Sea turtles can be found all over the world. In some parts of the world looking for Sea Turtles is part of a vacation and the sighting of these creatures is the highlight of a day at the beach or in shallow water.

Most species of sea turtles prefer tropical or subtropical waters, although some species can be found in much cooler bodies of water. Perhaps a lesser known fact is that most mature sea turtles spend the majority of their time in shallow coastal waters, even venturing into coastal bays and inlets.

Green Sea Turtles can be found throughout the Atlantic Ocean, the Gulf of Mexico, the Pacific and Indian Oceans, and in the Mediterranean Sea. They prefer shallower coastal waters near continents and islands, but are occasionally seen in open ocean waters.  While mating takes place in the sea, female turtles must come ashore to lay their eggs and usually go to the same place each year.  The eggs incubate in nests in the sand for roughly two months, after which they hatch en masse, usually at night, and the hatchlings then make their way to the ocean. There is evidence that light plays a role in their ability to locate the ocean; however it is uncertain if other sensory cues also play a role.

Sea turtle nesting is the only period during which sea turtle vocalization has been documented (turtles are considered the least vocal of all living reptiles). The process is physically exhausting, and sounds generated by the sea turtle are a result of breathing, pumping sounds from the turtle’s throat, and grunting.  At present, there are no reliable underwater recordings of sounds produced by turtles, nor do we know how well most sea turtle species hear sound either in air or under water.

The Hearing of Sea Turtles

All reptiles, including sea turtles, have a single bone in the middle ear that conducts vibrations to the inner ear. The opening into a sea turtle’s ear is covered by thick skin, known as the cutaneous plate, which is a ring of scales that are similar to but smaller than those on the rest of the head. The eardrum (tympanic membrane) of the sea turtle is covered by the skin of the face (the cutaneous plate). This area is relatively soft and yielding and can be felt with the finger. The outer layer of the eardrum lies at the side of the head, well behind the eye, and above the level of the corner of the mouth. The middle layer of this membrane is particularly thick and contains a large amount of fatty tissue. The inner layer is formed by a plate of cartilage, which is the main part of the extracolumella.

The eardrum is attached by a heavy posterior ligament and a thin anterior ligament. The extracolumella is attached to the columella. This pencil-shaped rod attaches directly to the cochlea. This middle ear arrangement is odd compared to that of mammals due to the lack of a mechanical transformer to match impedance of aerial sounds.  The tympanic cavity, the cavernous sinus, and the Eustachian tube of the sea turtle ear all contain enclosed air pockets and would be extremely sensitive to any pressure pulse, especially from seismic airgun arrays used to explore for oil and from distant explosions.  The outer ear of turtles is connected directly to the inner ear by an almost straight rod. Obviously, sea turtles would not hear well in air. They may actually “feel” the vibrations via the soil when on land, rather than via an air conduction channel through the middle ear system.  A mechanical transformer in a well developed middle ear has an important secondary function. It serves to protect the delicate inner ear. The muscles of such a system tighten automatically by a pre-programmed reflex response whenever vibration exceed certain limits, thereby, restricting movement to afford a great degree of protection. This total lack of a protective system in sea turtles is a major weakness of the ear underwater for this species.

Researchers have found that sea turtles respond to low-frequency sounds and vibrations.  Our best data regarding sea turtle hearing come from studies using auditory brainstem responses (ABRs). From this work, scientists know that sea turtles can hear low- to mid-frequency sounds, but with poorer sensitivity than mammals. Sea turtles appear to hear best between 200 and 750 Hz and do not respond well to sounds above 1,000 Hz. To determine the hearing capabilities in an adult green sea turtle (Chelonia mydas), researchers obtained a behavioral audiogram for an older female green sea turtle, “Myrtle”, housed in the New England Aquarium’s Giant Tank exhibit. Based on her responses, it was concluded adult green sea turtles hear best between 200 Hz and 500 Hz.

While we think that that this is how turtles hear, the “rest of the story” deals with the possibility that Barotrauma can be creating deafness among these turtles.

Barotrauma Among Sea Turtles

In order to determine the potential for deafness it is necessary to examine the method of reception. This should indicate the areas of the ear most likely to suffer barotrauma, and the types of pressure waves that would be most damaging.  When the anatomy of the ear in sea turtles is considered it would appear that intrusion from high-intensity sound could easily destroy their hearing.  It is also easy to understand why their ears have not evolved to provide this protection. The coastal water habitat of turtles was a silent world, and void of excessive pressure changes before the recent invasion by man.  Water also tends to transmit pressure without significant loss. Peak over-pressure from underwater explosive events is much higher at the same distance from their epicenters than the corresponding over-pressure in air. For example, a 1 megaton explosion underwater will cause a 500 psi over-pressure at 6.2 miles from the epicenter, but less than 1 psi for the same distance in air.  Sound travels as a series of pressure variations. Traveling in air, the variations of pressure are low and the light weight air molecule is moved a great distance. Just the opposite is true in water. The acoustical pressure is sixty times higher, but the water molecule moves only a short distance (1/60th the distance of the air molecule). Sound underwater carries a short punch; but, it is sixty times more powerful!  High-intensity sound energy (a series of rapid pressure changes) underwater would interact more violently with areas of anatomy that offers the greatest mismatch to sound in water (impedance). Flesh and bones, composed mostly of water, would offer less mismatch, and suffer the least damage. Sound energy (a series of rapid pressure changes) would be more destructive in the air pockets of the lungs, the tympanic cavity, the cavernous sinus, and the Eustachian tube of the sea turtle ear. Exposure to excessive pressure underwater would rupture the alveoli in the lungs and mutilate the entire hearing apparatus.  Moving down the scale of intensity, destructive pressure would cause minor damage in the lungs, and destroy the ear.

With a lesser degree only the ear would suffer barotrauma, and the turtle would be deafened, yet appear outwardly healthy.  This would not kill them, nor would a deaf turtle die within a few days. How long a deafened turtle could survive is a mystery. It is conceivable that he could survive for years or until he became entrapped in a shrimper’s net or struck by a boat.