This entry will focus on discussing carapace functions, geometry and composition. Along with that the exoskeleton of the crab is explained.
Functions
Basically, the carapace is the shell on back of the crab that is made of a hard bone called chitin[1]. Chitin is a polymer which is the main component of arthropod's exoskeletons such as crabs [2].
A = antenna. AB = abdomen (underneath). C = claw. CA = carapace. E = eye. SL = swimming leg. WL = walking leg
Figure 1. Shows the anatomy of the Crabs
Figure 1. Shows the anatomy of the Crabs
A study on the materials of the carapace was done by a chemist, P. Romano, shows that the main composites of the shell are calcium carbonate and inorganic mineral [3].
Table 1. The Compositions of the Crab’s Carapace.
Based on the composition the structure of the carapace will give optimal performance in resisting high stress. Since the yield strength of the calcium carbonate material is relatively high that will provide a high tensile strength which will work as a protection boundary on the crab’s body, it protects crabs from outsides hits. Besides that, the materials found in the carapace act as insulation board to preserve the crab from high temperature considering crabs hide under rock and sand to refuge from predators.
Exoskeleton Composite Material
In crab exoskeletons, the minerals are in the form of calcite or amorphous calcium carbonate, deposited within the chitin-protein matrix [4]. The numbers of calcium carbonate particles determine the hardness of the exoskeleton, which may vary both according to species and within one shell [5].
Helge Fabritius revealed that there are other compositions contained in the crab shell. The crab shell shows region with low calcium content while containing large quantity of magnesium. They also reveal areas with hardly any magnesium. Magnesium atoms in calcium carbonate crystal cause irregularities in the crystal structure, thus increasing the stiffness of the mineral which in turn hardened the crab carapace [5].
The author also demonstrated that the claws have high mineral content as it need to withstand particularly strong forces. However, carapace contains fewer minerals making it less heavy. There are more resilient compounds in carapace to form a thicker shell.
Helge Fabritius revealed that there are other compositions contained in the crab shell. The crab shell shows region with low calcium content while containing large quantity of magnesium. They also reveal areas with hardly any magnesium. Magnesium atoms in calcium carbonate crystal cause irregularities in the crystal structure, thus increasing the stiffness of the mineral which in turn hardened the crab carapace [5].
The author also demonstrated that the claws have high mineral content as it need to withstand particularly strong forces. However, carapace contains fewer minerals making it less heavy. There are more resilient compounds in carapace to form a thicker shell.
Elements represented with colors: Energy dispersive X-ray spectroscopy reveals the crab's carapace contains relatively little calcium in the outer layer, which has a greater magnesium and phosphorus content.
Carapace Geometry
The rainbow crab is known to reach sizes of 15 – 20 cm across carapace [6]. One of the crabs that we adopt, has a maximum top view carapace width of 4.5 cm and the smallest width at the back equals 2 cm. The medium top view carapace width at the front near the eyes equals 4 cm. The side view width is around 2 cm, and this makes the size of our crap across carapace around 8.5 cm.
Top-view drawings of the carapace, mathematically approximated. The medium width is at the top, the maximum width lies on the x-axis and the smallest width can be seen at the bottom.
Side-view drawings of the carapace.
In a study conducted on the carapace structure of shore crabs Carcinus maenas [7], it was found that the male carapace is thicker at sites with higher density of individuals. This was attributed to the higher possibility of competition for sexual partners and food at sites with higher density of individuals. This indicates that the geometry of the carapace is affected by the environment. Thicker carapace is advantageous in agonistic encounters. Interestingly, it was found that female crabs have thicker carapace compared to male crabs. This was attributed to that carapace of males are more susceptible to damage due to frequent disputes which force them to moult more often than females. However, there is still a possibility that the thicker carapace in females is independent of external factors and thick carapace may be beneficial in reproduction for females.
While the results of this study may or may not be applicable to the rainbow crabs, but it shows how the carapace geometry is influenced to favor certain dimensions in order to cope with functional groups (male or female) and sites conditions.
While the results of this study may or may not be applicable to the rainbow crabs, but it shows how the carapace geometry is influenced to favor certain dimensions in order to cope with functional groups (male or female) and sites conditions.
