While usually being associated with mutated crabs, the term Mirelurk refers to a range of mutated aquatic species.
The crab-like mirelurk is considered both fearsome and formidable by wastelanders. Mirelurks are generally social with others of their kind, although they tend to be aggressive to other creatures and one another if one gets too close to another.
Most adult mirelurks appear to have a hardened carapace on their backs, which is suggested to be their primary defense mechanism. While their eyesight appears to be poor, mirelurks have an extraordinary sense of smell. After using their crab/spider-like legs and large claws to reach and catch prey, they use a smaller set of abdomen claws to strip their prey down to the bone within minutes. Most mirelurks grow remarkably fast, and the shelled-variant has been observed to reach maturity within weeks.
The base variant of the mirelurk attacks exclusively with melee. They will get close, usually in a group of three or more and attempt to hit the player character with their large claws. They will duck into their hard shell as a way to block incoming melee damage. They do this often and it can become an issue if the player character is being attacked from multiple sides, as when they turn to attack a different foe, the crouched mirelurk will leap up and attack almost instantly. They are vulnerable to explosives and headshots. Mirelurk spawn and hatchlings are weak. They can be killed easily, especially with explosives. They are used mainly as a distraction.
Hunters are the next variant, with high mobility and melee damage. The lack of a hardened carapace means that they have slightly less concentrated armor protection, however, they have superb overall protection, meaning they are not nearly as vulnerable to headshots. They can also utilize a poisonous spit attack, similar to that of the mirelurk queen, but on a much smaller scale. This ranged ability means they will often hang back from the main pack and force the player character to have to approach them.
Mirelurk kings are highly agile and well-armored foes that are almost always encountered nearby a mirelurk queen. Though usually alone, it is not uncommon to find more than one king at a mirelurk den. They will run up and swipe, as well as perform a powerful sonic blast attack from range, with a very high chance to stun. They also have a high HP pool and damage output.
The mirelurk queen is the largest enemy one can encounter in-game besides the scorchbeast queen and behemoth. She will attack with powerful melee strikes, and a very effective poison spit attack, which can blanket an entire area in a matter of seconds. This creates an area of effect pool, which ramps up damage the longer the target stays inside. She can also surprise unsuspecting player characters when she rises from a nearby water source other mirelurk are gathered around, but the roar she makes to alert them of her appearance can send them into a panicked run to get away once they hear it. They can also release mirelurk spawn like grenades, using them as a distraction while she kills the opponent. Being one of the few boss enemies, she is very difficult to kill. Her high armor and HP can make her a tough enemy for most player characters, but she is vulnerable in her head, spouts and legs.
This type of mirelurk has mutated from the lobster population native to Chesapeake Bay. 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.
Mirelurk kings do not resemble the crablike mirelurks, instead appearing to be heavily mutated frogs based on sounds they make and the webbing between their fingers and toes. They are very fast and can easily kill a low level player character in seconds.