One of the most common variants of the mirelurk, Scylla serrata horrendus is the result of mixing between Chesapeake Bay horseshoe and blue crabs. Regional variations exist: for example, crab mirelurks in the Capital Wasteland tend to be bipedal and slimmer, while those in the Commonwealth and Appalachia - migrating from Chesapeake Bay - are quadrupedal and rounder.
Their most noticeable feature is the large, chitinous shell that provides excellent resistance to firearms, melee weapons, and even ray guns. Soft body parts, such as their faces and sections of their abdomen, are much less resistant. Of course, that only applies to adult mirelurks. The shell's chitin takes time to harden and young mirelurks are quite vulnerable at that stage of their life.
An interesting feature of crab mirelurks is that they instinctively know how to protect themselves with their shell. When faced with prey that can attack them at range, a crab mirelurk will lower its head to cover its body with the chitinous shell, greatly increasing its ability to survive. Other animals tend to avoid mirelurks, perhaps instinctively realizing the danger posed by the shell.
When hunting, mirelurks rely on their extraordinary sense of smell, with their poor eyesight a secondary aid. They catch their prey with their primary large claws. Once immobilized, the prey is then stripped down and consumed with the claws on their abdomen, capable of stripping animals down to the bone within minutes.
They are ferocious apex predators and will pursue their prey incessantly. They mostly attack with there huge and powerful claws as well as slamming into prey in order to stagger them; therefore, it is best to keep mirelurks at a distance.
Mirelurks instinctively keep their vulnerable faces down, protected behind their hardened shell, until they are within striking distance. To this end, explosives are very effective against them until they close distance and show their faces; after this, shotguns and automatic weapons have the best chance to hit the small target.
- Quercus: " Oh, lovely! The classic mirelurk, migrated all the way from the Chesapeake Bay. Your friends have taken up quite a residence here, haven't they?"
- Morphological differences between the two crab mirelurks.
- Enclave Field Research Terminal, Field Entry: "Mirelurk": "Kingdom: Animalia
Class: Merostomata / Malacostraca / ???
After going to enormous lengths to procure DNA samples of hypothesized forebears, we cannot explain why ancestry evidence is present from two vastly different creatures; the Limulus polyphemus (horseshoe crab) and the Callinectes sapidus (blue crab). It's obvious that a mutation occurred, giving this creature bipedal movement and massive growth, but I cannot explain the presence of multiple subphylum markers in DNA/RNA. Given their mystifying origins, I have decided the species deserves its own classification, and have thus dubbed them "Scylla Serrata Horrendus." They can often be found in sewers and caves and should be considered hostile and very dangerous, as their chitnous shell provides excellent resistance to both firearms and hand-held weapons. They are disgusting creatures, and I cannot fathom their popularity as a food source among the citizens of the Wasteland."
- Fallout 4 loading screen hints: "Most Mirelurks are heavily armored and resistant to most forms of damage, but their faces are vulnerable."
- Tenpenny Tower terminals; Exploration Database, Mirelurks
- Softshell mirelurks
- Behavior of mirelurks in Fallout 4.
- Mirelurk Research