Although the games do not specifically state the origins of mirelurks, it can be discerned from their appearances that they are the products of shellfish who have been living and breeding in irradiated waters for over 200 years following the Great War.
Given this level of diversity, it can be assumed that the term "mirelurk" is more than just the common name for one species, but rather a colloquial term for any foul, marsh-dwelling creature living together in a symbiotic relationship as with each other. Before the war the mutation was predicted by researchers at Nahant Oceanological Society, the dumping of radioactive waste would aparently lead to greater size and toxicity.
Most mirelurk variants have shells resembling those of horseshoe crabs. The queen, on the other hand, has a shell that resembles that of a lobster or spider crab. The hunter appears to be a shrimp or a much smaller lobster compared to the queen. Finally, the king is unique in that it bears no resemblance to any sort of shellfish, and may instead be a mutated frog or fish.
All mirelurks (excluding mirelurk kings and softshell mirelurks) are protected by a thick shell which is difficult to penetrate, even with powerful ballistic weapons. The face is unarmored but a small target, and when charging a mirelurk will often lower its head to protect this area. However, the face will inevitably be exposed once the creature commences close combat. Extremities are also more vulnerable than the rest of the body, though not as much as the face.
Typical specimens in particular resemble mirelurk hunters from the Capital Wasteland, but also differ in several aspects, such as is seen with their legs. This specimen also walks in a mixed crab/spider-like motion, which is atypical of mirelurk variants documented so far; in general they have more realistic qualities correlating to appearance.
Mirelurks will bury themselves in dirt or mud, with the only visual indication that they are present being the rounded backs of their shells, which resemble large, smooth rocks from a distance. V.A.T.S. will be able to detect them from a distance when used. It is not uncommon to see mirelurks with netting and seaweed caught on their carapaces or have barnacles latched on to them. Typical mirelurks and their variants can occasionally be found feeding on sea creatures.
Mirelurks are mostly found in groups and thus often appreciate strength in numbers, and glowing mirelurks also deal radiation damage when in close quarters. Combine this with their generally high resilience they can easily prove a challenge for low-level player characters.
Because of their thick shells, they can be quite hard to defeat in a melee fight, as they tend to hide their weak points when the player character is about to strike, or when they charge before revealing their body parts in order to attack. Because of their unique fighting style, melee weapons with the legendary modifiers "Penetrating" (attacks ignore 30% of enemy's damage resistance) and "Exterminator's" (specific bonus damage to mirelurks), are very useful against them.
As some companions mention, mirelurks have no way to protect themselves against mines, or any attack from below. Even being on a lower platform that can be shot through give the player an advantage.
When approaching a mirelurk nest, attacking or stealing mirelurk eggs will force nearby eggs to hatch prematurely, revealing these small crab like creatures. The mirelurk queen can spawn several hatchlings to attack the player
character. In terms of overall combat they are practically identical to radroaches.
A more powerful variant of the mirelurk that may be encountered commonly as the player character levels up. It has double the health of a softshell mirelurk and deals significantly more damage per hit, making them potentially challenging opponents for those that are poorly-equipped.
A powerful type of mirelurk that can be encountered in any mirelurk-infested area, such as Salem, if the player character's level is high enough. It is much more resilient than the razorclaw, and is more powerful in combat.
An irradiated variant of the mirelurk which is increasingly encountered as the player character levels up. Due to steady absorption of large quantities of radiation these mirelurks glow with its superabundance; their undersides are positively luminescent and a notable build up of radiation in its eyestalks gives them a distinct feature even among "glowing" creatures.
Glowing mirelurks inflict radiation damage on top of the ballistic damage. This variant has a considerable amount of health (half of that of a super mutant behemoth), and deals considerable amount of damage in combat, making them challenging opponents for low-level player characters.
The most powerful type of the base mirelurk genus in terms of both resistances and damage potential, Bloodrage mirelurks were added to the game with the Far Harbor add-on. They have very dark carapaces, glowing red eyes and also emit a baleful reddish glow from their underside. Their presence is not exclusive to the Island - the Sole Survivor can encounter them in the Commonwealth as well once they reach level 42.
The mirelurk queen is a rarely seen mirelurk variant. The queen is considerably larger and more dangerous than her children and inflicts more radiation on the player character. The mirelurk queen's acidic spittle rapidly damages anyone caught in it and she can spawn new hatchlings. Crippling her spouts will prevent her from using her acid attack. She benefits from the same resistance as the regular mirelurks: an incredibly thick shell but a very soft skin.
This type of mirelurk has mutated from the local lobster population. They are significantly more resilient than regular mirelurks and gain the ability to spit acid at its enemies, as well as more powerful attacks. A few can be found at Spectacle Island, and others may spawn in the tunnels during The Big Dig.
Mirelurk hunters that have been born afflicted with albinism, giving them a distinguishable pinkish-white skin color signature to albino creatures. While their attacks are indifferent from that of a regular mirelurk hunter, they are protected by higher resistances. They, as with most albino creatures, are very rare and appear only by chance.
Mirelurk kings do not resemble the crab-like mirelurks, instead appearing to be heavily mutated frogs based on sounds they make. They are very fast and can easily kill a low level player character in seconds.
Unlike the other mirelurks, they have a ranged sonic projectile attack that is particularly effective at crippling the target's head. Mirelurk kings have less health than glowing mirelurks, mirelurk hunters or mirelurk queens, although they are much more agile and efficient at range. One can be found in a room alongside power armor, a few tunnels into the The Big Dig. Up to four can be found at Cutler Bend, and a few may be encountered at Spectacle Island. Another can be found inside a crypt to the north of Electrical Hobbyist's club underneath the skybridge. He will appear and attack after taking the lone mirelurk egg.