Headcrab Evolution

Kleiner’s headcrabs (also known as the Corvusbarmagnet headcrabs) are a group of three known species of alien spine puppeting monsters. They are often classified as the subfamily Leucochloridium. It is still not clear what dimension they belong to. They were first teleported to earth from Xen during and after the Black Mesa Incident in early 2000. They originally existed as Xen Wildlife but have since evolved and adapted quite remarkably to the conditions here on Earth.

The term “Kleiner’s Headcrabs” was first applied by yours truly, Isaac Kleiner. I’ve always had a deep fascination in the alternate evolutionary line of the Cranium Brachyuras. The most important differences between species are in the size, speed, and presence of venom. They have highly adapted to their environments and different food sources. The crabs are all dull-coloured.

Kleiner's theory

During the events following the Black Mesa Incident, Kleiner had no idea of the significance of the crabs of Xen. He had learned how to preserve crab specimens while bored and hiding from the combine behind a vending machine hideout. During the incident, he mostly left crab shooting to his co-worker Gordon Freeman. Nonetheless, these crabs were to play an important park in the inception of Kleiner’s theory of evolution in Xen creatures.

Kleiner was interested in the geographical and rapid distribution of headcrabs as a biological weapon from combine and how they evolved thereafter. Kleiner noted during a visit to Ravenholm that some of the species of headcrab were much darker and faster and could not breed with their chubby brown brethren. This lead him to believe that they had adapted to the dark zombie-ridden conditions accordingly.

Following his return from the voyage, Kleiner presented the crabs to his own domesticated pet headcrab, Llamar, because no one seems to be interested in how interdimensional spine controlling monsters evolved differently than our own pool of animals. It’s always ‘survival this’ and ‘we need more fresh water’ that.

Kleiner found that the Fast Headcrabs could not breed with Standard Headcrabs, meaning an entirely new species evolved record-breakingly rapidly on Earth.

Black Mesa, 1995, Pre-Incident

Molecular basis of crab evolution

The Standard Headcrab is the mother species. They are small and stumpy, but their back legs have evolved to allow them to leap up to ten feet forward toward a victim’s face. Once attached, the living host will begin to suffocate. If the host is already dead, then they skip straight to puppeting their host by attaching their mouths and “beaks” to the cranium. An example of how quickly Xen wildlife evolves, according to data I’ve scavenged before escaping black mesa, is how the Standard Headcrabs (and subsequent species) evolved to no longer have eyes in the span of roughly 20 years. We still aren’t sure why.

The Headcrab underbelly is dominated by a large “mouth” with a hidden beak-like latch used to attach to skulls of victims and to feast. This creature has also been noted to habitually burrow, similar to a tarantula.

The Fast Headcrab

The Fast Headcrab is a subspecies that was nonexistent before the Combine Invasion. This subspecies has longer legs, darker skin and a faster movement speed. There are a few theories of how this (arguably) superior headcrab came to be. The most prominent idea is that in heavily populated areas and hideouts survivalists began to understand the movement patterns of the Basic Headcrab and would shoot them on sight. Very quickly, the slower of the Headcrabs would die off. In darker areas like cave, hideouts, and Ravenholm, the bright tan coloring would make the Basic Headcrab a very easy target. Thus, dark skin began to dominate as environmental camouflage. The Xen species evolves so quickly that in an estimated matter of 5 years the Fast Headcrab was no longer able to bear children with the Basic Headcrab.

The Poison Headcrab is one of the most interesting mutations. They evolved to wait very patiently in high-security or dangerous areas surrounding combine, antlions, or even other headcrabs. They’re the slowest of the headcrab family, but have adapted a frightening projectile poison attack that will instantly bring you to critical conditions. Without an antidote, one shot from their affliction will have your average human crawling in pain. Any immediate physical injury thereafter will serve to kill their prey. The poison headcrab evolved the poisonous ability through excess exposure to radioactive decay. They, too, have the ability to latch on to corpses to reanimate them. However, unlike the other two species, up to three poison headcrabs will puppet one host at a time. One will latch to the skull and two will latch to the spine. This creates a very strong zombie with much more health than a Basic Headcrab, in addition to poisonous projectile attacks,

Three Poison Headcrabs working together.

"Have you ever seen such a magnificent species? These 'crabs' can completely control their host's nervous system. Can you imagine what the next stage of mutation looks like?"