This entry is focusing on describing and detailing out the claws functions, mechanisms, and geometries. The claws to the crab are as important as the hands to the humans. A plenty of functions, offensive and defensive can be carried using them. Along with that, the claws may be used as a stand or for grasping/catching food.
As crab is classified as arthropod animals its Legs and cheliped (arms) are the two parts of its limb. The cheliped of the crab is divided into three parts which are outer margin, carpus and claws.
Anatomy of cheliped of the crab [1]
Crab has a single pair of claws which is called chela. Each Chela has two fingers, fixed finger and movable finger.
Segmentation of the cheliped shows that the fixed and movable fingers [2]
The right claw of the crab is bigger and blunt used for crashing compared to the left one which is smaller and sharper that is used for cutting.
A photo of the crab shows that the left is bigger than the right chela
The joint of the cheliped part which is connected to the crab’s body is located besides the eyes of the crabs. The distance between the two limbs of the crab across the broadest part of the shell is 160mm for male and 150 for female [3]. Along with that, the end of the claw is close to the crab’s mouth. That is because the big chela will take the food and bring it to the mouthparts.
The distance between the two limbs of the crab across the broadest part
Mechansim
Crab's claws have only one moving part which is the dactyl. Basically, the crab claw has two fingers, one is moving and one is not. The dactyl is the movable finger, and it rotates about a hinge axis to the next fixed finger. The muscles are attached to an outer cuticle and a movable tendon or apodeme. The cuticle are attached to the end of the movable finger, moreover the cuticle serves as an exoskeleton support and a rigid support for the muscle to be attached. The apodeme is placed at the back end of the movable finger. As the muscle contracts, the apodeme pushes the movable finger from the back and at the same time the cuticle muscle expands allowing the movable finger to open. In the reverse process, the cuticle muscle contracts forcing the movable to rotate toward the fixed finger and the apodeme muscle expands allowing the movable finger to close [4].
Source [4]
Offensive Purposes
The first pair of limbs for the crab is called pincers (or chelae; also called "claws"). It is for fight or to carry food. The offensive mechanism is physical attack with their claws which have a mobile "finger", formed by the last segment (dactylus), that can be closed with another "finger" that extends from the previous segment (propodus). [8]
This enables the crab to have high tension stress when clipping onto enemies which will cause pain. Therefore, the predators will retreat. The crab also in its nature likes to fight with their own species. Their fighting strategies is to harm or break the opponent's leg which will weaken them. This give them the advantage over finding food and also set their territories.
As shown in the figure where rainbow crab has 2 different sizes of claws has different function (claw dimorphism). They are generally identified as the`crusher' (larger claw) and `cutter' (smaller claw) in accordance with presumed functional differences. This is to trade off between strength and speed which give rise of a strong and a fast claw during attacking. [9]
This enables the crab to have high tension stress when clipping onto enemies which will cause pain. Therefore, the predators will retreat. The crab also in its nature likes to fight with their own species. Their fighting strategies is to harm or break the opponent's leg which will weaken them. This give them the advantage over finding food and also set their territories.
As shown in the figure where rainbow crab has 2 different sizes of claws has different function (claw dimorphism). They are generally identified as the`crusher' (larger claw) and `cutter' (smaller claw) in accordance with presumed functional differences. This is to trade off between strength and speed which give rise of a strong and a fast claw during attacking. [9]
Defensive Purposes
Crabs have a number of ways to defend themselves from a predator. The special features of a crab such as pincers and hard shell are one of the methods to escape trouble from predator. A crab can use their pincer to pinch with force, so the predator is difficulty to get away from crab [5]. It will sometimes shedding a leg or claw and this is the reason why crab is often seen with legs missing. This strategy is called autotomy [6]. Crabs have hard exoskeletons, known as the carapace. Carapace is essentially to keep predator from biting down on it or must be cracked before it can be eaten. The crab also learns camouflage to protect themselves from enemy. The crab attaches external objects including plant matter or stones, to their bodies to allow them to better match their background [7]. Hiding is one of their defence strategies too. It protects itself by burying into the mud or sand [5].
References
[1] Cornwall, J “ Edible Crab Measuring Guide” , 2014, retrieved 3/5/2014 from: http://helfordmarineconservation.co.uk/os/regulations/edible-crab.php
[2] “True Crabs” Universidade Santa Úrsula, Brazil, 2014, p.329.
[3] “Basis”, Zipcodezoo.com, 2014, retrieved 3/5/2014: from http://www.zipcodezoo.com/glossary/basis.asp
[4] H. Kroening. (2000). Decapod Claw form and fuction (subphylum Crustacea, phylum Arthopoda). Available: http://www.biology.ualberta.ca/facilities/multimedia/uploads/zoology/CrabClaw.swf
[5] Beverly Hernandez. "What Is A Crab", 2014, retrieved 7/5/2014 from:
http://homeschooling.about.com/od/unitssubjsci/p/crabs.htm
[6] "Crabs", 2014, retrieved 7/5/2014 from:
http://www.pznow.co.uk/marine/crabs.html
[7] Brenton Shields. "How Do Crustaceans Protect Themselves", 2014, retrieved 7/5/2014 from: http://www.ehow.com/about_6328364_do-crustaceans-protect-themselves_.html
[8] "Offense and defense" 2014, retrieved 7/5/2014 from: http://speculativeevolution.wikia.com/wiki/Offense_and_defense
[9] Steve C. Schenk1 and Peter C. Wainwright, "Dimorphism and the functional basis of claw strength in six brachyuran crabs" 2011, Journal Zoological London, 255, 105-119.
[1] Cornwall, J “ Edible Crab Measuring Guide” , 2014, retrieved 3/5/2014 from: http://helfordmarineconservation.co.uk/os/regulations/edible-crab.php
[2] “True Crabs” Universidade Santa Úrsula, Brazil, 2014, p.329.
[3] “Basis”, Zipcodezoo.com, 2014, retrieved 3/5/2014: from http://www.zipcodezoo.com/glossary/basis.asp
[4] H. Kroening. (2000). Decapod Claw form and fuction (subphylum Crustacea, phylum Arthopoda). Available: http://www.biology.ualberta.ca/facilities/multimedia/uploads/zoology/CrabClaw.swf
[5] Beverly Hernandez. "What Is A Crab", 2014, retrieved 7/5/2014 from:
http://homeschooling.about.com/od/unitssubjsci/p/crabs.htm
[6] "Crabs", 2014, retrieved 7/5/2014 from:
http://www.pznow.co.uk/marine/crabs.html
[7] Brenton Shields. "How Do Crustaceans Protect Themselves", 2014, retrieved 7/5/2014 from: http://www.ehow.com/about_6328364_do-crustaceans-protect-themselves_.html
[8] "Offense and defense" 2014, retrieved 7/5/2014 from: http://speculativeevolution.wikia.com/wiki/Offense_and_defense
[9] Steve C. Schenk1 and Peter C. Wainwright, "Dimorphism and the functional basis of claw strength in six brachyuran crabs" 2011, Journal Zoological London, 255, 105-119.