Mirelurks have the dubious distinction of being one of the first radioactive mutants to emerge into the world, as a result of widespread radioactive pollution of the environment by United States megacorporations such as Poseidon Energy and General Atomics. Environmental organizations such as Nahant Oceanological Society, dedicated to saving the oceans, attempted a variety of last-ditch efforts to raise awareness of this problem and encourage radical action to curtail environmental destruction at the hands of reckless corporations. However, their efforts were in vain. Ignored by authorities, to the point where they refused to even take their phone calls (as was the case with the Boston Port Authority), they attempted to garner attention by reporting the situation to the Galaxy News Network. The media ignored their reports as well, except for running a story about the upcoming shellfish season and how great it was going to be. Of course, it neglected to mention that the average increase in mass on the order of 14% in crabs and 12% in lobsters meant that the available food supply would run out quickly, leading to colony collapse and localized extinction.
Although the Great War put an end to corporate pollution, it was replaced by overwhelming amounts of nuclear fallout instead. In the ensuing melting pot of mutations, the modern mirelurks emerged. Wasteland scholars generally assign it its own classification: Scylla serrata horrendus. The knowledge that mirelurks were much smaller before the bombs dropped and mutated them still persists among some wastelanders in the Commonwealth. Mirelurks are also known to have a pronounced sensitivity towards white noise, as seen in Vault 92 and on Spectacle Island.
Although mirelurks tend to inhabit polluted rivers, coastal waters, and sewers, their meat is considered a delicacy among wastelanders. Mirelurk hunters or herders can count on a steady demand for their meat and make a high profit selling it to wasteland traders. However, the size of the meat market means competition is stiff and suppliers can be targeted by rivals attempting to knock them out of the market (sometimes literally). They also grow remarkably fast, requiring just a few weeks for them to reach maturity. If corraled and domesticated, mirelurks can be a flexible supply of food.
Mirelurks can also be a blight on permanent settlements, as is the case with Rivet City. Mirelurks nesting below inhabited decks are a continued nuisance for denizens of the city, despite regular extermination operations by the city's security team. Infestations are made even worse by the fact that mirelurks have adapted to lay their eggs in multiple small nests throughout shallow water (unlike pre-War crustaceans that adhered their eggs to their body); if disturbed, the larval mirelurks within exhibit incredible predacious behavior and are ready to attack if need be.
Most adult mirelurk variants have shells and pincers resembling those of horseshoe crabs, while their offspring resemble mole crabs. The queen, on the other hand, has a shell that resembles that of a giant isopod or spider crab. The hunter variant appears to be a lobster.[Non-canon 1] Finally, the mirelurk king is unique in that it bears no resemblance to any sort of crustacean.
All mirelurks (excluding mirelurk kings and softshell mirelurks) are protected by a thick shell that 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 is the rounded backs of their shells, which resemble large, smooth rocks from a distance. They can also submerge themselves underwater when needed. 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 utilize strength in numbers. Glowing mirelurks also deal radiation damage when in close quarters. Combine this with their generally high resilience and 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. All mirelurk variants have a chance to be pacified through the Wasteland Whisperer perk.
When nearing a mirelurk nest, typically about half of its eggs will shake briefly and then hatch, revealing these small, aggressive crab-like creatures. The mirelurk queen can spawn several hatchlings to attack the player character. Their overall combat strength is practically identical to radroaches.
A Nuka-Cola imbued mirelurk hatchling which can emerge from eggs littering the World of Refreshment attraction or birthed by the Nukalurk queen. It shares the same appearance as a mirelurk spawn, with the exception of its blue glow.
A more powerful variant of the mirelurk that may be encountered commonly as the player character levels up. It has over double the health of a softshell mirelurk and deals significantly more damage per hit, making them potentially challenging opponents.
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 the steady absorption of large quantities of radiation, these mirelurks glow with its superabundance; their undersides are positively luminescent and a buildup 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.