Mechanism
Crab carapace has high fracture strength and toughness. The properties of this biomaterial is related to the scientific investigation fine nanostructures. Under the scanning of electron microscope (SEM) on the carapace crab, it shows that the carapace is a kind of natural bioceramic composite which consists of calcite crystal layers and collagen protein matrix. The thickness of this crystal's sheet is within nanometer scale which arranged in parallel distribution. The investigation on maximum pullaout energy of this sheet is important as it is closely related to the strenght and toughness of the shell . Research shows that the parallel distribution of the nanometer calcite crystal sheets with the long and thin shape bring large pullout energy and endow the carapace with the high fracture toughness. Therefore, the mechanism of carapase is to protect and give them support. [8]
References
[1] Cornwall, J. (2014). Edible Crab Measuring Guide. [online] Helfordmarineconservation.co.uk. Available at: http://helfordmarineconservation.co.uk/os/regulations/edible-crab.php [Accessed 3 May. 2014].
[2] Madigan, M., Martinko, J. and Parker, J. (2000). Brock biology of microorganisms. 1st ed. Upper Saddle River, NJ: Prentice Hall.
[3] The composition of the exoskeletons of large arthropods: example of two crustaceas: The American lobster Homarus americanus and the edible crab Cancer pagurus. (2007). Thermochimica Acta, [online] 463(1-2), p.2. Available at: http://www.mpie.de/index.php?id=2836.
[4] P.-Y. Chen, A. Y.-M. Lin, J. McKittrick, and M. A. Meyers, "Structure and mechanical properties of crab exoskeletons," Acta Biomaterialia, vol. 4, pp. 587-596, 5// 2008.
[5] A. STIRN. 10 May 2014). Bionanocomposites. MATERIALS & TECHNOLOGY.
[6] (2014, May). Rainbow Land Crab. Available: http://www.exotic-pets.co.uk/rainbow-land-crab.html
[7] A. T. Souza, M. I. Ilarri, J. Campos, J. C. Marques, and I. Martins, "Differences in the neighborhood: Structural variations in the carapace of shore crabs Carcinus maenas (Decapoda: Portunidae)," Estuarine, Coastal and Shelf Science, vol. 95, pp. 424-430, 12/20/ 2011.
[8] Chen, Bin; Yin, Dagang; Chen, Xi; Yuan, Quan; Zhang, Zhiling, "Nanostructural Mechanism of Toughness of Crab Carapace", Journal of Computational and Theoretical Nanoscience, Volume 10, Number 6, June 2013, pp. 1436-1440(5)
[2] Madigan, M., Martinko, J. and Parker, J. (2000). Brock biology of microorganisms. 1st ed. Upper Saddle River, NJ: Prentice Hall.
[3] The composition of the exoskeletons of large arthropods: example of two crustaceas: The American lobster Homarus americanus and the edible crab Cancer pagurus. (2007). Thermochimica Acta, [online] 463(1-2), p.2. Available at: http://www.mpie.de/index.php?id=2836.
[4] P.-Y. Chen, A. Y.-M. Lin, J. McKittrick, and M. A. Meyers, "Structure and mechanical properties of crab exoskeletons," Acta Biomaterialia, vol. 4, pp. 587-596, 5// 2008.
[5] A. STIRN. 10 May 2014). Bionanocomposites. MATERIALS & TECHNOLOGY.
[6] (2014, May). Rainbow Land Crab. Available: http://www.exotic-pets.co.uk/rainbow-land-crab.html
[7] A. T. Souza, M. I. Ilarri, J. Campos, J. C. Marques, and I. Martins, "Differences in the neighborhood: Structural variations in the carapace of shore crabs Carcinus maenas (Decapoda: Portunidae)," Estuarine, Coastal and Shelf Science, vol. 95, pp. 424-430, 12/20/ 2011.
[8] Chen, Bin; Yin, Dagang; Chen, Xi; Yuan, Quan; Zhang, Zhiling, "Nanostructural Mechanism of Toughness of Crab Carapace", Journal of Computational and Theoretical Nanoscience, Volume 10, Number 6, June 2013, pp. 1436-1440(5)