The Evolution of Turtle's Shell

The Evolution of Turtle's Shell

The video below shows the evolution of turtle's shell from the ancient to the modern era to adapt new environment and external trauma.

Mechanical Performance of Turtle's Shell

Mechanical Performance of Turtle’s Shell

Biological composite shields have been increasingly investigated in recent years. In turtle shell, there is a unique arrangement of alternating rigid rib and flexible suture elements that give rise to superior mechanical performance. The rigid structure of turtle shell protect the internal organ from external damage while some degree of flexibility allow the respiration and locomotion. The top dorsal of the turtle shell (namely carapace) was reported in the scientific literature and it found that carapace contains unique macroscopic configuration of complex alternating strips of rigid boney ribs. The carapace is consists of both alternating rigid (rib) and flexible (suture) elements in an arrangement of zigzag tips of rib connects to the flexible suture sites. The image of turtle shell has been shown in Figure 1. 

Figure 1. Ventral view (inside-out) of turtle shell. The red arrow mark the individual suture adjoining to rib

Flexural high stress cyclic loads were applied to both rib, suture, and complex specimens obtained from the carapace.  Static bending test were tested for specimens cut from the carapace to measure the strength for each specimen. The average strength for different part of the turtle shell has been shown in Table 1.

Table 1. Average strength for different part of turtle shell measured by quasi-static bending stress

Type of specimen	Strength (MPa)
Suture	51.3
Complex (Whole shell)	71.2
Rib	121.6

Based on the results, it showed that the rib is the strongest as compared to suture and the complex structure of turtle shell. The ribs demonstrate better fatigue resistance than sutures due to layered sandwich micro-structure (two perpendicular parallel-fibered sub-layers). However, the complex specimen made of a sequence of rib-suture-rib-suture-rib elements are able to withstand repeated loads due to its fast unlocking mechanism.

            According to the CT scan, it showed a decreasing mineral concentration from the shell toward the suture. The un-mineralized suture integrate with the rib allow extra degree of flexibility underload. This interdigitating nature of the structure of the sutures allows them to move freely towards each other under small load. However, the shell becomes rigid when adjoining dermal bones meets under critical deformation threshold. The concept of the nature of the shell structure is depicted in Figure 2. The SEM images of the complex 3-dimensional structure of suture joining the rib of the turtle shell have been shown in Figure 3a, b, c.   

Figure 2. Schematic depiction of unloaded and loaded deformed beam. The parameter D, W, and α are denote as pitch of the zigzag, gap of the suture, and maximal bending angle. 

Figure 3. SEM images complex 3-dimensional structure of suture joining rib. (a) at the interdigitating suture, (b) at the cancellous region adjacent to suture. The thin dorsal cortex region shown below the dashed green line is gradually thicken away from the center from suture to rib , (c) Higher magnification of (b). The red elipse marks the fracture region.

References

1. B. Achrai and H. Daniel Wagner, "The red-eared slider turtle carapace under fatigue loading: The effect of rib–suture arrangement", Materials Science and Engineering: C, vol. 53, pp. 128-133, 2015.

2. R. Shahar, S. Kraus, E. Monsonego-Ornan and P. Fratzl, "Mechanical Function of a Complex Three-dimensional Suture Joining the Bony Elements in the Shell of the Red-eared Slider Turtle", MRS Proc., vol. 1187, 2009.

General Introduction of Red Eared Slider

Background

Red-eared slider (scientific name: Trachemys scripta elegans), which is also known as red-eraed terrapin, is one of the most distributed turtle in the world. It is a subspecies of pond slider and belonging to family Emydidae. It is the most popular pet turtle in United Stated and also popular in rest of the world. Its originally came from Southern United States and Nothern Mexico, and now has available in many other places due to the pet release. Figure 1 shows the red-eared slider our group adapted. 

Figure 1: Red Eared Slider adpated by us

General Description

The turtle is a freshwater semi-aquatic turtle with a fairly flat, oval shell and a weakly keeled carapace. Soft waters with muddy bottoms, aquatic vegetation and suitable sites for basking are their prefer habitats. Their colors are range from bright greens and yellow to more muted olives, brown, and some even blacks. Small red stripe can be seen around their ears and this has become the origin of their name. The average length ranges of this species is around 15 to 20 cm (6 to 8 inch) and the longest length it can reach is 40 cm (16 inch). The female species is often larger than male. On the other hand, their lifetime is fall between 20 to 50 years and it has greatly influenced by their living environment.