Despite their power and threat potential, killing a bloodrage mirelurk awards significantly less experience than most of its lower-ranking variants.
The mirelurk queen is a rarely seen mirelurk variant, considerably larger and more dangerous than her offspring, and inflicts significantly more physical and radiation damage on the player character. Her arrival is announced with a great roar. In addition to her claws, she possesses a ranged acidic spittle that deals damage over time to anyone caught in it. Once combat is initiated, the queen will occasionally eject a barrage of eggs that spawn a wave of hatchlings, an ability that is considered a grenade attack and therefore comes with the respective warning icons on the screen. One queen was able to knock a large hole into the thick stone walls of the Castle in 2240. She benefits from the same resistance as the regular mirelurks, featuring an incredibly thick shell but very soft skin.
Subsequent respawns may have her either already emerged and idling in her area, or else submerged if her first spawning-in is triggered by killing nearby mirelurks. The Sole Survivor notes her presence (as well as that of hidden kings and hunters) by sneaking to triangulate her location, and then triggering her appearance with an explosive or a ground slam from power armor.
Mirelurk queens are considered giant creatures in-game, therefore, killing them counts towards the achievement ...The Harder They Fall.
While the mirelurk queens may seem similar to the common "horseshoe crab" variant of mirelurk at first glance, the mirelurk queen has a full crustacean telson, unlike that which the horseshoe crab has.
The Nukalurk queen can only be found outside the Nuka-Cola bottling plant. She is a unique version of the mirelurk queen and has a blue glow due to the strontium isotope from Nuka-Cola Quantum. She has a large health pool, but when the player character finally kills her she drops queen Nukalurk meat.
This type of mirelurk has mutated from the local lobster population.[Non-canon 1] They are significantly more resilient than regular mirelurks and gain the ability to spit acid at its enemies, as well as more powerful attacks. While the common mirelurk is vulnerable to headshots, the head of the mirelurk hunter is much stronger. 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 and the webbing between their fingers and toes. They also uniquely possess both gills and lungs that were developed independently, allowing kings to survive on both land or water easily.[Non-canon 2] 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 long range. One can be found in a room alongside power armor, a few tunnels into the Dig. Another one can be located behind Rook family house on the shore. 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.
A mirelurk king imbued with high amounts of Nuka-Cola Quantum, causing it to glow blue. It has greater health than other mirelurk kings as a result of this, and has stronger attacks as well. They can be found outside the World of Refreshment, near the mirelurk queen.
The following is based on unverified behind the scenes information and may be inaccurate.
The mirelurk model was created by Jonah Lobe.[Non-canon 3] Lobe has stated that he greatly prefers the Fallout 4 mirelurks compared to those from Fallout 3, which he also modeled. The Fallout 4 mirelurks were designed to be more realistic, yet also more original in shape, based on concept art by Ilya Nazarov. He also feels that the Fallout 4 mirelurks have more personality, being hungry and "stupid-looking." The Garthim from the movie The Dark Crystal were also a major inspiration to their design.[Non-canon 4]
The mirelurk queen was added late into development, creating another large enemy to encounter, specifically for the coastal regions of the Commonwealth.[Non-canon 5]
End of information based on unverified behind the scenes information
↑The Lone Wanderer: "They descended from local crabs. I'd call them "Scylla Serrata Horrendus."" Moira Brown: "That's very scientific of you! Personally, I wasn't sure if they were crabs or if they came from some sort of brine shrimp, perhaps. Some of these observations about their armor and camouflage gave me an idea for reinforced, neutral-colored headgear. Here, consider it thanks for not interfering with them. Oh, speaking of which, take these. So you can continue to avoid them in the future." (Moira Brown's dialogue)
↑John Hancock: "I'd heard Mirelurks were tiny before the bombs. Wonder what else might be plying those waters." (John Hancock's dialogue) Note: This line is spoken if Hancock is asked for his thoughts while on any large boat, such as the ones at Libertalia.