Crab Defense, also known as the Crab Shell or Crab Style, is a style of fighting primarily used in boxing. Variations of this style use the Cross-armed Guard, Reverse Cross-armed Guard and the Philly Shell or Michigan Defense. Crab Style fighters Floyd Mayweather Jr., Pernell Whitaker, James Toney, Sugar Ray Robinson, Bernard Hopkins and Nicolino Locche are widely considered some of the greatest defensive boxers of all time. Crab Style is considered one of the oldest styles in modern boxing, but variations of this defense are used in several martial arts.
Stance.
Crab Style relies primarily on a bladed, or side, stance. The fighter gives the appearance of using a sideways motion towards the opponent which resembles the sideways movement of a crab. When the Crab Style is used in certain martial arts, however, such as Muay Thai or Mixed Martial Arts, the fighter often adopts more of a squared stance to defend against leg kicks.
Guard.
Variations of Crab Style use different guards including:
Cross-armed guard.
The forearms can be placed on top of each other horizontally or diagonally in front of the face with the rear arm (right for an orthodox fighter and left for a southpaw) being on the top of the lead arm with the rear hand glove over the lead elbow or shoulder. The position of the lead arm (left for an orthodox fighter and right for a southpaw) is greatly varied when it rises vertically. The head is titled towards the rear shoulder to keep the head off of center-line, and to make space to use the shoulder to block. The lead shoulder is brought in tight against the side of the face. The forearms and elbows can be used to block, or the palm of the gloves can be used to catch or parry punches
Reverse cross-armed guard.
The forearms can be placed on top of each other horizontally or diagonally in front of the face with the lead arm (left for an orthodox fighter and right for a southpaw) being on the top of the rear arm with lead glove over the rear shoulder. The position of the lead arm (left for an orthodox fighter and right for a southpaw) is greatly varied when it rises vertically.
Philly shell or Michigan Defense.
The lead arm is placed across the abdomen, below the rear arm, to protect the body. The head is titled towards the rear shoulder to keep the head off of center-line, and to make space to use the shoulder to block. The lead shoulder is brought in tight against the side of the face. The rear hand can be placed next to the chin close to the rear shoulder to defend against hook punches, or placed in a cross-arm block position, with the rear hand over the lead shoulder to protect against straight punches. 
The term Philly Shell refers to Philadelphia fighters who use this guard such as George Benton, Bernard Hopkins and Tim Witherspoon. Whereas the term Michigan Defense refers to Michigan fighters known for this defense such as Floyd Mayweather Sr., Roger Mayweather and James Toney.
Blocks.
The Crab Style uses traditional boxing defense such as slipping and the Bob and weave. It also uses defensive techniques largely unique to this style in boxing, but which are common in other martial arts such as the wedge block, also known as the horizontal forearm block or leverage block. In Karate this block is called Age-uke and in Taekwondo it is called Ulgul Makgi or Olgul Makki. This block is used primarily with the lead arm to defend against straight punches and uppercuts in boxing as well as kicks in martial arts. While Boxers who use the conventional guard, high guard, or Peek-a-boo style use their elbows to block their bodies, Crab Style fighters are unique as they also use their elbows to block their heads.
Shoulder roll.
Crab style fighters often make use of the shoulder block and shoulder roll. To execute the shoulder roll a fighter rotates and ducks (to the right for orthodox fighters and to the left for southpaws) when their opponents punch is coming towards them and then rotates back towards their opponent while their opponent is bringing their hand back. The fighter will throw a punch with their back hand as they are rotating towards their undefended opponent.
Origins.
The origin of the Crab Style is unknown, however, it is considered one of the oldest styles in modern boxing. The Crab Defense may even predate the Marquess of Queensberry Rules since elements of the Crab Defense are found in boxing manuals from the bare-knuckle boxing era. 
A 1789 boxing manual depicts a boxer defending a combination of a strike to the body and then to the head with the use of the forearm across the abdomen to block the body and the cross guard to block the head in the same way a modern Crab Style fighter would use.
In modern boxing, variations of the Crab Defense were used by Boxers such as Tommy Ryan and Jim Jeffries. Its use has continued to the present day, although sometimes under alternative names such as the armadillo, stonewall, or hitman defenses.
Boxing.
Crab Style can be used at all ranges and in all of the four boxing styles : In-Fighter, Out-Boxer, Slugger and Boxer-Puncher.
In fighter: Crab Style often provides two levels of head defense. For example, the lead arm can be used to wedge block, elbow block, shoulder block or shoulder roll to defend against the jab and cross, while the rear hand can be used as a cross block to provide a second line of defense against the same punches in case the lead block fails. This allows an in-fighter to move forward while being able to adequately defend in the process. Examples of In-Fighters who have used the Crab Style include Joe Frazier and James Toney.
Out-fighter: The Crab Style relies heavily on one-handed or no-handed defense. For example, the lead wedge block can block both the jab and the cross leaving the rear hand free to punch even while blocking. Alternatively, the rear cross block can defend against both the jab and the cross leaving the lead hand open to punch even while defending. The shoulder roll uses no hands for defense and leaves both hands free to punch while defending.
