Thresher Maw

Thresher maws were enormous subterranean carnivores that spent their entire lives eating or hunting for something to eat, bursting up from the ground without warning when disturbed. Their spores could lie dormant for millennia and survive deep space and atmospheric re-entry, so the creatures appeared on worlds across the galaxy, spread by previous generations of space travelers. From the massacre of Alliance marines on Akuze to the legendary Kalros that roamed Tuchanka, the thresher maw was among the most feared predators Commander Shepard would ever face.

Biology#

Thresher maws lived alone in nests that spanned large underground areas. They could grow in excess of thirty meters above the ground, with a body nearly twice that size beneath the surface, and the adult body never entirely left the ground, only the head and tentacles erupting to attack. Aggressive and highly territorial, they fed by absorbing large quantities of solar radiation and survived best on planets, asteroids, or moons with little or no atmosphere. They also consumed minerals and ores from the ground for use in an unusual form of photosynthesis, a metabolic process that turned solar radiation and minerals into the fuel powering their biology and allowing them to move their great mass with surprising speed. Because of their size and burrowing movement they were mostly immobile above ground but could travel at incredible velocities below it. They absorbed tremendous damage and were very hard to kill, attacking with viscous acid that splashed on contact, by bursting up beneath their prey, and by smashing targets with their claws while emitting infrasound. The direct physical nature of their attacks rendered shields completely ineffective.

The Akuze Massacre#

The Alliance first encountered thresher maws on the colony of Akuze in 2177. After contact was lost with the pioneer team, marine units were deployed to investigate, only for the shore parties to be set upon by hungry threshers that killed nearly the entire force. Since then thresher maws were found throughout the galaxy, surviving even severely hazardous environments, and their physical and biological origins remained unknown, though it was widely assumed they had been spread by a previous generation of space travelers, possibly before even the asari.

Cerberus and the Survivors#

The truth behind Akuze proved darker than a simple predator attack. A second survivor of the massacre, Corporal Toombs, was later encountered at a facility on Ontarom, where he claimed that the black-ops group Cerberus had caused the disaster deliberately. According to Toombs, Cerberus scientists had wanted to study the thresher maws and so allowed them to kill both the colonists and the marine rescue team in order to observe them, then held him captive for years of experiments. His account was likely true, as Cerberus used a similar method six years later, luring Admiral Kahoku's marine team to their deaths in a thresher nest on Edolus. Thresher maws were known to inhabit a number of worlds, usually one to a planet but sometimes two, including Antibaar, Chohe, Nepheron, Nonuel, Presrop, Xawin, Trebin, and Edolus.

Threshers and the Krogan#

Thresher maws were deeply woven into krogan culture. A thresher emerged during the krogan Rite of Passage into Clan Urdnot, lured to the proving grounds by persistent ground-pounding from a giant piston and attacking as the final test of a warrior's skill. Slaying one on foot was a feat of legend; in one telling, no krogan had accomplished it in generations, the last being Urdnot Wrex. Such a victory was a matter of great pride, with jubilant krogan boasting of eating thresher steaks for months afterward.

Kalros and the Reaper War#

The greatest of all thresher maws was Kalros, known as the mother from which all other thresher maws spawned and the largest ever seen. She roamed the wastelands of Tuchanka near the salarians' Shroud facility, acting as a natural deterrent to the vital installation, and a mural in the nearby ancient krogan city depicted her, suggesting she was at least as old as the city itself. During the Reaper War of 2186, Kalros was summoned to attack a Reaper Destroyer guarding the Shroud. Shepard activated two maw hammers that pounded the ground to call her, and following her territorial instincts she coiled around the Reaper and dragged it into the earth, clearing the path to the Shroud. Kalros' sheer size suggested that thresher maws had grown far larger over the ages, having reportedly been small enough to ride in the time of the Protheans.