. The males produce low frequency sounds (<500Hz) by their swim bladder-sonic muscle mechanism (Figure 3). The spawning sounds they produce are loud and easily detectable by passive acoustic technology. These sounds are intentional vocalisations, used mainly for communication. Fishes also produce a variety of unintentional sounds by their swim bladder (drum sounds) or by rubbing skeletal parts. Such sounds can be a significant tool for science and conservation. Scientists use these sounds to locate the spawning grounds of Cods (Figure1, Figure 2) and Haddock in Norway and of the family Sciaenidae in Florida. Thus, they determine important regions for the management of the stocks and delineate boundaries of marine protected areas (MPA).
It becomes apparent that the acoustic underwater world is unique, impressive and could be highly illuminating. Sound could give us detailed information even from long distances and has been proven very valuable for the deeper knowledge of the marine environment and for its protection.
Fish, M.P. and Mowbray, H.M. 1970, “Sounds of Western North Atlantic Fishes.” Baltimore, MD: The Johns Hopkins Press. 205 p. Recordings of 153 species of fish from the Western North Atlantic compiled by Marie P. Fish and William H. Mowbray (also available on CD-ROM)
J Astrup 1999 Ultrasound detection in fish – a parallel to the sonar-mediated detection of bats by ultrasound-sensitive insects? Comparative Biochemistry and Physiology Part A 124:19-72
Akamatsu T, Matsushita Y, Hatakeyama Y, Inoue Y. Startle response level of Japanese anchovy Engraus japonicus to underwater pure tone signals. Fish. Sci. 1996; 62: 648–649.
Akamatsu T, Nanami A , Yan H Y. Spotlined sardine Sardinops melanostictus listens to 1-kHz sound by using its gas bladder. Fish. Sci. 2003; 69: 348–354