This allows an out-fighter to attack even while defending making it easier for them to disengage and land long-range punches even while defending. Examples of Out-Fighters who have used the Crab Style include Floyd Mayweather Jr. and Shakur Stevenson.
Slugger: Sluggers often throw predictable punching patterns (single punches with obvious leads) which can leave them open for counterpunching. Sluggers can, therefore, rely on the crossed-arm guard of the Crab Style to defend if they miss a power shot and are forced to defend. Examples of Sluggers who have used the Crab Style include George Foreman and Jim Jeffries.
Boxer-Puncher: As Boxer-Punchers often use tactics used by Out-Boxers and Swarmers, they can also benefit from use of the Crab Style. Examples of Boxer-Punchers who have used the Crab Style include Sugar Ray Robinson and Archie Moore.
Kickboxing.
Kickboxing includes elements of boxing and martial arts that use kicking techniques. Many kickboxers have, therefore, used the Crab Defense. Some examples are Bill Wallace, Don Wilson, Alvin Prouder and Manson Gibson. Unlike boxing, however, kickboxing often allows for spinning kicks and spinning backfists. Instead of a shoulder roll, the kickboxer may, instead, opt for spinning techniques.
Muay Thai.
Some Muay Thai fighters that also have boxing backgrounds have used the Crab Style in Muay Thai. Examples include Samart Payakaroon, who is also a former WBC boxing champion, and Somluck Kamsing who was an Olympic gold medalist in boxing.
Some Muay Thai fighters, such as Dieselnoi and Buakaw, use the Dracula guard, a variation of the Crab Style. The Dracula guard uses one arm, usually the lead arm, in a long guard and the other arm, usually the rear arm, in the cross arm guard position. Another variation of the Crab Style used in Muay Thai is the Diamond Guard.
Mixed Martial Arts.
Mixed Martial Arts also includes elements of boxing, and many Mixed Martial Artists have used the Crab Style including Sean Strickland,
Ilia Topuria, Dustin Poirier, King Green and Yoel Romero.
Sport Taekwondo.
Styles of martial arts have different techniques, and these techniques can vary even within styles. Some styles of martial arts have techniques similar to the Crab Style. Traditional Taekwondo is designed for self-defense. Practitioners of Taekwondo, or taekwondoin, often begin blocks without a guard, but in the ready position (joon bi or junbi) instead. Some schools teach that the Downward Block (Naeryeo Makgi), Twisting Block (Biteureo Makgi) and Scissor Block (Gawi Makgi) have a chamber position that is very similar to the reverse cross arm guard. While some schools teach the Upward Block (Ollyeo Makgi), Outward Block (Bakkat Makgi) and Knife Hand Block (Sonnal Makgi) have a chamber position that is very similar to the cross arm guard. Some schools consider this chamber position to represent a parry position like is used with the Crab Style.
Some Taekwondoin, may also use techniques similar to Crab Defense while sparring or kyorugi. Some Taekwondoin may adopt a guard for kyorugi where the lead arm is held across the abdomen to block against body kicks and punches. Their lead arm can also be used as a wedge block, or Olgul Makki, to defend against high kicks and punches. Whereas the rear hand is used in the cross arm guard position to defend against high kicks and punches and the rear forearm and elbow used against body shots. The shoulder roll is occasionally used, but spinning techniques can be used instead.
Sport Karate.
Traditional Karate is designed for self-defense so practitioners of Karate, or karateka, often begin blocks without a guard or in the ready position (yoi). Karate blocks often go from the yoi position to a chamber position as the first part of a defensive technique. Some schools teach that the Knife-hand block (Shuto-uke) has a chamber position that is very similar to the cross arm guard.
Some schools teach that the low sweeping block (Gedan barai) has a chamber position that is very similar to the reverse cross arm guard. Some schools consider this chamber position to represent a parry defense like is used with the Crab Style.
The Crab Style is often used in Karate, as it is used in Taekwondo, during sparring or kumite, but has additional benefits in Karate. Karateka often use a bladed stance during kumite similar to the Crab Style. Since sport Karate kumite rules, such as the World Karate Federation rules, often allow holding of the uniform, or gi, and hitting as well as takedowns, the Crab defense provides defense against grappling. Since the lead arm rests against the abdomen and the rear arm rests across the lapel, this provides a passive defense against upper and lower lapel grabs. Additionally, since the arms are pressed against the body in the Crab Defense, this makes it difficult for an opponent to use common grappling tie-ups in the clinch such as underhooks and overhooks, whereas because the Crab Defense uses a low lead hand this can be used for an underhook position to counter takedowns.
Sport Jujutsu.
As with Sport Karate, Sport Jujutsu uses punching, kicking and grappling from the standing position as well as a gi, Keikogi. The Crab Defense is sometimes used in Sport Jujutsu to defend against strikes and takedowns. The Crab Defense is used from the standing position and should not be confused with ground fighting techniques such as the Crab Ride and the Boston Crab.