Turtle's Anatomy - Ears

1. Introduction

The red-eared slider (Trachemys scripta elegans) is a semiaquatic turtle that inhabit in fresh water or pond. The red-eared slider is omnivores but highly rely on aquatic plants as their main source of food, thus they normally found in pond, lakes, swamps, or even slow-flowing rivers [1]. This turtle is poikilotherms which is unable to regulate its body temperature independently. Therefore, they need to sunbathe frequently to keep their body temperature at certain level. As an amphibious animal, the inner/middle ear structure determine their sensitivity to airborne and underwater sound via vibration stimuli.  

Figure 1. A photo of red-eared slider and the tympanum is shown (arrow) [2]

2. Anatomy

2.1.1 Middle Ear Structure [3]

The red-eared slider ear is characterized by tympanum (Figure 2a), a thin skin with a soft ovoid subdermal layer that is continuous with a cartilaginous tympanal disc that forming the extracolumella (Figure 2c). The extracolumella is attached with a posterior ligament and a thin anterior ligament. Two bilaterally symmetrical middle ear cavities are in the form of ovoid with long axis directed anteroposteriorly (Figure 2b,d). The volume of the cavities for small turtle are 0.22 ml (left and right) and 0.5/0.44 ml (right/left) for large turtle respectively.

2.1.2 Tympanic disc and middle ear cavity

The tympanic disc and middle ear cavity is the major parts of the ear for underwater hearing of red-eared slider. The tympanic disc vibrates with 40 dB larger amplitudes than adjacent region to underwater sound with a frequency of 400-600 Hz. The vibrations are reduced by filling of water inside cavity.  It is believe that air-filled cavities in the ear of turtle resonate in the underwater sound field and drive the tympanic disc with sound pressure. Due to the fact that sound pressure thresholds in air is lower than in water, and sound energy in air is greater than in water by 30 dB, the ear of turtle has better respond to sound in water than air. Moreover, the impedance mismatch from tympanic disc may reduce sound sensitivity of turtle in air. 

Figure 2. The ear structure and laser vibrometry of red-eared slider. (a) The tympanum on the turtle (arrows). (b) The head of the turtle (top). (c) The tympanic disc with overlying epidermis. (d) CT scan of turtle head skull. (e) Laser vibrometry chart of tympanic disc vibration frequency in a function of eardrum vibration transfer functions. (f) Laser vibrometry chart of disc vibration after middle ear cavities are filled with water  [3]

2.2.1 Inner Ear Structure [4]

The inner ear of red-eared slider can be divided into two parts: (1) Pars superior (top) and (2) Pars inferior (bottom). Pars superior (Figure 3b) consists of three semicircular canals that each contains an otic ampulla. Inside the ampulla, it contains a crista ampullaris with a sensory epithelium, utricular macula. Pars inferior (Figure 3b) is a large sac consists of lagenar macula, saccular macula, and basilar papilla.   

2.2.2 Utricular Macula Structure

Utricular Macula consists of hair cells, nerve fibres, and supporting cells. The supporting cells are spanning the entire epithelium in the form of ciliary rod. There are two major types of hair cells: (1) hair cell I and (2) hair cell II. The hair cell I is in the form of amphora shaped and enclosed by a nerve calyx. Hair cell II is in cylindrical shape and innervated by bouton-shaped nerve endings. The scientific study found that there are roughly 5000 hair cells inside utricular macula of a juvenile turtle. The main function of hair cell is acts as a transducer to convert mechanical stimuli (such as vibration) into neurochemical signals.   

Figure 3. The inner ear structure of red-eared slider. (a) The position of inner ear and tympanum of red-eared slider. (b) The lateral view of inner ear and position of the organs. ca: crista anterior; cl: crista lateralis; cp: crista posterior; ml: macula lagenae; ms: macula sacculi; mu: macula utriculi; pb: papilla basilaris; d:dorsal direction; a: anterior direction. (c) The overview of utricular macula. c: cotillus; r: rampa; s: striola; m: medial direction; a: anterior direction [4]

References

[1] "Red-eared slider", Wikipedia, 2016. [Online]. Available: https://en.wikipedia.org/wiki/Red-eared_slider.                          [Accessed: 23- Apr- 2016].

[2] "Species: Red Eared Slider - Turtle Guide & Community", Pet Turtle & Tortoise Guide with Community, 2014.                  [Online]. Available: http://turtletanksetup.com/red-eared-slider/. [Accessed: 23- Apr- 2016].

[3] S. Avall Severinsen, J. Morup Jorgensen and J. Randel Nyengaard, "Structure and Growth of the Utricular Macula            in the Inner Ear of the Slider Turtle Trachemys scripta", JARO - Journal of the Association for Research in                       Otolaryngology, vol. 4, no. 4, pp. 505-520, 2003.

[4]  J. Christensen-Dalsgaard, C. Brandt, K. Willis, C. Christensen, D. Ketten, P. Edds-Walton, R. Fay, P. Madsen and C.         Carr, "Specialization for underwater hearing by the tympanic middle ear of the turtle, Trachemys scripta elegans",           Proceedings of the Royal Society B: Biological Sciences, vol. 279, no. 1739, pp. 2816-2824, 2012.