Sexual Dimorphism

There is some dimorphism between male and females of this species. It is difficult to identify the sex of young red-eared slider because their looks similar. However, the sex of its adults are easier to differentiate as the shells of males are smaller than of females. Male species reached sex maturity when their carapaces reach 90 to 110 mm and females reach maturity when their carapaces reach 160 mm to 200 mm. Besides that, the claws of males are longer than females, which makes them easier to hold on to female for mating. The male tails is usually thicker and longer. Figure 2 has clearly illustrated the differences between male and female species. 

Figure 2: Male and Female Red-Eared Slider

Habitat and Behaviour

The red-eared slider is very adaptable to the environment, even has the ability to hibernate in the winter. It can inhabit in a source of stagnant, warm water, for example lakes, ponds, swamps, creeks, streams or rivers. They can tolerate brackish water, thus they also can be found in coastal waterways. They stay in calm water area where there are some rocks or tree trunks for them to climb on it and rest in the sun. On the other hand, they eat everything, from fish, frogs, insects, vegetation to ducks. Figure 3 shows that the turtles are sun-bathing on the root. 

Figure 3: The turtles are sun-bathing on root

Threats and Control

Due to pet released and their high adaptable ability, their population has been increase in several areas. This turtles are aggressive and bold, and they will compete for foods and habitats with native turtle species. Besides that, they may interbreeding with other similar species and diluted their gene pool. They also can carry some unusual pathogens to their new environment, which will have some great damage apart from turtle themselves. In order to control these turtles, hunting, trapping and collecting eggs is encouraged in some areas. However, these methods have caused overhunting and decline of the species. A better solution in urban areas would be capture-sterilization-release, which would take a long time to work and be an ongoing one. 


References

1. J. Burger, “Red-eared slider turtles ( Trachemys scripta elegans ),” no. December, 2009.
2. www.bcreptiles.com, “Red-eared Slider Scientific name : Trachemys scripta,” pp. 3–5. [online] Available at: http://www.bcreptiles.ca/docs/76E182A811776B7B.pdf [Accessed: 30 April 2016]

Turtle's Anatomy - Shell

Figure 1: Shell of a red-eared slider turtle

The shell which can be seen in Figure 1 consists of bony plates that are mostly covered with shields called scutes (osteodem). The shields are entirely made of keratin. The shell of the turtle is divided into 2 sections:

• Upper or dorsal carapace
• Lower, ventral carapace or plastron

These are similar to scales and they are derived from the red-eared slider’s epidermis. However, the top layers of the scute will fall off when the red-eared slider begins to shed. A bridge connects the carapace and the plastron.

The carapace (top shell) is nicely rounded and smooth. The carapace has a black and yellow pattern. Hatchlings start with a bright green carapace that is slightly soft whereas adult red-eared sliders have a darker green carapace. As the red-eared sliders age, the colour of the carapace will grow darker and the patterns will be less visible. Hence, this is one way to determine the age of the red-eared slider. The carapace of a red-eared slider can be seen in Figure 2.

Figure 2: Carapace, upper section of the shell

The plastron is similar to the carapace. it is also smooth but it is usually yellow in colour with a distinct and unique pattern that can be seen in Figure 3.

Figure 3: Plastron, lower section of the shell

Let’s go in depth on the carapace and the plastron. Both section of the shells have their own specific name at all corners of the shell which can be seen in Figure 4.

Figure 4: Illustrative structure of carapace and plastron scutes respectively

Carapace Scutes

• Vertebral scutes cover the spinal region of the turtle.
• Pleural scutes cover the ribs of the turtle.
• Marginal scutes are located at the outermost scutes of the turtle.
• Supramarginal scutes fall between the marginal and pleural scutes.
• Cervical scutes cover the neck region of the turtle.

Plastron Scutes

• Intergular scutes cover the throat of the turtle and the anal scutes.
• Gular scutes are the first lateral scutes followed by the humeral scutes.
• Pectoral scutes are laterally paired near to the pectoral girdle of the turtle.
• Abdominal scutes cover the abdomen of the turtle.
• Femoral scutes cover the femur of the turtle.

Reference

1. Newman, S. (2016). Red-Eared Slider Turtle Facts, Habitat, Diet, Pet Care, Pictures. [online] Animalspot.net. Available at: http://www.animalspot.net/red-eared-slider-turtle.html [Accessed 23 Apr. 2016].
2. Virginiaherpetologicalsociety.com. (2016). Red-eared Slider. [online] Available at: http://www.virginiaherpetologicalsociety.com/reptiles/turtles/red-eared-slider/red-eared_slider.php [Accessed 23 Apr. 2016].

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.