Marine invertebrates lack the anatomical features of fishes or marine mammals that allow them to "hear." Nevertheless, when presented with sound during experiments some marine invertebrates do show an apparent response to the stimulus. How do they do it? Many marine invertebrates have special sensory organs known as chordotonal organs, which are a type of internal mechanoreceptor. These organs sense pressure, movement, and tension. They detect cues generated from vibrations that may be associated with sound.

Most marine invertebrates spend most of their time on some type of substrate rather than swimming about in the water. Stimuli produced by other organisms (e.g. predators or prey) can be transmitted through the substrate and detected by chordotonal organs. For example, the fiddler crab, Uca pugilator uses chordotonal organs to sense movement, which can make it aware of feeding opportunities or the presence of a nearby predator. Research on the spiny lobster, Palinurus elephas, has also demonstrated its use of chordotonal organs to sense vibrations produced by sound.

Marine invertebrates, such as this spiny lobster, sense vibrations that are produced from underwater sounds. Photo courtesy of Sheila Patek

Although marine invertebrates do not hear in the same way vertebrates do, they are able to sense vibrations and movements associated with sound production. This ability assists them in survival by allowing them to detect potential predators, prey, and even sense the activity of tides and currents.

References
• Salmon, M. and Horch, K. 1973. Vibration reception by the fiddler crab, Ucaminax. Comparative Biochemistry and Physiology 44A: 527-541
• Salmon, M, Horch, K. and Hyatt, G.W. 1977. Barth's myochordotonal organ as a receptor for auditory and vibrational stimuli in fiddler crabs (Ucapugilator and U. minax). Marine Behavior and Physiology 4: 187-194.

For more information on how other marine animals hear sounds use the following links:
Marine Mammals	Fish	Invertebrates